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MORPHOLOGY OF INSECTS
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PREFACE
A simplified
version of Insect Morphology is presented for the purpose of quickly
instructing those interested in the identification of insects, particularly
those with predatory or parasitic behavior.
The evolutionary format used is to ease the means by which the various
insect structures may be learned.
Admittedly, some of the trends hypothesized may not be universally
accepted as valid. The text is
produced or paraphrased from cited references. It was developed over the
years by the author while at the University of Wisconsin, Utah State
University, Wilson College in Chicago, Texas A. & I University in
Kingsville and at the University of Illinois. The diagrams were derived and modified from those provided of
the author and Dr. Robert Dicke at the University of Wisconsin, Madison and
Dr. Donald Davis, Utah State University.
The terminology of Snodgrass (1952) is generally used.
Acknowledgment and appreciation are made to the following who assisted
during the course work and later developmental
phases: Dr. D. P. Annecke, Dr. Blair
R. Bartlett, Dr. Robert F. Brooks, Dr. Donald W. Clancy, Dr. Curtis P.
Clausen, Dr. Harold Compere, Dr. John Falter, Dr. Stanley E. Flanders, Dr. C.
A. Fleschner, Dr. Dan Gerling, Dr. Gordon Gordh, Dr. Marcos Kogan, Dr.
Clayton W. McCoy, Dr. David Rosen & Dr. G. Zinna. Special appreciation is extended to Dr.
Dorothy Feir who supplied some of the early drawings of Dr. Dicke, which had
become lost. - - - - - - - - - - - - - -
- - - - - - - - - - - - - - Introduction Insect
identification to the specific level requires a substantial knowledge of
morphology. The following is an
introduction to the gross, comparative morphology of insects. The term, morphology as developed in this work is a study of the functional
form of an insect, although details of anatomy
or the specific parts of an insect must be described before the functional
whole can be grasped. It is a comparative morphology restricted to
seven representative species that were chosen to broadly represent the
complex spectrum of insect forms.
These are in ascending evolutionary sophistication, Silverfish, Thermobia
domestica (Packard) ‑ Thysanura; Madeira roach, Leucophaea
maderae (Fabricius) ‑ Orthoptera; Milkweed bug, Oncopeltus
fasciatus (Dallas) ‑ Hemiptera; June beetle, Phyllophaga
rugosa (Melsheimer) ‑ Coleoptera; Noctuid moth, Heliothis
zea (Boddie) ‑ Lepidoptera; House fly, Musca domestica
(Linnaeus) ‑ Diptera; and the Honey bee, Apis mellifera
(Linnaeus) ‑ Hymenoptera The general plan
of this study establishes a typical insect form for comparative purposes,
which basically represents most insects, as we know them today. The cockroach, Leucophaea maderae, was
arbitrarily selected by Dr. Robert Dicke as such a "typical" form.
This selection was based on concepts of the evolutionary changes that
probably occurred from a hypothetical worm‑like ancestor through the
primitive silverfish, to the very highly evolved or specialized house fly and
honey bee. A primitive
structure or system is one that has occurred early in the evolutionary
history of insects, while a specialized structure is a more recent elaboration of a
primitive form. The establishment of
a concept of A primitive structure facilitates comparisons or homologies and
allows an understanding of specializations that have given insects as a group
such a wide range of successful adaptation to their environment. However, the concept or designation of
primitive does not imply relative uselessness. A vestige
is a useless relic of postevolutionary development. Although a primitive structure may have
occurred early in evolutionary history as a very useful, it may be retained
by an otherwise highly evolved form.
The giant tropical cockroach, Leucophaea maderae, representing
a group of Orthoptera which probably evolved very early in insect history
will serve as the typical form. Thermobia
domestica represents a group of primitively wingless Thysanura
illustrates many of the theoretical primitive structures. The milkweed bug, Oncopeltus fasciatus is
an insect that has retained the primitive wing development and metamorphosis
of Leucophaea
maderae, but also shows considerable evolutionary change in the
structure of the head and mouthparts.
Phyllophaga rugosa, Heliothis zea, Musca
domestica and Apis mellifera are representatives
of the four major orders of insects.
These illustrate many specializations, especially in the metamorphic
forms or larvae that precede the adult stage. The detailed
drawings in the text are useful during dissections and study of preserved and
living insects in the manner that an artisan would employ a set of blueprints
in his construction of a building or machine. The descriptive text should be studied, the structures
identified, and the concepts verified by examination of the drawings. However, all this effort is incomplete at best
until one has personally dissected, manipulated and identified the animal's
structures and systems. Theoretical
concepts are alluded to and then thoroughly discussed in Section IV. All technical terms are in bold faced type
and specifically described in Section VII,
Morphological Terminology. Dr. Robert Dicke in his course
"Insect Morphology" at the University of Wisconsin, concluded with
the following introductory comments, "Proceed carefully and diligently
with your study and dissection of these insects. You will be rewarded by a fascinating display of an ingenious
and beautifully created machinery that can sense and adapt itself to a
complex environment, that can ingest and synthesize a wide range of organic
matter, and that comprises a vast group of animals which probably will reproduce
and survive in spite of the intentional or incidental efforts of man to
exterminate them." EXTERNAL MORPHOLOGY
SECTION I ‑ THE BODY WALL
Metamerism and the Principal Body Regions A major
characteristic of an Arthropod is the division of its body into
segments. This trunk segmentation is
usually referred to as metamerism. Each body segment may then be identified
as a metamere. Considerable evidence exists that all
Arthropods including insects probably evolved from a segmented, worm‑like
ancestor or prototype
comprising about 20 distinct but undifferentiated metameres./1 Each metamere probably was cylindrical or
ring‑like in form, and in a series coextensive with the gut or
intestinal tract was joined together by transverse invaginations of the body
wall. The anterior opening to the gut
or mouth was probably
situated ventrally between the first metamere or prostomium and second
metamere, while the posterior opening to the gut or anus was borne by the last metamere or periproct. With the exception of the periproct, each
metamere acquired a pair of ambulatory appendages by means of lateral expansions of the
body wall. It is then believed that
this prototype evolved into the present day insect form through a series of
specializations in which distinct functions of the organism became the
responsibility of certain body regions.
These body regions or tagmata are the head (region
of ingestion and principal sensory perception), the thorax (region of locomotion),
and the abdomen
(region of visceral function and reproduction). The prostomium and first four metameres are thought to have
coalesced into the head region. The
locomotory appendages of the prostomium probably evolved into sensory
structures or antennae
and the three appendages of the posterior metameres of the head complex
became modified into organs of ingestion or, the mouthparts. Fusion of the metameres of the head region
has been so complete that no evidence of their separate entities exists in
present day forms. The 6th, 7th and
8th metameres comprise the thoracic region.
In most insect forms, lateral appendages of this region were retained
and further specialized to become the principal organs of locomotion. Wings, as additional expansions of the
body wall, provided highly specialized and unique forms of locomotory
structures. Complex external and
internal modifications of the thoracic metameres were required to support and
propel the leg and wing mechanisms.
The remaining metameres of the hypothetical prototype were evolved
into the abdominal tagma. With few
exceptions, the ambulatory functions of the lateral appendages of the
abdominal metameres were lost or modified into specialized appendages,
especially for the reproductive function.
The abdominal region, devoted primarily to housing the principal
visceral systems, retained many of the features of the undifferentiated
primitive metamere. A preliminary
examination of the body form of the representative insect species included here
will demonstrate that the three body tagmata are distinct even in the
caterpillar of Heliothis zea.
However, extreme modifications are quite apparent in the illustrated
sagittal sections of Leucophaea maderae (Fig 1), Apis
mellifera (Fig 2) and Phyllophaga rugosa (Fig
3).
The body of Leucophaea
maderae is flattened, or dorso‑ventrally compressed, and an
outline of the thoracic and at least the first eight abdominal metameres are
comparable in size and form. In
contrast, the abdomen of Apis mellifera is cylindrical, and
the number of abdominal metameres is reduced. An extreme modification of the first abdominal metamere has
occurred (fusion with the thorax, e.g., propodeum, and narrow petiolated
constriction). A disproportionate
development of the 2nd thoracic metamere has evolved along with wing development
at the expense of the first and 3rd (prothorax and metathorax). The
Exoskeleton
The body wall or
integument is the external covering of an organism which maintains its
characteristic form and contains the body fluids and tissue systems (Fig. 152):
In an insect,
the integument further serves the purpose of support as a skeletal system and
is an integral part in the mechanism of locomotion. The inner cellular layer or epidermis of the integument
secrets an external layer or cuticula./2 This cuticula is composed principally of
a complex of polymerized proteins, a nitrogenous polysaccharide commonly
referred to as chltin,
pigments and lipids. The entire
external surface of the insect (as well as such invaginations of the body
wall as the fore and hind gut and genital pouch) is covered by a layer of
cuticula. This continuous envelope of
cuticula, which incases the insect, is part of the integument, which is caste
and replaced when the body size is increased by growth. Cuticula may be soft and flexible or hard
and rigid. The degree of hardening
and inflexibility is known as sclerotization. A sagittal section of an insect's body
demonstrates that the integument serves as its skeletal structure. Compared with the internal bony skeleton
of a vertebrate, this structural mechanism is the exoskeleton. Thickness of cuticula and the degree of
hardening or sclerotization varies considerably. In Phyllophaga rugosa, the cuticula of the head and protergum is
much thicker than similar areas in Leucophaea maderae. The skeletal
structure of a metamere is not a simple inflexible ring of cuticula. Although the abdominal metameres are the
least modified from the hypothetical form, at least two divisions of the
metamere are apparent as shown in the cross sectional illustrations of Leucophaea
maderae (Fig 1)
and Apis mellifera (Fig
2). A dorsal plate
or tergum is separated
by a longitudinal infolding of the body wall from a ventral plate or sternum. This comparatively thin and flexible
infolding of the body wall is termed a suture. Each of these plates or other areas of the
body wall defined or separated by a suture are collectively termed
sclerites. The metameres of the
thoracic region are further subdivided into sclerites to make up the complex
ambulatory and flight mechanism. A
thoracic metamere is almost box‑shaped, and besides a tergum and
sternum there is a side area or pleura. The tergum, sternum and pleura are rarely
simple plates, but are further subdivided into sclerites especially on the
wing bearing metameres. The
Endoskeleton
The cuticula is
more than an outer skin or protective armor.
The body wall may be invaginated to form cuticular ridges or rods
wherever additional rigidity of the skeletal structure is advantageous or
where supplementary points for muscle attachment are required. These cuticular invaginations are usually
hardened or heavily sclerotized. They
are called apodemes
and collectively comprise the endoskeleton. Apodemes may be simple internal ridges such
as the dorsal invaginations between the thoracic metameres of Leucophaea
maderae (Fig 1). These dorsal thoracic invaginations may
be greatly expanded into a broad plate‑like structure or phragma for
muscle attachment as illustrated for Apis mellifera (Fig 2)
or Phyllophaga
rugosa (Fig 3). Rod‑shaped apodemes may combine to form an effective
brace or strut bridging the anterior head cavity. This structure is the tentorium situated at the base
of the mouthparts in Leucophaea maderae (Fig 1). Sternal
apodemes may be rod‑shaped or forked such as the sternal and
intersternal apodemes of Leucophaea maderae (Fig 1), or they may be a greatly expanded median plate such as
the sternal apodeme #3 of Phyllophaga rugosa (Fig 3), or sternal apodeme #2 + 3 of Apis mellifera. If the apodeme is an internal ridge or a
phragma, the external evidence of such an invagination is an impression of
the body wall. If this is a shallow
groove or impressed line, it may be properly referred to as a suture. However, if the site of this invagination is a deep furrow, it
is usually referred to as a sulcus. Where the apodeme is a rod or tubular structure, its
point of invagination may be called a pit,
e.g., tentorial
pits of the head
tagma. Not all of the cuticular
invaginations are sclerotized. Soft,
flexible invaginations or intersegmental membranes occur between the metameres. These membranes may be pleated and folded
as illustrated for the abdominal metameres of Leucophaea maderae (Fig 1). The intersegmental membranes permit
articulation of the metameres and expansion of the abdominal cavity. This abdominal expansion in insects is
rarely accomplished by a stretching of the body wall. Cuticula when stretched does not fully
regain its original form. Expansion
of the abdomen is accomplished by an unfolding of the intersegmental
membranes. A longitudinal suture
accomplishes articulation or expansion between the tergal and sternal
sclerites of the abdomen. Protuberances
of the Body Wall
The external
surface of the cuticula is rarely smooth.
In addition to the more apparent protuberances, the cuticula may be
variously sculptured with minute depressions, corrugations and striations, or
by irregularly alternating concave and convex surfaces. The cuticula may be produced into heavily
sclerotized spines
such as in the caterpillar of Heliothis zea (Figs
4 & 5):
The spines may
be sharply pointed or they may be blunt and irregularly shaped knobs. Spines often resemble minute hairs and are
referred to as microtrichia
(Fig 6). The veins and wing membrane of Musca
domestica have a scattered covering of microtrichia (Fig 10). Although spines
usually occur in an irregular pattern, they may be arranged in well-defined
lines such as on the tibial spurs of Leucophaea maderae (Fig
8). or on the ental surface of
the labrum in the grub of Phyllophaga rugosa (Fig 108):
All of these structures are
collectively referred to as noncellular processes since the
protuberance is composed entirely of heavily sclerotized cuticula and are
fixed to and confluent with the exoskeleton. Frequently, the
epidermal cells of the body wall may become modified for the specialized
function of secreting single hollow protuberances or unicellular processes. These may exhibit a variety of forms and
are referred to by many descriptive terms.
The hair like movable structures that are found on all insects are
usually designated as setae
(Fig 6); and the flattened, spatulate structures may be
correctly identified as scales
(Figs. 7 & 11). All unicellular processes arise from a
well-defined socket and are seated in a flexible membrane. The socket of a unicellular process
distinguishes these structures from the fixed cuticular microtrichia, which
they frequently resemble. Unicellular
processes may be further modified into sensory and protective
structures. Setae may be associated
with nerve cells and accomplish a tactile or olfactory function.
The importance
of numerous sensory structures scattered over the surface of the body is
evident when it is understood that the sclerotized integument effectively
isolates the insect from its environment.
A modified hypodermal cell may secrete an urtication fluid into a
hollow setae. When such a seta is
broken in the tissues of a predator, it serves as a deterrent. Setae may be found profusely scattered or
in constant patterns on the insect's body or appendages wherever cuticular
structures occur. They are abundant
on the compound eyes of Apis mellifera, on all of the
mouthparts of most insects, on the relatively naked wings of Leucophaea
maderae, and on the external genitalia of Phyllophaga rugosa. Most setae occurring on the body probably
serve only as a protective covering and as such appear to be scattered
without any particular design. These
may be referred to as secondary
setae. However, certain setae may be heavily
sclerotized and pigmented, and appear bristle‑like and conspicuously
larger than the more numerous secondary setae. These setae, commonly called primary setae, are usually arranged in a constant and bilaterally
symmetrical pattern peculiar to a species (e.g., Fig 5). The setal
design or positioning of setae on the left side of a metamere is a mirror
image of the setal arrangement on the right side. Their arrangement may be so constant that the design may be
employed as taxonomic characters (Fig 5). The study of
setal arrangements, their use in identifying insect species, and the
nomenclature applied to these setae is known as chaetotaxy. The dorsal thoracic setae of Musca
domestica may be used to distinguish primary from secondary setae (Fig
9). The relatively small setae illustrated are
secondary setae. It should be noted
that they are numerous and that they do not occur in a constant pattern. The large conspicuous setae (designated bristles by
descriptive entomologists) are differentiated as primary setae. These setae are arranged in a bilaterally
symmetrical design peculiar to Musca domestica. The nomenclature employed in chaetotaxy
varies considerably from one taxonomic group to another. Primary setae of muscoid flies are
designated by terms that are descriptive of their position on the thorax,
e.g., anterior dorsocentral bristles (Fig 9)
(situated on the anterior sclerite of the thoracic tergum on more or less a
central line), acrostical bristles (setal rows in parallel lines or across
from each other), etc. Chaetotaxy has
been extensively employed in the taxonomy of such naked larvae as the
caterpillars of Heliothis zea (Fig 5). Comparative arrangements and size of setae
are plotted on a rectangular setal map. The
left side of a particular metamere from the mid‑dorsal to the mid‑ventral
line is included. The positions of
the primary setae in relation to each other are good taxonomic characters
since they are constant for a species but quite variable between
species. Primary setae of insect
larvae are usually designated by letters of the Greek alphabet (Fig 5), although various numeral and/or letter systems are also
encountered in the literature. Setal
patterns are not the same on all of the metameres. The first thoracic metamere is distinct from the 2nd and 3rd. In Heliothis zea, one seta, RHO
situated above the spiracle, is more prominent than others since it is
usually seated on a raised and distinctly pigmented area (Fig 5). Using RHO as a
central point for Heliothis zea, it will be noted from the drawing that four
prominent setae occur above it on the first (prothoracic) metamere (ALPHA,
BETA, GAMMA, and DELTA). It also
occupies a pigmented area with an additional smaller seta (EPSILON). On the 2nd & 3rd thoracic metameres
(mesothorax and metathorax), two setae (GAMMA and DELTA) are absent. On the mesothorax, seta ALPHA lies
directly above BETA in comparison to its more anterior position on the
prothorax. The setal arrangements on
the first seven abdominal metameres are uniform but are not comparable with
the thoracic metameres. To
illustrate, seta EPSILON lies dorsad of the spiracle on the prothorax but
anterior to the spiracle on the abdominal metameres. The position and number
of setae below the spiracle is also quite different when a comparison is made
of the thoracic and abdominal regions.
Abdominal metamere 9 is comparatively narrow, does not bear a
spiracle, and has a reduced setal pattern.
Taxonomists usually figure
as the most diagnostic, the first and 2nd thoracic metameres, the 2nd and 3rd
abdominal metameres (the 3rd bearing an abdominal appendage, the proleg), the 8th
metamere, and the reduced 9th.
Although secondary setae are arranged in a constant pattern on many
species of insects, occasional variability can be expected. In the thoracic illustration of Musca
domestica for example (Fig 9),
the 2nd anterior dorsocentral bristle is absent. The socket in which it was previously seated may identify a
broken seta. However, these should
not be confused with naturally occurring punctures in the cuticula. These punctures are referred to as pits as illustrated
on the prothorax of Heliothis zea (Fig 4). Pits are usually external openings associated
with chemical sense receptors situated in the cuticula.
Tubular, hair
like setae are the more common unicellular protuberances encountered in
insects. However, they may be
modified into spatulate or plate‑like structures referred to as scales. These may represent a variety of shapes from elongated fringe
scales to broad plates as illustrated by the wing scales of Heliothis
zea (Fig 7). Body scales are also abundant in some
insects as illustrated by the broad thoracic scales of Thermobia domestica (Fig 11). The scales may be pigmented and precisely
arranged in an overlapping pattern comparable to the placement of shingles on
a roof. Parallel ridges that form
minute striations usually mark the flat plane of the scale. This sculpturing of the scale may produce
a physical coloration due to interference of reflected light. Protrusions of the entire body wall
including the formative epidermis comprise the relatively conspicuous multicellular processes. Such a process may be a simple elevation
of the integument bearing a unicellular seta at its apex. The illustration of seta ALPHA in Heliothis
zea (Fig 6) is an example
of a simple multicellular structure termed a chalaza by descriptive entomologists. Common examples of the more conspicuous
multicellular processes are the heavily sclerotized, spiny structures termed spurs that are encountered
on the legs of many insects. These
spurs may be fixed and confluent with the cuticula. Others may be set in a membranous ring and are therefore
movable as illustrated by the tibial spurs of Leucophaea maderae (Fig 8). Multicellular processes may bear fixed
spines as the microtrichia on the spurs of Leucophaea maderae (Fig 8) as well as single or numerous unicellular setae.
= = = = = = = = = = = = = = = = = = = SECTION II ‑ THE HEAD
Evolution of the Insect
Head
The principal
regions of the insect body are thought to have evolved as composites of
cylindrical metameres, each of which in the primitive form bore a pair of
ambulatory appendages./1
(See Figs. 148-151):
While this
theory seems plausible for the abdomen and in most forms for the thorax, it
appears at first examination to be a rather remote assumption for the head
region. The head capsule has become a
highly evolved or specialized
structure involving at least five primitive or generalized
metameres. The first metamere or prostomium probably
bore the mouth opening at its posterior margin in addition to a pair of
appendages that evolved into the sensory antennae. A
study of the brain of present‑day insects and the head region of
certain related arthropod forms such as the Crustacea has led morphologists
to assume that the prostomium and the next following metamere (first
postoral) both developed sensory antennae.
With later evolution, the principal sensory structures were then
situated on the first two metameres. These
metameres may have fused early in the evolution of the head to form a theoretical
protocephalon. The development
of the photo
receptors or eyes
is not clear, although these sensory structures are believed to have
developed on the prostomium. From a
comparative study of the morphology of present‑day insect mouthparts
and the nerve centers associated with them, it may be concluded that these
organs of ingestion probably evolved from ambulatory appendages. Since three pairs of structures make up
the generalized feeding mechanism, it may be assumed that three metameres
were involved in the formation of a second primitive head complex or gnathocephalon. In the present‑day insect, the
sensory protocephalon and the ingestive gnathocephalon have coalesced and
have become completely fused into a composite structure. Unlike the thorax and abdomen,
segmentation of the head is obscure and the sutures as we know them today
have little correlation with the metameres that were involved in its
formation. The Typical or
Generalized Insect Head
The head of Leucophaea
maderae may be used to illustrate a typical, generalized form of head
capsule (Figs. 12-16):
Essentially, the head is an ovoid envelope of sclerotized integument
enclosing the brain centers, certain glands, and muscle systems for the
operation of the head appendages. The
head capsule is open at its posterior juncture with the thorax to permit a
passageway for certain connectives such as the ingestive tube, which connects
the mouth with the digestive system.
This opening is called the occipital foramen. The thin,
flexible cylinder of integument connecting the margins of the occipital
foramen with the thorax is the neck or cervix. A mouth opening is situated on the ventral
aspect of the capsule that is also depressed to form a pocket or oral cavity to accommodate the
operation of the mouthparts. Internally, an A‑shaped,
composite apodeme formed by invaginations of the integument, braces the head
capsule before the oral cavity. This
brace is the tentorium, and the
points of invagination of the integument are the tentorial pits. Usually, the tentorium is well developed
in insects that have powerful biting and chewing mouthparts to form an
internal strut, to prevent the moving jaws from collapsing the head
capsule. In Leucophaea maderae, the
anterior invaginations or anterior tentorial arms unite mesally to form a bridge,
while the posterior invaginations form at the base of the occipital foramen
a posterior
tentorial bridge (Figs 15 & 16). The fused
anterior tentorial arms and posterior tentorial bridge are united into a common,
A‑shaped structure leaving a median opening for the passage of nerve
trunks. The conspicuous
photoreceptors or compound
eyes occupy the dorso‑lateral
aspects of the head, and the antennal sockets are situated on the frontal
surface between the eyes. A suture
outlines and separates the compound eye and antennal socket from the
adjoining sclerotized areas. These
sutures may also enclose a sclerotized area forming a ring about the sensory
structure. In Leucophaea maderae,
there is an ocular
suture enclosing an ocular sclerite (Fig
13), and an antennal suture enclosing an antennal sclerite (Fig
12). The anterior
surface of the head lying between the compound eyes is designated as the frons (Fig 12). Although the frons is usually easily
identified as the broad frontal area between the eyes, an accurate
identification of facial areas is best made with reference to the sutures
lining the integument of the head. It
should be emphasized that while certain head sutures are relatively constant
in position, they do not represent the primordial divisions of the metameres
that originally formed the head region.
Ventrad of the
frons in Leucophaea maderae is a short suture bearing at its mesal
ends the anterior
tentorial pits. This is the epistomal suture (Figs 12 & 13). In most
insects, the epistomal suture is continuous across the face and is probably
the most constant frontal suture to use for the identification of facial
areas. The anterior arms of the
tentorium are usually anchored on the apodeme or an epistomal ridge formed by the invagination of this suture. When anterior tentorial pits are present,
they will always be found on the epistomal suture. If the anterior pits are not developed, the suture may be
identified by dissection of the head that may reveal that the tentorial arms
are anchored on the epistomal ridge. In some species, the tentorial pits are readily identified, but
the epistomal suture is absent, or incompletely developed as in Leucophaea
maderae. An imaginary line
drawn between the two pits will represent the absent suture and will serve to
identify the facial areas usually separated by it. The facial area above the epistomal suture is the frons; the
area below the suture is the clypeus. Occasionally, the distal portion of the clypeus is
membranous. The proximal sclerotized
portion of the clypeus is then identified as the postclypeus and the distal,
membranous portion as the anteclypeus
(Fig 12). An oblong sclerite freely articulating at
its proximal margin with the clypeus, is the labrum. This sclerite serves as an upper lip for
the mouth cavity. Although the
labrum is generally considered as a part of the organs of ingestion, it is a
true sclerite of the head and was not evolved from an appendicular
structure. The gena or cheek is a poorly defined area in most insects,
but usually lies below and immediately behind the compound eyes. In Leucophaea maderae, this area is
set off by a short subocular
groove (Fig 13). An area immediately above the
articulations of the mandibles may be heavily
sclerotized to support the powerful jaws.
This area margined by a subgenal suture is designated as the subgena. The subgenal suture is usually continuous
with the epistomal suture. A frontal
suture resembling an inverted Y is
common in immature insects and is known as the epicranial suture. This is
actually an ecdysial
suture or a point of
rupture in the integument during the molting process. The epicranial suture is uncommon in adult
forms, although it is faintly represented in Leucophaea maderae (Fig 14). The stem of the Y is referred to as the coronal suture and the arms as the frontal sutures. When this
suture is developed, the area enclosed by the frontal sutures is designated
as the frons. The top of the head as a poorly defined area is
the vertex. When an epicranial suture is present, the
vertex is the area immediately to either side of the coronal suture. Identification of the posterior areas of
the head is best accomplished by locating the posterior tentorial pits (Fig 14). These mark the point of invagination of
the posterior tentorial bridge. The
pits are always situated on a postoccipital suture. As for the
epistomal suture in the frontal region, the postoccipital suture is usually
the most constant suture of the posterior region. The sclerite enclosed by the postoccipital suture is the postocciput
which serves as a sclerotized ring about the occipital foramen. The neck membrane or cervix is attached to
this sclerite, and a mesal projection or occipital condyle
serves as a point of articulation for the sclerites of the
cervix. An ---------------------------------------------------- 1/ Refer to Section IV ‑ Origin of the Principal Body Regions.
additional suture may occur anteriorly to the postocciput and
margins the flat posterior aspect of the head. In Leucophaea maderae this suture is more of a marginal ridge,
but it may be referred to as the occipital suture and the area enclosed by it as the occiput. Usually, the term occiput is used only to
describe the posterior area immediately behind the vertex. The lateral, ventral portion of this
sclerite is then referred to as the postgena. However, technically the entire sclerite
may be correctly referred to as the occiput. THERMOBIA DOMESTICA:
The head of Leucophaea
maderae was described as the "typical form.” But this does not imply that the head of Leucophaea
maderae is primitive in the sense of being but little elaborated in
comparison with a hypothetical prototype.
Thermobia domestica is a relatively primitive insect compared
with Leucophaea maderae.
The conspicuous epistomal sulcus of Thermobia domestica
will readily distinguish the facial areas (Figs 17 & 18). Note that the frons and clypeus are large,
well-defined sclerites. The gena,
however, is a small area immediately before the antenna and below the
eyes. All of the other head sclerites
described for Leucophaea maderae are absent. The postocciput as a sclerite is inconspicuous, but the
invagination of the postoccipital suture forms a large apodeme or postoccipital
ridge (Figs 19 & 20). The tentorium of Thermobia domestica is
of special interest to the morphologist.
Previously, this was defined as a cranial brace formed by the fusion
of two anterior and two posterior invaginations of the exoskeleton forming
the head capsule. In Leucophaea
maderae, the tentorium forms an A-shaped structure comprising a
posterior tentorial bridge and two anterior arms. However, the posterior tentorial bridge of Thermobia domestica has
not fused with the anterior arms although a large central plate has been
formed by the posterior fusion of the anterior arms. If the theory on the formation of the
tentorium is correct, it may also be assumed that in Thermobia domestica
this is a relatively primitive structure. Specializations
in the Adult Head Structure
Further
modifications of the insect head from the typical form may occur in 1) the
fronto‑clypeal region, and 2) the posterio‑ventral region. For many of the highly evolved forms,
these modifications may progress to the point where it is difficult, and in
some forms impossible to compare or homologize the sclerites with the typical
form. This is especially evident in
species that have evolved highly specialized sucking mouthparts, or in the
larvae of immature forms of the Endopterygota. Where the structures cannot be identified, it may then be
necessary to borrow a descriptive term from the taxonomic literature. When the epistomal suture is intact, there
is little difficulty in identifying the facial sclerites. The area above the suture is the frons,
and the sclerite below is the clypeus.
The epistomal suture is not always in a transverse line. In the adult of Apis mellifera (Fig 27) and the larva
of Heliothis
zea (Fig 57), this suture is
strongly arched dorsad and resembles the epicranial suture. Since the tentorial pits are situated on
the suture, the area enclosed by it would resemble the frons but would be
incorrectly identified as such. In
the absence of the epistomal suture, the tentorial pits may determine the
relative areas since the anterior arms of the tentorium are always anchored
in position on the epistomal ridge.
Dissection of the head will also determine the position of the
tentorial invagination should the pits be indistinct. Certain muscles of the sucking apparatus
and ingestive canal arise from either the frons or the clypeus, and these
sclerites can be identified by their muscular attachments. Where the tentorial arms are greatly
modified or where they are absent as in Musca domestica, a study of the
musculature of the sucking apparatus is the only clue to identification.
The posterio‑ventral
aspects of the head are modified in many forms so that the mouthparts may
project forward. In the generalized form, the facial area is directed
forward and is anterior and vertical in position. The mouthparts are pendant or hang ventrally in position, and the
labium that forms
the floor of the oral cavity is attached to the cervix. This position of the head is referred to
as the hypognathous
form. Direction of the mouthparts
forward is advantageous to many species.
The head is rotated upward with the mouthparts directed anteriorly,
and the facial region is now in a relative horizontal or dorsal
position. This modification is known
as the prognathous
form. In order that the occipital
foramen will retain its vertical plane, the ventral surface o, the head must
be elongated. This is accomplished by
1) the formation of a gula
that is a sclerotization of the neck membrane at the base of the labium, and
2) by a lateral expansion of the subgenae.
The expanded postoccipital suture always encloses the gula. When a gula is present, the postoccipital
suture is often referred to in descriptive literature as the gular suture. As the ventral aspects of the head are
expanded in the prognathous form, the attachment of the labium becomes
further removed from its original attachment to the cervix. PHYLLOPHAGA
RUGOSA (Fig
3).
The head capsule
of Phyllophaga rugosa is
oval in shape, flattened dorso‑ventrally, and the facial area is
essentially like that of the typical form.
It is heavily sclerotized and further strengthened by a TT‑shaped
tentorium. The posterior tentorial
bridge is weak, but the anterior arms are well developed and have become
fused with the ventral sclerites.
Posterior tentorial pits lying on the gular suture are well developed,
but the anterior pits at the base of the compound eyes are difficult to
demonstrate. However, the anterior
tentorial arms are attached at the outer margin of the epistomal suture. Unlike Leucophaea maderae, a
well-developed gula has projected the mouthparts forward. The head of Phyllophaga rugosa is
therefore of the prognathous form.
Two other modifications distinguish this species from the typical
form: the clypeus is strongly reflexed to produce a ledge which overhangs the
labrum, and a slender sclerite given the descriptive term of canthus projects into the
ocular region (Fig
3). APIS MELLIFERA:
At first
examination, the head of Apis mellifera appears like the
typical form previously described including a "typical" epicranial
suture. It was already noted that the
epistomal suture sometimes is strongly arched upward enclosing a triangular
sclerite that is often incorrectly identified as the frons. This is definitely the epistomal suture
since the anterior tentorial pits are situated on it at a point below the
antennae. Therefore, the area
enclosed by this suture is the clypeus (Fig 27). Unlike Leucophaea maderae, the
antennae are considerably removed from the margins of the compound eyes, and
a cluster of three simple eyes (the ocelli)
is situated on the vertex.
Posteriorly, the occipital foramen is greatly reduced in size compared
with Leucophaea maderae or Phyllophaga rugosa, an occiput is
not clearly defined, and the postocciput is a pair of small sclerites on
either side of the foramen. These are
clearly identified by the posterior tentorial pits. On the ventral aspect of the head, the postgena has become
deeply invaginated to form a pocket within which the base of the mouthparts
is seated (Figs 22 & 24). This pocket may be referred to as the postgenal
inflection. The mouthparts
of Apis
mellifera will be discussed in considerable detail in the following
section III, but it should be noted at this point that the mouthparts of the
typical chewing form have been modified into a complex sucking
mechanism. However, the mandibles
have been retained as functional structures comparable to those of Leucophaea
maderae and Phyllophaga rugosa. The tentorium is a typical TT‑shaped
brace with a posterior tentorial bridge and strong anterior arms. A sexual dimorphism is very evident in the head of Apis
mellifera. The heads of the
queen and the worker (in which the sexual organs are retarded) are comparable
in form (Figs 21 & 28). In the male, or drone,
the compound eyes are greatly expanded at the expense of the frons and gena,
giving the head an appearance that at first would seem quite unlike that of
the female sex (Fig
27). HELIOTHIS ZEA:
The head of Heliothis
zea is densely covered with setae, and is conspicuous for its large
compound eves which occupy much of the head surface, long antennae, and a
coiled sucking tube or proboscis
(Fig 34). When the head is denuded of its setae,
only the frons remain of the facial sclerites. The epistomal suture and the anterior tentorial pits are absent,
but the anterior arms of the tentorium are anchored at the posterior margin
of the facial sclerite correctly identifying it as the frons. The gena appears to be absent, although
this may be the area described by taxonomists as the mandibular
sclerite (Fig 30). The labrum is greatly reduced to an
inconspicuous flap. On either side of
the labrum are two small sclerites given the descriptive term of pilifers (Fig 31). These sclerites are of unknown
morphological origin although they are said to be remnants of mandibles. Two simple eyes or ocelli are situated
between the antennae and dorsal margin of the compound eyes. Posteriorly, a dorsal sclerite appears to
be the occiput. A small postocciput
identified by the posterior tentorial pits occurs above and rings the
occipital foramen. Postgenal
sclerites make up the flat, lateral and ventral aspects of the posterior head
capsule. The tentorium is a typical
TT‑shaped structure, although the posterior bridge and anterior arms
are weak. Of special interest is that
the anterior arms of the tentorium are inflated midway into weakly
sclerotized bulbular structures. The
function of these expansions is unknown.
The remaining identified sclerites of the head such as the postmentum
and ligula and appendages such as the proboscis and palps are modified from
and associated with the organs of ingestion and will be discussed in the
following section. Oncopeltus fasciatus :
Although
taxonomists have placed Oncopeltus fasciatus relatively
low on the phylogenetic scale, it is actually a highly evolved form. The organs of ingestion are an efficient
piercing‑sucking apparatus; the head has been rotated forward by the
development of an extensive gula, and the facial sclerites associated with
the mouthparts have been modified to the extent that it is difficult to
homologize many of them with the typical form. An oblong sclerite given the descriptive name of tylus by specialists of
Heteroptera is probably the anteclypeus (Fig 42). This sclerite is confluent with the
integument of the head capsule at its posterior end and is laterally margined
by a deep sulcus, which is probably the epistomal suture. That this sulcus is the epistomal suture
may be assumed since the anterior arms of the tentorium are anchored on the
walls of this inflection. Actually,
this is not a suture in the sense that it is an invagination between two
sclerites, viz., the clypeus and the frons.
The lateral margins of the tylus are not united with the head capsule
and the entire sclerite is fixed only at its posterior end, and lies freely
in a groove formed by the inflection of the integument, which was tentatively
identified as the epistomal sulcus.
The sclerotized walls and partial floor of this groove (best seen by
removing the anteclypeus) is identified as the maxillary plate (Figs 37 & 43). The two plates or sclerotic areas lying
between the anteclypeus and the base of the antenna are probably an
expansion of the gena. But, this area
has been given the descriptive name of jugum (Fig 42). Since the muscles and apodemes associated
with the mouthparts are also associated with this sclerite, morphologists
have referred to this area as the mandibular plate (Fig
40). Pigmentation of
the head of Oncopeltus fasciatus is such that a light, triangular area is
formed on the facial region.
Demarcation of the black pigmentation in the adult is so distinct that
some specialists have assumed the presence of an epicranial suture, and have
named the light triangular area the frons (Fig 42). A distinct epicranial or ecdysial suture does occur in the immature form or nymph, but there is no evidence of
such a suture in the adult. The
dorsal surface of the head (or facial area since this is a prognathous head)
is the frontoclypeus
(Fig 35). Later it will be shown that the muscles,
which operate the highly evolved sucking pumps, are anchored on the facial
sclerites. The origin of these
muscles in Oncopeltus fasciatus indicates that both sclerites are
present. Modification of the head has
been such that an epistomal suture does not separate them, and the entire
area must be identified by this composite term. The compound
eyes protrude from the head capsule by expansion of the genae. Two simple eyes occur at the bases of the
large compound eyes. In Oncopeltus
fasciatus, the labrum is not a simple oblong upper lip as in Leucophaea
maderae, but has been modified into a sharply tapering flap that
covers the basal portion of the proboscis (Fig 41). The ventral floor of the head comprises a
large sclerite termed the gula, and margined by sutures referred to as gular
sutures. Again, this identification
is uncertain since the gular sutures do not appear to be homologous with a
postoccipital suture bearing the posterior tentorial pits. The tentorium is
modified into two arms without a posterior tentorial bridge. The anterior tentorial pits may be found
on the epistomal sulcus, but the posterior arms are free. Each arm is anchored posteriorly to the
head capsule by lateral projections which are fixed to the head capsule at a
point below the compound eyes, but not on the gular suture. The proboscis is set in a membranous area
margined by sclerotized ridges. These
ridges or elevated plates are referred to as the buccula (Fig 41)., a descriptive term since their identification is
obscure. The occipital foramen is
large (Fig 38), and is
margined by what appears to be a postoccipital sclerite. Since the posterior tentorial bridge is
absent, this sclerite is also difficult to homologize.
The head of the
adult fly is ovoid and hypognathous with the complicated sucking apparatus
pendant in position. Similar to Oncopeltus
fasciatus, the facial areas of Musca domestica are also difficult
to identify. The tentorium is greatly
reduced and is without anterior arms or a posterior tentorial bridge. A posterior tentorial ridge has been tentatively identified as a modified part of
the tentorial structure (Fig 51). To further complicate the head structure,
the ptilinum, a
peculiar invagination of the head capsule, has required further modifications
of the facial region (Fig 51). The ptilinum is an invaginated sac which
is protruded (along with a distension of the frontal region) bubble‑like
during the emergence of the adult from its pupal case. Although the ptilinum is used only during
emergence, its suture or invagination remains intact. For want of a better term, this suture is
referred to as the frontal
suture (Fig 44). However, this
frontal suture of Musca domestica is not homologous with the anterior arms of
the previously described ecdysial or epicranial suture. The area enclosed by the frontal sutures
is possibly the true frons. An
epistomal suture is absent, although a sclerite termed the hinge plate on the ventral margin of
the frons may represent the epistomal region (Fig 45). A true clypeus, identified by the muscle
attachments of the pumping mechanism (the cibarlal pump), does occur on the
proboscis and articulates with the hinge plate (Fig 51). Thus far in the description of the facial
area, the clypeus is the only sclerite that can be identified with any degree
of certainty. At the apex of the
frons is a distinct triangular sclerite given the descriptive name of frontal lunule (Fig 45). The marginal sutures of this sclerite lead
directly to the ptilinum. A pair of
highly modified antennae lies on the frons with their base attached to the
apex of the frontal lunule. The
sclerotized areas between the compound eyes and frontal sutures are the gena
(the "cheeks" of descriptive entomologists). Dorsad of the frontal lunule is a
sclerotized area identified as the vertex.
At the apex of the vertex is a distinct protuberance or chalaza bearing 3 simple eyes in a
cluster. A sexual dimorphism is
evident in that the vertex of the male is narrow compared with that of the
female (Figs 46 & 47). Since the eyes appear to be set close
together in the male, this condition is referred to as holoptic. In the female, the head is dichoptic, or a
condition in which the eyes are set comparatively wide apart. In the absence
of a distinct tentorium, the posterior regions are also difficult to
homologize. The occipital foramen is
comparatively small. A ridge margins
its ventral aspect, which is probably the postocciput. An occipital suture is absent, although
the dorsal area of the broad posterior aspect of the head is usually referred
to as the occiput. Its lateral and
ventral aspects are identified as the postgenae (Fig 48). The fleshy proboscis is divided into two
distinct parts referred to as the rostrum and haustellum (Fig 50). This complex structure will be discussed
in detail in the following section III. Specializations in the
Head Capsule of the Immature Insects
The body form of
a grub or caterpillar might suggest that these worm‑like immatures are
primitive in form, but this assumption would be far from accurate. The larvae of the holometabolous insects
are highly evolved forms modified to meet a particular food niche. The head structure ranges in complexity
from the grub of Phyllophaga rugosa to the maggot of Musca domestica. Little difference would be observed in the
head of an adult or immature Thermobia domestica, and the
nymphal head of Leucophaea maderae is comparable with the adult. The nymph of Oncopeltus fasciatus is
also comparable with the adult. The
presence of a well-developed epicranial suture in the nymph is the important
exception, and the gular region may be incompletely developed. The epicranial suture in Oncopeltus
fasciatus is obviously an ecdysial suture. It is absent in the adult of Oncopeltus fasciatus
although it is retained in the adult of Leucophaea maderae. PHYLLOPHAGA RUGOSA Larva:
The facial
regions are readily identified in the grub of Phyllophaga rugosa. A distinct epicranial suture is
present. Both compound and simple
eyes are absent. The tentorium forms
a strong posterior tentorial bridge, but the anterior arms are weak and do
not extend to the epistomal suture (Fig 52). As a compensation for these weak tentorial
arms, it should be noted that the margins of the mouth cavity are strongly
sclerotized to accommodate the articulation of the chewing mouthparts. The epistomal suture also forms a strong epistomal ridge (Fig 54) which is an
apodeme bracing the ventral aspect of the head. A distinct labrum is present, and the clypeus is divided into
an anterior membranous anteclypeus and a posterior sclerotized
postclypeus. Unlike the adult, no
posterio‑ventral development has occurred, and the chewing mouthparts
of the hypognathous head are pendant as in Leucophaea maderae. The larval head
is unique in its posterior development.
A postoccipital suture is present along with a narrow, laterally
flanged postocciput. Fixed to this
sclerite is a broad
plate which is attached to the posterior aspect of the head. This plate is identified as a cervical plate since it is a
sclerotization probably derived from the cervix (Fig 53). The membranous cervix proper is joined to
the outer margins of the cervical plate giving the cervix a broad truncate
attachment. Actually, the occipital
foramen is considerably smaller than is indicated by the broad attachment of
the cervix. HELIOTHIS ZEA Larva:
At first, the
facial region of the larval Heliothis zea appears as the
generalized form of an immature insect.
There seems to be a distinct epicranial suture enclosing a triangular
frons. In fact, quite incorrectly,
the head of a caterpillar such as Heliothis zea has been illustrated
as a form with a "typical" epicranial suture. However, examination of the tentorium
shows that the small anterior arms are attached about midway to the so‑called
frontal sutures. This condition,
then, is similar to the adult of Apis mellifera. The suture may now be correctly identified
as a strongly arched epistomal suture, and the area enclosed by it is the
clypeus (Fig
57). The partially
sclerotized area between the triangular clypeus and the labrum may be
correctly identified as the anteclypeus.
An examination of the endoskeleton reveals that the stem of this
suture forms a strong internal apodeme.
It seems, then, that the incorrectly identified coronal suture is in
fact an invagination of the frons to form a strong internal plate‑like
ridge. This suture is correctly
termed the frontal
sulcus (Fig 60). The tentorium
forms a fairly strong posterior tentorial bridge, although the anterior arms
are weak. In the absence of an
effective tentorium, invaginations of the epistomal suture and frontal sulcus
‑ the frontal
brace ‑ provide the
strong endoskeleton necessary for strengthening the head capsule (Fig 60). The combined epistomal suture and frontal
sulcus do serve as an ecdysial suture during molting of the larva. Functionally, then, this combined Y‑shaped
suture could be identified as an epicranial suture. However, anatomically the suture is not homologous with the
epicranial sutures of other immature forms such as Oncopeltus fasciatus, Phyllophaga
rugosa or Apis mellifera.
Peculiar to lepidopterous larvae is a secondary weak suture, which
parallels the epistomal suture. It
has been suggested that this suture represents the frontal arms of a
primitive ecdysial suture and that the area enclosed by them is a remnant of
the frons. Since there is little
evidence to support this, the suture must be identified for the present by
its descriptive name, the adfrontal suture,
and the area enclosed by it as the adfrontal area (Fig
59). The regions
laterad of the clypeus are the genae and dorsad, the vertex since it was
shown that the frons is invaginated.
Compound eyes are never found in the immature forms of such
holometabolous insects as Phyllophaga rugosa, Heliothis
zea, Apis mellifera and Musca domestica. However, in Heliothis zea six
simple eyes (ocelli) occur in a semicircle on the lateral aspect of the head
(Fig 61). Posteriorly, as in Phyllophaga rugosa, the
head is broadly joined to the cervix.
However, unlike Phyllophaga rugosa the occipital
foramen of Heliothis zea is very
broad. An inflexed bridge, which
margins the occipital foramen, may be referred to as either the occiput or
postocciput. Modifications in the
antennae and mouthparts as will be described later, also indicate that the
hypognathous head of Heliothis zea is highly specialized. APIS MELLIFERA Larva:
Unlike the free‑living
larvae of Phyllophaga rugosa and Heliothis zea, the grub of Apis
mellifera is the ward of a socialized system and is cared for by the
worker bees within a protective comb cell.
It might be assumed that there would be little need in this larva for
the development of strong, efficient organs of ingestion or effective organs
of sensory perception. Actually, the
head of Apis mellifera has evolved in the direction of simplification. The mouthparts with the exception of a
silk organ, are greatly reduced, and the organs of sensory perception are
reduced to functionless vestiges. A
well-developed epicranial suture encloses a fronto‑clypeal region (Fig 62). However, identification of this region is
somewhat uncertain, because an epistomal suture is absent, and the anterior
tentorial pits appear at the distal ends of the frontal sutures. But, an examination of the endoskeleton
gives little reason to assume that the arms of the Y‑shaped suture are
morphologically comparable to the condition described for Heliothis
zea. The tentorium is a
typical TT‑shaped structure with distinct anterior and posterior
tentorial pits. Both the posterior
tentorial bridge and anterior arms are weakly sclerotized. The occipital foramen is very wide, and
the head is broadly joined to the thorax without a distinct cervix. MUSCA DOMESTICA Larva:
The larva of Musca
domestica is a very active, free‑living form. But unlike Phyllophaga rugosa and Heliothis
zea, the head has been greatly modified and reduced drastically from
the typical form (Fig
65). A conspicuous
anterior segment, which may be readily confused as a cylindrical head, is
actually the first thoracic metamere.
This metamere may be identified by a pair of respiratory structures
(anterior spiracles) which are never known to occur on the head region. The small lobe anterior to this metamere
represents the head of the maggot since there is a mouth opening on its
ventral aspect. A series of grooves
lead to and occur on either side of the mouth opening. These grooves are the so‑called food channels,
and may have the function of conducting liquids to the mouth opening (Figs 67
& 68). Two pairs of small projections or papillae
occur on the dorso‑anterior aspect of the head. These are identified as the dorsal sensory papillae and the ventral sensory papillae. The papillae are apparently sensory in function, but are not in
any way homologous with antennae or eyes.
Protruding from the mouth opening is a hook‑like structure
identified as a mouth
hook used for procuring and
ingesting food. Dissection of the
larva reveals structures such as the sucking pump (cibarlal apparatus), which
are homologous with similar structures in the adult (Fig 68). These will be discussed in detail in the
next section III. For the present, it
is apparent that the mouthparts and certain sclerites of the head are deeply
invaginated within the body cavity.
It is also apparent that all of the head capsule but the food
ingesting apparatus has been retarded in development. The primordial cells for the adult head
including the sensory structures are found within internal sacs known as the frontal sacs (Fig
69). These primordial sacs are retracted deep
in the body cavity. Careful
dissection shows that the anterior channels of these sacs actually open into
the mouth cavity. During the pupal
stage, the primordial cells within the sacs grow very rapidly and to the
extent that the sacs are evaginated to the exterior. The head capsule and frontal region of the
adult are then placed in an external, anterior position. It then becomes apparent that the
functional head of the maggot is not comparable with a typical head capsule,
and that such structures as the mouth hooks and sensory papillae are secondary
but highly evolved organs. SECTION III -
THE MOUTH PARTS
The organs
developed for the ingestion of food collectively referred to as mouthparts, may for most insects be
functionally classified as either mandibulate or haustellate. Mandibulate mouthparts probably occurred
early in the evolution of insects and for the most part were primary modifications
of existing appendages remodeled through the process of selection for
grasping, biting and chewing solid foods.
Haustellate mouthparts probably were further elaborations of the
mandibulate types for the purpose or rasping or piercing and for sucking
liquid foods. While mandibulate
mouthparts usually occur in such primitive forms as Thermobia domestica or Leucophaea
maderae, they may be retained in part by such highly evolved forms as
Apis
mellifera or by the larvae of Heliothis zea. Mandibulate Mouthparts
The true mouth of an insect is the anterior
opening of the gut track and is represented in the hypothetical prototype as
the oral opening, situated ventrally
between the prostomium and the 2nd metamere.
It was suggested that the ambulatory appendages of the 3rd, 4th and
5th metameres of the evolved head (collectively, the gnaphocephalon) became associated with the mouth as organs of
ingestion. Actually the segments of
these mouthparts although highly modified can be identified with legs./1 The appendages of the 3rd metamere
probably evolved into a pair of mandibles
which serve as a cutting and grinding mechanism. Appendages of the 4th metamere evolved
into a pair of digitate structures referred to as the maxillae. And finally, the fused appendages of the
5th metamere or labium evolved
into a plate‑like structure underlying the mandibles and maxillae. A cranial sclerite, the labrum, serves as an upper lip, and a
lobe of the head, the hypopharynx
serves as a median tongue‑like structure. All of these
mouthparts precede and enclose the true mouth, forming an ingestion cavity
identified as the preoral cavity. The
preoral cavity is best visualized as box‑like in formation with the top
covered by the labrum and the bottom enclosed by the fused labium. Because the mandibles and maxillae unlike
our own jaws articulate on a horizontal plane, these appendages enclose the
sides of the cavity and regulate the opening and closing of the anterior
aspect. The posterior aspect of the
cavity bears the true mouth or opening into the gut and the base of the
median tongue‑like hypopharynx.
A sagittal section of the head as in Leucophaea maderae
illustrates this relationship (Fig 70).
Certain areas of
the preoral cavity are identified further.
The cavity lying directly below the clypeus and above the base of the
hypopharynx is the cibarium. It may be observed in the sagittal section
of Leucophaea
maderae that strong cibarial dilator muscles operate
between the dorsal wall of the cibarium and the clypeus. These muscles probably serve an important
function in assisting mandibulate insects to swallow food. Of considerably greater importance is
their specialization into a sucking or cibarial pump in the haustellate
species such as Apis mellifera, Heliothis zea, Oncopeltus
fasciatus and Musca domestica. The cavity formed by the ventral surface
of the hypopharynx and the ental surface of the labium is identified as the salivarium. In many mandibulate forms such as Leucophaea
maderae, the duct of the salivary gland is situated at the posterior end of this
cavity. ------------------------------------------------ 1/ Refer to Section VI ‑ Origin
of the Mouthparts. The mandibulate
mouthparts of Leucophaea maderae provide a good example of the generalized biting and chewing
mechanism (Figs 70, 71, 72, 73, 74, 75,
76, 77, 78, 79, 80, 81
& 82). The mandibles are the true jaws designed
for cutting, tearing and grinding solid foods. In composition they are hollow, unsegmented and usually heavily
sclerotized. The tips of the
mandibles are toothed, and about midway the mesal edge is flattened into a
grinding surface designated as the molar area or mola
(Figs 71, 72 & 73). These mastication areas of the mandible
are asymmetrical so that the distal teeth and the mola will effectively work
against each other for cutting and grinding.
In Leucophaea maderae, the basal portion of the mandible is
modified into a soft, resilient lobe or oral flap. The oral
flaps seem to have an important part in the process of swallowing by forcing
food particles into the cibarium as the mandibles are closed together. The masses of setae on the mesal edges
probably serve to hold the food particle as it is being forced backward. Outside of the oral flap, few setae occur
on the mandible in Leucophaea maderae.
In other species
such as Phyllophaga rugosa setae may be
distributed profusely over the mandibular surface (Fig 83). Each mandible is attached to and
articulates with the head capsule at two points. This dual attachment is referred to as a dicondylic articulation. All other appendages are attached to the
metamere of their origin at only one point or by means of a dicondylic articulation. Apparently,
the articulation of the primitive mandible was monocondylic, and in fact this
condition does exist in some of the more primitive Thysanura. The powerful jaws of Leucophaea maderae and Phyllophaga
rugosa require a dicondylic articulation so that the mandibles may be
rocked horizontally and can accomplish a strong mesal thrust. While the monocondylic mandibles of the
primitive Thysanura are comparatively weak, they appear to be sufficiently
effective, however, to maintain these ancient and quite successful
forms. In Leucophaea
maderae, the primary (or primitive) point of articulation is
accomplished by means of a knob situated on the posterior angle of the
mandible. This is the posterior condyle which fits into a
pocket provided by the ventral margin of the postgena (Fig 73). The anterior articulation of the mandible is a much less prominent
projection, which is accommodated by a notch in the lateral margins of the
postclypeus. Two apodemes accommodate
the movement of the mandible. The
adductor tendon is a broad apodeme
connecting the mesal margin of the mandible with a set of powerful
muscles. These adductor muscles close the jaws in the
cutting or grinding function. Opening
the jaws is accomplished by a comparatively weak set of abductor muscles attached to the abductor tendon which operates on the
outer angle of the mandible. The
mandibles, then, are rocked forward with a powerful stroke and backward on a
horizontal plane by two opposing sets of muscles, while the two points of
articulation serve as a hinge. The second pair
of mandibulate appendages are the maxillae.
These can be reasonably well homologized and most nearly resemble a
typical leg. The maxilla is broadly
united and articulates with the ventral margin of the postgena. In Leucophaea maderae this hinge‑like
articulation, the cardo, is 2‑segmented, and its proximal extremity
fits into a notch or maxillary articulation
in the posterior margin of the postgena (Figs 78 & 79). The base of the
maxilla, or stipes
bears laterally a 5‑segmented palpus and distally two prominent
lobes. The ectal
surface of the stipes is heavily sclerotized and the ental surface is
membranous. The palpus, designated as
the maxillary
palp, is a finger‑like
structure with two short basal segments, and three long distal segments. The distal portion of segment 5 is
membranous and is probably sensory in function. In Leucophaea
maderae, the maxillary palp articulates directly with the
stipes. Where a distinct sclerite
occurs for articulation as in the adult and larva of Phyllophaga rugosa,
this articulatory sclerite is referred to as the palpifer. The outer lobe of the stipes is the galea. It is weakly sclerotized except at the
base on its ental surface, and is probably sensory in function. The inner lobe is the lacinia, which in contrast with
the galea is heavily sclerotized and serves as an adjunct to the mandibles as
a second cutting and tearing instrument.
Importantly, its distal end is armed with three sharp teeth and its
mesal margin bears numerous stout setae.
Articulation of the opposing maxillae is on the same horizontal plane
as the mandibles. Underlying the
mandibles and maxillae is the labium.
This is a composite structure which readily can be homologized with
the maxillae and traced ti its origin as a fused pair of typical legs. The broad basal portion of the labium
articulates directly with the cervix, and appears to be closely associated
with the postocciput as the sclerite of its origin. The articulatory portion of the labium is referred to as the postlabium and
is comparable with the cardo of the maxilla.
Where the postlabium is a single sclerite as in Leucophaea maderae, it
is usually termed the postmentum
(Fig 74). When two distinct sclerites comprise the
postlabium as in Phyllophaga rugosa the most proximal is the submentum and
the distal sclerite is the mentum. In Leucophaea maderae the postlabium
is composed of a basal sclerite and a distal membranous area. This membranous area probably is not a
true mentum. The distal portion of
the labium is the prelabium. Its proximal
sclerotized area is the prementum
comparable to the maxillary stipes, the inner distal lobes are the glossae and the outer lobes
the paraglossae
(Figs 74 & 75). These lobes are
homologous with the galea and lacinia of the maxilla. Development of the glossa and paraglossa
in Leucophaea
maderae is best seen in an ental view of the labium (Fig 75). The deep cleft between the glossa suggests
that the labium originated from a pair of appendages following fusion of the
basal segments. A pair of 3-segmented
palps, the labial
palps, is borne by the
lateral margins of the prementum.
These palps articulate with a sclerite (best viewed in Leucophaea
maderae from a lateral view) designated as the palp bearing sclerite
or palpiger. The labrum or
upper lip is
an integral part of the chewing mechanism although unlike the labium, it was
not modified from appendages. This
ovoid sclerite probably represents a portion of the old prostomium overhanging
the mouth. The labrum simply serves
as an upper lip for the preoral cavity, connected with the head capsule only
along its proximal margin and freely articulating with the clypeus. A mass of sensory pits and setae may occur
on its ental surface as in Leucophaea maderae (Fig 77) and
particularly in the larva of Phyllophaga rugosa (Fig 108). The hypopharynx in Leucophaea maderae is a
fleshy lobe of the cranium lying in a median position as a tongue and
occupying a large portion of the preoral cavity (Figs 81 & 82). Its dorsal surface
forms the ventral floor of the cibarium, and its grooved base or sitophore leads
directly into the mouth (Fig 80). The ventral surface of the hypopharynx
forms the dorsal wall of the salivarium, and the salivary duct empties into
the salivarium at its base. For the
most part, the hypopharynx of Leucophaea maderae is soft and
membranous. The lateral sclerite and hypopharyngial
suspensorium are sclerites,
which articulate with the oral arm, an
apodeme upon which the retractor muscles arising from the tentorium are
inserted. A second apodeme, the oral arm, provides insertion for retractor muscles arising from the
frons. While the hypopharynx in Leucophaea
maderae is a relatively simple median tongue, this structure may
become highly modified in other forms with mandibulate mouthparts and finally
may become an integral part of the salivary apparatus in insects with
haustellate mouthparts. THERMOBIA DOMESTICA:
The mouthparts
of Thermobia
domestica are very similar to Leucophaea maderae but are
comparatively simple in structure and represent many more of the primitive
features. It was assumed that the
articulation of the primitive mandible was monocondylic. This condition does exist in some of the
more primitive Thysanura, and Thermobia domestica represents a
transitional stage from the primitive monocondylic to the more highly evolved
dicondylic articulation. The primary
point of articulation is the well-developed posterior condyle. A second but very weak anterior articulation
does occur along the anterior, lateral margin. While the mandible of Thermobia domestica appears to be
a very weak structure compared with Leucophaea maderae or Phyllophaga
rugosa, it must have served its purpose well through the millions of
years of this animal's existence (Fig 94). The maxillae and
labium are typical in form although very simple in composition when compared
with Leucophaea maderae (Figs
95, 96 & 98). A palp-bearing
sclerite is absent in both the maxilla and labium. The postlabium is attached to the cervix by means of a very
broad base, and the prelabium bearing the glossae and paraglossae is greatly
reduced. Unlike most insects, the
labial palps are 4‑segmented.
The hypopharynx is simple and poorly sclerotized, although there is a
distinct division between its basal and apical aspects (Fig 99). PHYLLOPHAGA RUGOSA:
The mandibles of
adult Phyllophaga rugosa are blunt, powerful grinding instruments
with a broadly developed molar area (Figs
83, 84 & 85). They are dicondylic with a conspicuous
ball‑shaped posterior condyle.
The anterior articulation of the mandible is a pocket, which fits over
a ball‑shaped condyle on the ventro‑lateral margins of the
clypeus. Both of these ball-and‑socket
joints fit so securely that it is difficult to dissect the mandibles from the
head capsule. The adductor apodeme is
very large, and the mandibles are closed by means of powerful muscles. A fleshy ridge or prostheca extends along the
ventro‑mesal margin of the mandible.
Its surface is weakly sclerotized, but does bear a mass of soft,
bright yellow setae. The prostheca is
a distinct sclerite (absent on all other insects examined here). It appears to be homologous with the
lacinia of the maxillae. Although the
mandibles are heavily sclerotized, they are covered with setae. The maxillae are
typical in form although the galea is heavy sclerotized and apparently
augments the lacinia as a cutting instrument (Figs 91, 92 & 93). A large palpifer bears a 4‑segmented
maxillary palp. The maxillae
articulate with the head capsule by means of a groove on the posterior
ventral margin of the gena. The head
of Phyllophaga
rugosa is prognathous (Fig
3), and the ventral aspect is composed of a gula and an
expanded postmentum. The submentum
and mentum together are about 3X as long as the prementum. A glossa and paraglossa are absent, and
the entire premental area is identified as a ligula. A distinct palpiger is absent. Unlike Leucophaea maderae and Thermobia
domestica, the hypopharynx is strongly sclerotized on its dorsal
surface, asymmetrically lobed and covered with a mass of setae, and is
attached to the labium on its ventral surface (Fig 87). The mouth is
posterior to the dorsal lobes, and the base of the hypopharynx is attached to
the oesophagus
(Fig
90).
The labrum is
inconspicuous, underlying the projecting clypeus (Figs 88 &
89). It is deeply
cleft on its anterior margin, and the two lateral lobes are asymmetrical in
shape especially when viewed from the ventral aspect. Also, the cibarial region is densely
clothed behind the labrum with recurved setae. Mandibulate Mouthparts of
Holometabolous Larvae
PHYLLOPHAGA RUGOSA Larva:
A generalized
mandible occurs in the grub of Phyllophaga rugosa which is
comparable to Leucophaea maderae with a similar distal tooth, mola or oral
flap. The articulations are strong
and well developed and resemble those of the adult Phyllophaga rugosa. Also, the molar area is dentate and
asymmetrical, providing a very effective grinding surface (Figs 101, 102 & 103). In all of the
species discussed thus far, the maxillae and labium articulate independently
of each other. In the larva of Phyllophaga
rugosa and Heliothis zea these appendages are
fused at their base and produce a highly evolved labial‑maxillary
complex. As illustrated in the drawing, the base of
the maxilla is provided with an abductor apodeme, but it should be noted
that the base cannot articulate independently of the labium (Figs 105, 106, 107, 108, 110 & 111). The galea and lacinia are heavily
sclerotized, provided with stout spines and setae, and are completely fused
except for a membranous fissure on the dorsum. In many of the species of Phyllophaga rugosa, a stridulating
field occurs on the ventral surface of the mandible. This structure is absent on the mandible
of Phyllophaga
rugosa although a row of stout spines or stridulating teeth line the meso‑dorsal margin of the
stipes. It is not known if Phyllophaga
rugosa can produce a sound by rubbing these teeth on the overlying
mandibles. A palpifer, separated from
the stipes on its ventral surface, bears a 4‑segmented maxillary
palp. Fused at its lateral margins
with the maxillae is a small labium.
The head of the larval Phyllophaga rugosa is
hypognathous, and the large postmentum articulates directly with the
cervix. The ligula is small and bears
2-segmented labial palps. The distal
portion of the hypopharynx is an irregular, horny plate that is fused to the
dorsal surface of the labium (Fig.
106) A pair of
apodemes are attached to the walls of what appears to be the pharynx. These may be the oral arms of the
retractor muscles, and the membranous floor of the pharynx may be the basal
portion of the hypopharynx. The
labrum is smooth and ovate on the dorsal surface. Ventrally, the labrum is developed into a complex sensory field
(Fig 108). Sensory pits,
grooves and setae are arranged with sclerotized rods and ridges to form a
constant pattern. These structures
are arranged in a constant pattern enabling taxonomists to employ
"maps" of the ental surface for species identification. An extensive descriptive nomenclature
appears in taxonomic literature that is similar to the previously described
chaetotaxy of lepidopterous larvae. HELIOTHIS ZEA Larva:
The mandibles of
the herbivorous larval Heliothis zea are well developed
but quite simple in structure compared with the adult and larva of Phyllophaga
rugosa or with Leucophaea maderae. However, these are highly specialized
dicondylic mandibles, and are not
simplified to the extent of the primitive mandibles of Thermobia domestica. The labial‑maxillary complex reaches
its greatest degree of specialization in Heliothis zea (Figs 110 & 111). Basal sclerites of the maxillae such as
the cardo and stipes are difficult to identify. The galea and lacinia are indistinguishable and are represented
only by minute papillae. An
inconspicuous 2‑segmented papilla may be the remnant of a maxillary
palp. The labium comprises a soft,
membranous postmentum. The prementum
is a complex fusion with the hypopharynx and is probably represented by a
sclerotized ring enclosing the specialized salivary duct or spinneret. Two small papillae may be remnants of the
labial palps. The larvae of Heliothis
zea are capable of spinning silk, and the salivary gland has evolved
from a gustatorial to a silk‑producing gland (Fig 112). More will be
mentioned about this gland in Section IV.
The ventral view of the labial‑maxillary-hypopharynx complex (Fig 112) illustrates the extreme degree to which these structures
have fused. The labrum is a simple,
oblong flap with little or no sensory modification (Fig 113). APIS MELLIFERA Larva. Larval larvae of
Apis
mellifera, unlike Phyllophaga rugosa or Heliothis
zea are nursed throughout their immature stages of development by
adult bees of the colony. Only a very
simple ingestive mechanism is required for their diet of honey and
pollen. The mandibulate mouthparts
are recognizable as such, but are greatly reduced. The labrum is a simple flap, and the mandibles are soft and
weak. Fusion of the maxillae, labium
and hypopharynx has progressed to such a degree that the structures are
undifferentiated and difficult to recognize.
Papillae at the distal aspects of the maxillary lobe and the labium do
not appear to be homologous with palpi.
The grub of Apis mellifera is also capable of
spinning silk produced by the modified salivary glands. A description of this spinneret along with
that of Heliothis zea will be reserved for a later section. Haustellate Mouthparts
The absences of
fossil records and scarcity of example species in existence preclude giving
satisfactory clues to the intermediate steps that may have occurred in the
evolution of the haustellate mouthparts.
Since the feeding mechanisms of Apis mellifera, Heliothis
zea, Oncopeltus fasciatus and Musca domestica are not
comparable; it would appear that their evolution was completely independent
of each other. An entirely different
approach is taken in the elaboration of the basic mandibulate structures into
a sucking tube. All of the
mandibulate structures have been preserved in Apis mellifera and Oncopeltus
fasciatus, but their physical appearance is so unlike that of Leucophaea
maderae that they are identified with considerable difficulty. The mandibles in Heliothis zea and Musca
domestica have completely disappeared. One development appears to be constant for all of these
species, and this concerns the modification of the cibarium into a pumping
apparatus referred to as the cibarial
pump. Even in the larvae of Musca domestica, a
readily recognizable cibarial pump operates the complex rasping‑sucking
mouthparts. APIS MELLIFERA:
The labrum is a
simple flap (Fig 124),
and the mandibles are preserved as in the typical mandibulate form (Fig 125). Although the
mandibles are strong dicondylic structures, they are no longer employed for
tearing and grinding in the ingestion of food. Their function in the drone and queen is obscure, but in the
worker the mandibles are employed for rasping, cleaning brood cells, shaping
wax, and other duties of the hive. A
sexual dimorphism is evident in the form of the mandibles. The ental surface of the worker bee
mandible is a flat and apparently effective spatulate tool. The drone, of course, has little interest
in the labors of the hive, and the hairy mandibles have relatively less
functional form. It has been observed
that the maxillae, labium and hypopharynx in some holometabolous larvae may
combine to form a complex, especially where the salivary glands are modified
for the production of silk. These
same structures in Apis mellifera have been greatly
modified into a sucking tube. The
head of Apis mellifera is hypognathous and the postmentum is a very
small triangular sclerite in comparison with the greatly expanded prementum. The prementum is a completely sclerotized
half‑cylinder (Fig 117). This cylindrical structure is completed by the membranous
hypopharynx on its dorsal surface. A
sagittal section of the prementum‑hypopharynx complex reveals the
salivary gland and a small salivarium which empties at the apex of the
hypopharynx between two suspensory rods or ligular arms. At the apex of
the prementum is a small triangular plate described as the distal plate of the prementum but which may be the ligula
(Figs 118 & 119). Two pair of appendages and a median tube
completes the labium, which may be referred to in the literature as the
tongue or proboscis. The large,
segmented outer pair are the labial palps.
The basal two segments are elongated, flat and L-shaped in cross‑section. The distal two segments of the labial
palps are small and typically palpiform. A median pair of spoon‑shaped
appendages is the paraglossae. The
salivary duct opens at the base of the paraglossae, and when the paraglossae
are drawn together, they close ventrally over the base of the median
glossa. Although the median tube is
referred to as the glossa, it is an unpaired structure and is only remotely
homologous with the glossae of Leucophaea maderae or Thermobia
domestica. The lumen of the
glossa that is ovoid in cross‑section, is open by means of a
longitudinal slit along the entire ventral surface. A long rod, which is
U‑shaped or deeply grooved on its ventral surface traverses the entire
length of the tubular glossa along its dorsal aspect opening proximally at
the distal plate of the prementum (Figs
115, 116 & 117). The rod is
attached to a membrane, which is usually folded within the lumen of the
glossa. Under certain circumstances
that are not apparent, the rod may be extruded from the lumen through the
longitudinal slit. The membrane
attached to the rod and the margins of the slit are then expanded into a
large sac with the grooved rod attached along its ventral surface. At the tip of the glossa is a spoon‑shaped
segment given the descriptive but anatomically incorrect name of labellum (Fig 117). It is concave
on its dorsal surface, and is margined by stout setae recurved toward the
depression. The entire surface of the
glossa is circularly grooved. Stout,
bristle-like setae ring each of the grooves, and the setae are all pointed
anteriorly and alternate in position with each preceding row. The proximal end of the glossa is notched
and appears to swing freely between the bases of the paraglossae. The hypopharynx
is actually a part of the labial complex, and its anterior portion
identified by the lateral, sclerotized hypopharyngial suspensoria is the dorsal wall of the cylinder partially
enclosed by the prementum (Fig 122). The posterior portion of the hypopharynx
is the floor of the bulbular cibarial pump.
It is interesting to note that the median portion of the hypopharynx
before the cibarial pump is expanded or looped outward into a flap or hypopharyngial
lobe. A patch of sense organs and paired ducts
or food glands occurs just posterior
to the hypopharyngial lobe. The
hypopharyngial lobe should not be confused with a similar membranous flap
given the descriptive name of epipharynx. This fleshy flap is a special feature of
certain Hymenoptera and apparently is derived from the ental wall of the
clypeus. The maxillae are
readily identified as the flat appendages on either side of the labium (Fig 120). The cardo
articulates directly with the postmentum by means of a V‑shaped
sclerite or yoke described as the lorum,
and with the postgena by means of an articulatory flange. The stipes bear a greatly reduced 2‑segmented
maxillary palp, a weakly sclerotized and lobular lacinia, and an elongated
and flattened galea. In cross‑section,
the galea is U‑shaped and similar to the labial palp. In addition, there is a conspicuous median
ridge on its ental surface. The
distal end of the galea is pointed and armed with stout setae. All of the functional details of this
sucking mechanism except for the cibarial pump are by no means clear. When the flat maxillae and labial palps
are closely appressed around the prementum and glossae, a tightly sealed food canal is formed as illustrated
by the cross‑section of the proboscis (Fig 116). The fleshy
lobes of the lacinia and the dorsal flap of the epipharynx serve as sealing
devices before the cibarial pump.
However, the function of the glossa is difficult to explain. It is sometimes believed that the ventral
groove of the glossa serves as a salivary channel, although the salivary duct
is dorsad of this slit in the walls of the tubular structure. The role of the rod is vague in the
description of authors especially when it is distended from the lumen of the
glossa. Possibly the ventrally
grooved rod serves as a salivary channel when it is distended from the
glossa. Saliva could flow from the
dorsal duct ventrally around the bases of the paraglossae even though this
appears to be an illogical development.
The lumen of the glossa with the rod distended might serve as the
actual food canal as far as the bases of the paraglossae. However, the lumen of the glossa appears
to open on the ventral aspect of the labium.
This again appears to be an illogical development for such a highly
specialized apparatus. The food cana1
formed by the galea and labial palps ends a considerable distance from the
labellum, and the glossa, and therefore, must serve as the ingestive device
beyond this point. It has been
suggested that the labellum serves as a lapping device. How liquid food is transferred from the
labellum to the food canal is difficult to visualize. The role of the anteriorly directed setae,
which cover the ectal surface of the entire glossa and margin of the ventral
slit, further confuses a logical explanation of the sucking mechanism. Liquid food does get to the cibarial pump,
either by devious channels or more likely, by an efficient but a poorly understood
route. ONCOPELTUS FASCIATUS:
Although the
sucking apparatus of Oncopeltus fasciatus is a highly
evolved mechanism, its functional morphology is much clearer than that of Apis
mellifera. The long and
conspicuous, 4‑segmented labium is not involved in the elaboration of
the actual plercing‑sucking tube (Fig 132). It is an ovate
cylinder with heavily sclerotized walls and a shallow dorsal groove or a dorsal gutter. This is such a highly specialized
structure that the elements of a typical mandibulate labium are obscure. The purpose of the labium is simply to
serve as a sheath for the sucking mechanism that lies encased in its dorsal
gutter while at rest. During feeding,
the labium is actually withdrawn and does not enter into the tissues of a
plant host. There is a cluster of
papillae at the distal end of the labium.
These papillae are probably sensory in function and the labium or
proboscis is used as a probe while the insect searches for a desirable food
source. The piercing‑sucking
mechanism is a closely appressed bundle of four, hairlike shafts or stylets. An examination of the musculature at the
base of these stylets suggests that they were modified from typical
appendicular mandibles and maxillae.
In gross appearance, the stylets appear to be a single, hairlike
bristle. However, cross‑section
reveals that four distinct heavily sclerotized and elaborately sculptured
structures are present (Fig 131). Longitudinal cavities in the stylets
demonstrate that they were formed from heavily sclerotized tubes. The outer pair is the mandibular stylets
which are grooved to fit an inner pair of maxillary stylets. The mandibles are the principal piercing
stylets. The tips are pointed and
provided with sharp cutting plates and recurved spines for anchoring the
stylets in host tissue (Fig 134). The tips of the
maxillae also are pointed, but their structure would indicate that they are
secondary to the mandibles as penetrating organs (Fig 135). The cross‑sectional
view illustrates the longitudinal grooves and mortising (= channeling) that
holds the maxillary stylets together, forming longitudinal tubes (Fig 131). The dorsal tube or
food canal leads to the cibarium, while the ventral tube or salivary canal opens
into the salivarium. Mortised joints
also hold the mandibular stylets securely to the maxillae. Although the stylet bundle is securely
united, each of the grooved mandibular stylets may move freely and
independently upon the maxillae on a longitudinal plane. This forward and longitudinal movement of
the mandibles is accomplished by protractor muscles arising
from the mandibular
apodeme and attached to the
base of the stylet. The maxillary
stylets as a unit are also protracted by muscles attached to a maxillary apodeme anchored on the
posterior tentorium. Penetration of
host tissue is accomplished by the alternate and individual protraction of
each mandibular stylet. When the pair
of mandibular stylets have reached a maximum protraction, the pair of
maxillary stylets are protracted to a position that is even with them, and
the cycle is repeated. Recurved barbs
on the tip of the mandibular stylets serve to hold the entire bundle in
position in the host tissue during each cycle of protraction. When the stylets have reached a desirable
feeding site, saliva
is pumped down the salivary canal by means of the elaborate salivary syringe, and liquid food is
pumped up the food canal by means of the cibarial pump. The highly evolved salivary syringe will
be discussed in greater detail in Section IV. The cibarial pump (Fig 41) is trough‑shaped and is formed by a sclerotization
of the posterior hypopharynx. The
membranous roof of the cibarium, derived from the ental wall of the
anteclypeus, is provided with numerous vertical spine‑like apodemes for
attachment of the cibarial dilator muscles.
The pumping action is simply a raising of the membranous top of the
cibarial trough by the dilator muscles, and a snapping back of the elastic
membrane into a resting position within the trough upon relaxation of the
dilator muscles. The opening or
closing of the trough is accomplished by a forward to backward series of
contractions and relaxations of the dilator muscles because valves are not
provided in the cibarial pump to prevent an opposing backward or forward flow
of fluids. The labrum is a sharply
pointed flexible flap articulating at its base with the anteclypeus and lying
over the dorsal gutter of the entire first basal segment of the labium (Fig 133). This modified
labrum is more than a simple covering flap.
Examination of the ental surface reveals a deep groove which
ensheathes the basal portion of the stylet bundle. This is the labral stylet groove
that serves to hold the stylets in position before their separation in the
head cavity. HELIOTHIS ZEA:
The proboscis of
Heliothis zea illustrates
considerable simplification in the formation of a sucking tube. The long, coiled tubular structure bears a
superficial resemblance to the ensheathing proboscis of Oncopeltus fasciatus,
but close examination reveals that the proboscis of Heliothis zea is
actually the sucking tube and that it is not a modification of the
labium. In cross‑section, the
proboscis is composed of two ovoid cylinders, deeply grooved on their mesal
surface, and fitted together by mortise joints so that the longitudinal
grooves form a channel or food canal (Figs 126
& 130). Examinations of the basal structures on each half of this
proboscis provide satisfactory clews to its origin. Two articulatory sclerites can be readily identified as the cardo
and stipes. Identification of the
proboscis as a modification of the maxillae is assured by the inconspicuous,
2‑segmented maxillary palp borne by the stipes on its ventral
margin. The long, sucking tube must,
then, be one of the maxillary endites, and each half of the tube is usually
identified as the galea. At the base
of the proboscis, the galea are divided and the food canal empties through a
narrow canal into a large, bulbular cibarial pump (Fig 127). The cibarial
pump is provided with two sets of dilator muscles as illustrated by the
sagittal section of the head. A small
set attached to the head capsule dorsad of the labrum are probably the true
cibarial dilator muscles arising from the clypeus. A large set of muscles identified as the frontal muscle is probably the pharyngeal dilator arising from the
frons. Apparently, the pump is a
composite structure involving both the cibarium and the pharynx. If this interpretation of its morphology
is correct, then the facial area identified as the frons is actually a
composite sclerite involving both the clypeus and the frons. The salivary gland empties into the narrow
channel connecting the food canal and the cibarium. The surface of the galea is lined with minute, irregularly
parallel ridges. These sclerotized
r1dges or annulations
give the proboscis its spring like characteristic that retracts the structure
while at rest into a tightly rolled coil.
The mechanism
involved in the expansion of this coil during feeding is not entirely
clear. This feat is probably accomplished
by muscles within the lumen of the galea, which change its shape upon
contraction. When the dorso‑ventral
muscles contract, the ventral surface of the galea is flattened
considerably. This reduces the volume
of the lumen, and fluids within the cavity of the galea are compressed. A basal constriction of the lumen prevents
a back flow of fluids. A compression
of fluids within the lumen may unroll the coil and extend the proboscis in
much the same manner as uncoiling a cylindrical paper toy by blowing air into
its cavity. In the dorsal view of the
proboscis, cross‑sections of the proboscis illustrate this change in
the shape of the galea. The basal
portion of the proboscis of the specimen used for the drawing was
curved. The cross-section of the ventral
surface is deeply grooved. The distal
portion of the proboscis was distended or straightened. In the cross‑sections of this
portion, the ventral surface is flattened and the volume of lumen is
appreciably reduced. The two sections
of the proboscis are separated at the distal end. The ectal surface is covered with stout setae and minute,
sclerotized protuberances. The tip of the
proboscis then, is provided with an abrasive surface for rasping plant tissue
prior to feeding upon the exudate.
All of the other typical mandibulate parts are reduced or wanting
except for the conspicuous labial palps.
These palpi are 3‑segmented and covered with a mass of
setae. The labium itself is greatly
reduced to a simple oblong postmental sclerite articulating with the cervix,
and a plate‑like prementum occupying the ventral aspect of the head and
bearing the labial palps (Fig 128). The mandibles are wanting, or possibly
they are represented as vestiges by two sclerites arising at the lateral
margins of the labrum (Fig 129). These flap‑like sclerites, covered
with stout setae, are pilifers of
descriptive entomologists. The labrum
is a small flap at the base of the proboscis. A fleshy distal tip of the labrum may serve to cover the food
canal as it extends into the cibarium.
There appear to be no remnants of the hypopharynx. MUSCA DOMESTICA:
The fleshy proboscis of adult Musca
domestica is a composite unit that is entirely different from any of
the other haustellate structures that have been mentioned. Only the sclerites associated with the
cibarial pump can be identified with certainty, and it is necessary to use
many descriptive terms to identify structures and areas of doubtful
anatomical origin. The entire
proboscis resembles a stubby, foot-like organ when it is protracted for
feeding (Fig 137). In this position,
there are three distinct regions: 1) a comparatively soft, cone‑shaped
basal region or rostrum,
2) a cylindrical median region or haustellum, and 3) a distal
pair of fleshy lobes forming the foot or labella. When at
rest, the haustellum is partially retracted within the rostrum and is folded
anteriorly upon it, while the ventral surface of the labella is tipped upward
and posteriorly from its horizontal feeding position (Fig 50). The base of the
proboscis or the rostrum is largely membranous except for a large U‑shaped
anterior sclerite identified as the clypeus, and two small lateral sclerites
or maxillary plates bearing unsegmented palpi which are the maxillary palps (Fig 136). The maxillary
plates may be simply remnants of the maxillae or a palpifer. It appears, then, that the rostrum is a
composite structure involving elements of the cranium and the maxillae. While the maxillary palps cannot be identified
with certainty, there is little doubt about the identity of the clypeus
although its appearance is quite unlike that of the typical mandibulate Leucophaea
maderae. Internal dissection
reveals that the dilator muscles of the cibarium are attached to lateral
invaginations of this U‑shaped sclerite. These invaginations or apodemes are referred to as the lateral plates best seen in a sagittal
section of the head (Fig 138). Since it was concluded that the cibarial muscles always arise from the clypeus, the U‑shaped sclerite
and its apodemes must be the true clypeus considerably removed from the head
capsule. The cibarium is a
sclerotized trough, and that a membrane lying in this trough is the actual
pumping diaphragm that is operated by large muscles laterally attached to the
lateral plates or apodemes of the clypeus (Fig 144). The haustellum
is a fleshy cylinder, which is entirely membranous except for a posterior
plate descriptively identified as the thecal sclerite, and a sclerotized dorsal groove or labial gutter (Figs 140 & 141). Overlying the
labial gutter is the labrum. The
labrum is ovoid in cross‑section and deeply grooved on its ental
surface (Fig 139). The hypopharynx is stylet‑like in form and underlies the
labrum and lies within the gutter.
The dorsal surface of the hypopharynx is depressed and the lumen is a
longitudinal tube (Fig 138). The salivary gland provided with a pumping
mechanism or syringe
empties directly into the tubular hypopharynx. The hypopharynx of Musca domestica, then, not only
forms a salivarium but completely encloses it as well. With the hypopharynx closely appressed to
its ental surface, the longitudinal groove of the labrum forms a short
sucking tube, leading from the labellum to the cibarial pump. Two rod‑like apodemes, the labral apodemes, attached to strong
muscles articulate the labrum at its base.
Lying between the labral apodemes is a narrow canal leading to the
cibarium. A small plate identified as
the hyoid
sclerite apparently
distends this narrow passage, which serves as the mouth. The sucking tube formed by the labrum and
hypopharynx does not actually penetrate a food medium. It simply serves to conduct fluids from the
labellum, which is the actual collecting mechanism, to the cibarium. Also, saliva is conducted to the distal
end of the labial gutter by the salivary
canal enclosed within the
hypopharynx. The labella are
fleshy lobes forming a foot‑like pad at the distal end of the proboscis
(Fig 142). It is assumed by morphologists that these lobes are
modifications of labial palps although their resemblance to palpi appears
quite remote. When the proboscis is
at rest, the ventral surfaces of the two-pad‑like labella are folded
mesally as illustrated by the frontal view of the proboscis. During feeding, the labella are broadly
expanded and directly contact the food source. The labellum is deeply incised on its anterior margin, and the
incision corresponds with the groove of the labial gutter. A V‑shaped sclerite, the discal sclerite, margins the apex of
this labellar incision (Fig 143). The orifice enclosed by the discal
sclerite is the prestomum
or functional mouth. Food
passes through this orifice and directly into the food canal formed by the
labrum and hypopharynx. A series of
tubes transversely lines the labellar lobes and empty into large collecting
channels which parallel the
discal sclerite. These tubes are
referred to as the canaliculi,
or pseudo tracheae since they remotely resemble the tracheal tubes of the
respiratory system (Fig 143). Sclerotized rings distend the canaliculi. The rings are incomplete on the ectal
surface, and they alternately terminate in a U‑shaped fork. A longitudinal slit occurs along the ectal
surface extending between the expanded ends of the sclerotized rings. This slit is best demonstrated by a cross‑sectional
view of the canaliculus
(Fig
143).
The dorsal slit is believed to be further expanded between the U‑shaped
expansions of the rings. Sclerotized
rings also distend the collecting channels, but these tubes apparently do not
have a dorsal slit. While the mechanics
of the sucking apparatus described for the rostrum and haustellum is not
difficult to understand, the function of the labella is quite vague. It is apparent that the canaliculi serve
as collecting tubes since the dorsal slit permits fluids to enter the hollow
tube by capillarity. The fluids would
then "flow" into the collecting channels. It has been assumed that the collecting channels (and the four
canaliculi independent of the collecting channels) empty directly into the
prestomium. However, these tubes,
which are attached to the discal sclerite, narrow sharply before their
attachment. Dr. Robert Dicke, who
performed most of the initial dissections for the illustrations, states that
he has not been able to demonstrate an orifice through which liquids could
flow into the prestomium. But, we
must assume that liquids (and particles of solid food) do flow from the
canaliculi into the prestomium from where the sucking apparatus of the
haustellum and rostrum deliver the food material to the oesophagus. Musca domestica apparently can
scarify a food medium to some extent since five sclerotized teeth are
anchored on the discal plate on the mesal margin of each labellum. These minute plates are the prestomal teeth (Fig 143 When the labella are pressed against a surface and
presumably rotated, the prestomal teeth may serve as a cutting and rasping
device. A unique sucking
and rasping device has evolved in the maggot or immature stage of Musca
domestica that is only remotely comparable with that of the
adult. Previously, it was established
that the typical head region is incompletely developed in the larva. In fact, all of the cranial structures of
the adult are represented only by primordial cells deeply invaginated within
the body cavity. The functional head
of the maggot is provided with rasping structures that are unlike those of
any other insect form. Within the
thorax is a large, trough‑shaped apparatus (best seen in a dorsal view)
which in lateral view appears to be a pair of flat sclerites with prominent
posterior expansions (Fig
145). Descriptive
taxonomists most commonly refer to this structure as the cephalopharyngial skeleton. The nature of this structure is not
apparent in taxonomic preparations cleared in a caustic solution. The first clew to its identification in
an uncleared dissection is the attachment of the oesophagus to its posterio‑ventral
aspect. A cross‑sectional
examination will immediately identify it as a cibarial pump (Fig 147). Actually, the
cibarium of the larva is comparable to that of the adult. The lateral walls of the trough to which
the strong, dilator muscles are attached are similar to the lateral plates of
the adult. The floor of the larval
cibarium is homologous with the posterior wall of the adult. Certainly, the pumping diaphragm, its
position in the trough and the attachment of the dilator muscles is the same
in structure as they are in the adult cibarium. If these homologies are correct, the anterior aspect of the
cibarium must be the clypeus, and the lateral walls of the maggot cibarium
must be the apodemes or invaginations of the clypeus, which were identified
as the lateral plates in the adult.
In the illustration (Fig 145),
an anterior prominence on the cibarium of instar-III is identified with some
reservation as a labrum. In instar-I
this is a distinct sclerite separated from the cibarium; it is distinct but
fused in instar-II, and finally becomes an integral part of the cibarium in
instar-III. Although the cibarium is
greatly reduced in the first instar, it is comparable in form for all of the
three larval stages. The feeding
structures anterior to the cibarium appear to have no counterpart in other
more primitive forms. A hook‑like
structure, the mouth hook, protrudes from the functional
mouth of the maggot. In Musca
domestica, this appears to be a single structure unlike other muscoid
species. The dorsal view reveals that
the mouth hook is actually a fusion of a pair of hooks, which can be
separated. The two hooks are
asymmetrical. The right hook is the
larger of the two and its distal aspect accounts for the bulk of the anterior
hook. The left hook is relatively
weak and closely appressed to its companion.
A small sclerite (the dental sclerite of taxonomists) is attached to
its ventral aspect. Intervening
between and articulating with the mouth hook and the cibarium is a small
sclerite identified as the hypostomal sclerite (Fig 146). This sclerite is notched at its posterior
margin to accommodate and support the salivary gland. The hypostomal sclerite is grooved on its
dorsal surface to provide a salivary canal.
The mouth hook and hypostomal sclerite are progressively modified in
design from the first to the last instar.
Additional sclerites of unknown morphology also occur in
instar-II. Since the head capsule in
the maggot is retarded in development, it is probably unwise to attempt to
homologize beyond the cibarial apparatus.
It is very unlikely that the mouth hooks are homologous with mandibles
or the hypostomal sclerite with a hypopharynx. Certainly, these are not precursors of any adult structures,
and they are completely discarded during the pupal instar. The mechanics of
the apparatus are not difficult to visualize. The mouth hook and hypostomal sclerite are enclosed in a
membranous sac, which serves as a functional mouth or atrium. The mouth hook may be protracted and
withdrawn by which action it serves to scrape a food medium. Fluids and particles of solid food are
drawn into the atrium by the sucking action of the cibarial pump, mixed with
saliva, and finally pumped into the oesophagus. SECTION IV
- ORIGIN OF THE PRINCIPAL BODY REGIONS
Dr. Robert Dicke
dealt in considerable detail with evolutionary aspects of the insect body
development. The following is derived
largely from his account to students at the University of Wisconsin. Fossil records indicate that insects were
probably as numerous on earth 150 million years ago as they are today. But the time of their origin, or the
stages through which they progressed in their evolution, are obscure. There is some fragmentary evidence that
Collembola-like arthropods may have occurred in the Devonian geological age
ca. 350 million years ago.
Unquestionable fossil records have been recovered from rocks dating to
the Upper Carboniferous Age of ca. 250 million years ago. Even at these ancient times, insects had
their wings fully developed.
Outgrowths of the prothorax on some of these fossil forms do provide
evidence that wings may have evolved from similar paranotal lobe structures.
It certainly appears that all of the important evolutionary changes in
insects were completed before the beginning of the Permian Age, dating from
215 million years ago. Because of the
incomplete palaeontological records, most evidence on the origin of insects
must be drawn from three sources of study: 1) a comparative morphology of
ancient and modern arthropods, 2) a comparative morphology of insects as we
know them today, and as they relate to fossil forms recovered from the
deposits of carboniferous and subsequent geological ages, and 3) the study
of embryonic forms of present‑day insects. The Trilobita
are ancient arthropods that lived about 550 million years ago, appearing as
fully developed animals in early Cambrian rocks and continuing to exist
beyond the Carboniferous Age and into the Permian. Living as companions with the trilobites were the now extinct
Eurypterida and the Xiphosura or horseshoe crabs of which living
representatives may still be found along coastal waters. Comparing structures and systems of these
ancient forms with present-day arthropods such as the Arachnida, Crustacea,
Chilopoda, Diplopoda, Pauropoda and Symphyla has allowed for speculation on
the pattern of development that may have occurred in the evolution of
insects. Much of the theory on the
origin and development of locomotory appendages and the organization of the
principal body regions may be derived from such a study. The antiquity of structures such as the
compound eye, antennae or chewing mouthparts helps to establish whether these
organ systems are relatively primitive
(have occurred early in phylogenetic history) or specialized (of relatively recent origin). A study of the
comparative morphology of ancient and modern insects further establishes
relatively primitive or specialized structures. Of the present‑day insects, cockroaches or the Blattoidea
are probably the earliest occurring forms.
These appeared in great numbers in the Upper Carboniferous of about
250 million years ago, and are essentially the same morphologically as the
forms that have adapted themselves to such an intimate relationship with our
present human civilization. The
morphology of the roaches may then he contrasted with the Hymenoptera or
Diptera, which probably appeared in the Jurassic Age some 95 million years
later. A study of internal morphology
furnishes further clews on the probable evolution of the principal body
regions. For example, the organization
of the central nervous system indicates the probable metameric organization and
the fusion of these metameres into the main body regions of a typical insect. Embryologists
have suggested that "ontogeny
[development of an individual] repeats or gives evidence to phylogeny
[history of a race]. Therefore, a study of embryonic forms reveals the abortive
development of structures long discarded by the individual during its
evolution into a present‑day adult form. Usually these studies are much more fruitful in the relatively
primitive forms such as the Collembola or Orthoptera. Embryonic phylogeny has become obscure in
groups in which a highly specialized form of metamorphosis has evolved such
as in the Holometabola. These studies
do provide sufficient evidence to support such theories as a head development
involving the fusion of five metameres, and suggest the probable occurrence
of such appendages as a second pair of
antennae. STAGE I. WORM‑LIKE PROTOTYPE
The phylum
Arthropoda is probably most closely related to the phylum Annelida, and
phylogenists have generally agreed that the arthropods and annelids probably
evolved from a common prototype. A hypothetical depiction of such a
prototype would be a 2O‑segmented, worm‑like animal in which the
mouth was situated posterio‑ventrally in the first anterior metamere
generally designated as the archeocephalon or prostomium
(Fig148).
In this concept, the
prostomium and all of the postoral metameres were relatively uniform in size
and composition. The body served
primarily to house the long intestinal tract which terminated as an anus in
the 20th metamere designated as the periproct. However, the prostomium and periproct probably should not be
designated as true metameres. The
prostomium may be considered as a sensory lobe or "head" derived
from the first anterior metamere, while the periproct may be considered
simply as a lobe bearing the anus as an outgrowth of the last metamere. In any case, the composition of the animal
may be depicted as a series of uniform, undifferentiated divisions
coextensive with the intestinal tract.
It may also be assumed that this animal could have been the prototype
for the earthworm as well as for the cockroach. STAGE II. DEVELOPMENT OF APPENDAGES
Probably the
first major change in development, which separated the arthropods from the annelids,
was the acquisition of paired appendages by all of the major divisions of the
body (Fig
149). Latero-ventral
protuberances of the body wall probably developed uniformly on all of the metameres
from 3 to 18, and were employed for locomotion. Appendages
developing on the prostomium, and on the first post‑oral and the 19th
metameres were sensory in function.
The anterior sensory structures are designated as antennae while the
posterior pair probably were the cerci of present‑day primitive insects. The antennae and cerci of present day
insects cannot be readily homologized with typical walking legs. Therefore, it may be assumed that while
all appendages arose as simple outgrowths of the body wall, antennae and
cerci evolved directly as sensory structures and were not modified from
ambulatory appendages at a later stage in development. Well-developed filiform antennae (primary antennae)
and cerci certainly occurred early in the evolution of insects. Although there are no known insects
including extinct species bearing two pairs of antennae, there is sufficient
evidence that the second antennae or postoral pair may have developed in such
an early prototype. In the embryos of
certain primitive species, a reduced second antennal protuberance may be
identified, but this structure is completely suppressed before completion of
embryonic development. In a few adult
forms, small lobes situated before the mandibles may be vestiges of such
appendages. The postantennal
appendages in the Crustacea are frequently referred to as structures
homologous with the hypothetical second antennae of insects, although these
appear to have been modified at a later period from leg‑like
appendages. The evidence proposed to
support the existence of a prehistoric second pair of antennae is weak,
although the supposition cannot be completely ignored. Photoreceptors
probably evolved early, and it should be recalled that well developed compound
eyes occurred in the Trilobita, Eurypterida and Xiphosura during the early
Cambrian Age. It has sometimes been
assumed that these eyes evolved from a pair of appendages in addition to the
antennae, and therefore there is the probable existence of an additional
metamere in the head complex. The
stalked eyes of the crayfish Cambarus
are frequently referred to in support of this theory. Since the dioptric apparatus of the eye is
simply a modification of the integument, development from an existing appendage
would appear to be an illogical step in their evolution. Ocelli also appeared in these early
Cambrian forms, and it would seem equally illogical that these were evolved
from appendages and would account for still other additional metameres. STAGE III. CEPHALIZATION AND DIFFERENTIATION OF
APPENDAGES
The term cephalization in
this phase of development implies the coalescence or unification of sensory
structures and the mechanisms designed for food ingestion into a composite
unit usually identified as the head (Fig
150).
Unification of sensory
structures would be a logical first step, combining the prostomium bearing
the primary antennae and photoreceptors with the first postoral metamere and
its second antennae. This primitive
head, combining only the principal sensory structures, is referred to as the protocephalon. As this prototype became a more highly
organized animal, the anterior appendages were utilized and subsequently
modified to aid in the ingestion of food.
The locomotory appendages of metameres 3, 4 and 5 gradually evolved
into the three principal appendages of the mandibulate mouthparts. Along with this specialization of
appendages it may be assumed that a coalescence of the metamere bearing them
probably occurred, bringing the feeding structures closer to the mouth. This combined region is designated as the gnathocephalon. Prior to the development of the
gnathocephalon, the second antennae may have served both a sensory function
and as an ingestion device. As the
appendages of the gnathocephalon developed into the more efficient mandibles,
maxillae and labium as they are known today, the utility of the second
antennae decreased and the structure was eventually discarded. A study of the
central nervous system leaves little doubt that at one time each metamere was
innervated independently by a central nerve center or ganglion. Eventually these ganglia were united by
the interconnected ventral
nerve cord. Examination
of this system in present‑day insects provides evidence of the probable
divisional composition of the prototype.
The composition of the gnathocephalon by the coalescence of three
metameres appears to be reasonably well established since the suboesophagial ganglion
that innervates the mandibles, maxillae and labium appears to be a composite
or three ganglia. However, there is
some question about this three‑segmented gnathocephalon in relation to
the superlinguae
(or paragnatha)
which in certain primitive insects appear as paired lobes associated with the
hypopharynx. There is some evidence
that these lobes may be the vestiges of a pair of post mandibular appendages,
which may have united to form between them the median hypopharynx. The suboesophagial ganglion does innervate
the hypopharynx. If this evidence is
sufficient, it could be assumed that four metameres were involved in the
composition of the gnathocephalon.
The protocephalon is that portion of the definitive insect head, which
is innervated by the supraoesophagial ganglion.
This ganglion is composed of three distinct parts: the proto cerebrum with its large optic
lobes, the deutocerebrum
which innervates the antennae, and the tritocerebrum which innervates
the labrum and is connected with the frontal ganglion and the stomodaeal nervous system. It appears that the origin of the
tritocerebrum was postoral since the commissure uniting
the two lobes of this ganglion, the suboesophageal commissure,
loops around the oesophagus and lays ventrad and posterior to the mouth. This ganglion probably represents the
first post oral metamere which probably bore the second antennae. The proto cerebrum and deutocerebrum
definitely are preoral and probably innervated the sensory structures of the
prostomium. As was previously
suggested, it seems unlikely that the compound eyes evolved from appendicular
appendages. The proposal has been
made that the proto cerebrum is simply an expansion of the original
prostomial ganglion to accommodate the highly evolved compound eyes. It was assumed
in Stage II that all of the appendages of metameres 3 through 18 were
utilized as walking legs similar to those of the Chilopoda and
Diplopoda. In Stage III, the legs of
metameres 3, 4 and 5 were modified into mouthparts, and still other
specializations of these appendages probably developed in other metameres of
the body. Segmentation of the
appendages would result in a much more effective walking leg. Development of the legs on metameres 6, 7
and 8 and a corresponding reduction of legs on the posterior metameres would
also increase the efficiency of the ambulatory mechanism. The primitive genital pore of the female probably was situated on the conjunctival membrane behind
the sternum of the 15th metamere.
However, in modern insects this pore is found on the 16th (8th
abdominal) wherever there is a special mechanism provided for
oviposition. There is good evidence
that the prototype of present‑day insects was equipped with an ovipositor, and
that two pairs of walking legs were modified into the valvulae of this
structure. A corresponding modification
of the appendages on the seventeenth metamere (ninth abdominal) of the male
evolved into a clasping device of the copulatory mechanism. Since the metamere of Stage II, visualized
as an inflexible ring retarded motion, a longitudinal suture may have
developed dividing each metamere into a dorsal tergum and a ventral sternum. STAGE IV. DIFFERENTIATION OF THE PRINCIPAL BODY
REGIONS
A division of
the body into specific regions or tagmata
was a logical development following specialization of the appendages (Fig 151).
Fusion of the
protocephalon with its sensory organs and the gnathocephalon with its
specialized appendages surrounding the mouth comprised the head tagma where the division of function was then related to
sensory perception and food ingestion.
The appendages of metameres 6, 7 and 8 evolved into an efficient
walking mechanism along with a corresponding expansion and elaboration of the
metameres. Important developments
were of the broad lateral surfaces, the pleurae, for better manipulation of
the leg base. Lateral expansions of
the terga probably occurred early in the evolution of the Pterygota. These were the paranatal lobes that may have been the precursors of wings on the 7th
and 8th metameres. In this area of
the body, the thoracic
tagma, division of function
was related to locomotion. The
remaining metameres were involved in relatively little elaboration since
their appendages gradually lost their locomotory function and were eventually
discarded. Only the appendages of the
16th and 17th metameres were retained and developed into a functional
ovipositor or copulatory mechanism.
With loss of locomotory or sensory function, the terminal metameres
were reduced in size or tended to coalesce.
This area of the body evolved into the abdominal tagma, and its division of function was related to
reproduction and to the centralization of such visceral systems as the
digestive tract, the principal respiratory tract, circulation, storage and
elimination. It was suggested
that present‑day insects evolved from a 20‑segmented prototype in
which function has dictated the formation of 1) a head of five metameres
centralizing sensory perception and the mechanism for ingestion of food, 2) a
thorax of three metameres designated for a both terrestrial and aerial
locomotion, and 3) an abdomen composed of the remaining 12 metameres which
houses the important visceral systems and serves as the seat of reproduction.
SECTION V
- COMPOSITION OF THE CUTICULA
The layer of cuticula
which envelopes the insect body and its appendages serve both as an integument for
containing tissues, and as a skeleton by providing support for organ systems
and muscles (Fig
152).
It is an exoskeleton since it is an external
structure, but it is also an endoskeleton
since the cuticula may be infolded or invaginated to form internal
ridges and rods. The cuticula may
have served simply as an integument for the worm‑like ancestor of
insects. Modification of the cuticula
into hard plates or sclerites
and the expansion of it into appendages brought about the
complex mechanisms that in insects evolved into the locomotory, sensory,
ingestive and reproductive structures which are unique in the Animal Kingdom.
The cuticula
comprises three distinct layers: an outer, thin layer of epicuticle, a median layer of
pigmented and usually hard and inflexible exocuticle, and an inner layer
of soft and flexible endocuticle
(Fig 152). A single layer of cells identified as the
epidermis secretes these layers. The
cuticula appears to be composed of irregular horizontal layers. Minute vertical striations in the cuticula
suggest that protoplasmic strands projecting from the epidermal cells secrete
these layers. A thin, fibrous sheet,
the basement
membrane underlying the epidermis
completes the body
wall. The epicuticle is a surface film about one
micron in thickness made up primarily of proteins and lipids. It is a protective film resisting the
effects of such environmental stresses as excessive humidity and
desiccation. One of the basic constituents of
the exocuticle and endocuticle is a polysaccharide polymer commonly known as chitin and chemically identified as a
polyacetylglucosamine. Chitin is a
colorless, transparent, soft, flexible material insoluble in water, alcohol,
ether, dilute acids and alkalies. A
second constituent or groups of constituents are complex, long chain
proteins, which apparently serves as a framework for all other chemicals
deposited in the cuticula. The
endocuticle is composed primarily of chitin and unmodified proteins. Endocuticle, then, is the principal
component of areas of the integument that are soft and membranous and at all
points of articulation between hard plates or sclerites, and metameres or appendicular segments. The exocuticle is composed of the same
framework of chitin and proteins but is usually hardened or sclerotized by
a polymerized protein referred to as sclerotin. The exact chemical nature of the
proteinaceous sclerotin and the tanning process or hardening of this protein
is still under vigorous investigation.
Exocuticle also is impregnated with lipids and is variously colored by
pigments. The cuticula is
seldom smooth, and microscopic examination will reveal many raised areas,
corrugations, ridges and blunt or sharp projections. Much of this sculpturing is due to
expansions or protrusions of the exocuticula such as the spines and microtrichia illustrated (Fig 152). These are the
fixed, usually inflexible, non‑cellular
processes of the cuticula. Other more macroscopic processes may
involve an invagination or a protrusion (evagination) of the entire body
wall. The apodeme of the illustration (Fig 152)
is such a multicellular process where the invagination
involves not only the cuticula, but the epidermis as well, A chalaza, which is a descriptive term
for an external protrusion, may be heavily sclerotized (e.g., involves the
exocuticle primarily), but its evagination also involves all of the other
elements of the body wall. Some of the
epidermal cells may be greatly enlarged and specialized for the secretion of unicellular processes instead of a laminated cuticula. Hollow, tubular setae arise from trichogen cells. A long,
protoplasmic strand arising from the trichogen cell extends through the
formative integumentary cuticula, and lays down around it a layer of
cuticula. When the protoplasmic
strand recedes back into its cell, a hollow tube remains composed primarily
of exocuticle. The cuticula of the
integument provides a circular pocket or alveolus, and a second
specialized epidermal cell, the tormogen cell, secretes a cuticular setal membrane at the base of the alveolus. The seta, then, is firmly seated in the alveolus
and is capable of articulation with the integument by means of the flexible
setal membrane. The tormogen cell
completely surrounds the base of the seta and at least a portion of the
trichogen cell. Additional
specialized cells may be associated with the trichogen and tormogen
cells. A poison cell may secrete an urticating fluid into the hollow
seta. When such a seta is broken in
the skin of a predator, the urticating fluid is released. Some setae are provided with sensory
nerve cells. An axon extending to or
within the setae modifies it into a simple tactile sense organ. Many of these tactile setae overcome one
of the serious disadvantages of an impervious and confining integument by
providing the insect with a means of contact with its environment. When a seta is broken and removed, the
circular alveolus remains as a pit.
Not all of the pits or punctures that occur in the cuticula are
provided with setae. Many of the pits
are simply ducts leading from a gland
cell in the epidermis. Secretions from these gland cells simply
issue from the pore and spread over the external surface of the
cuticula. Chemoreceptors, which are
simply modified setae, may be found enclosed within a deep alveolus. A cuticular pit, then, may simply be the
orifice for a chemoreceptor provided with a sensory cell. The depth of the alveolus and the size and
sclerotization of the modified seta enclosed within it will determine the
sensitivity of the chemoreceptor. SECTION VI -
ORIGIN OF THE MOUTHPARTS
The primitive
leg of the insect prototype probably was a simple, tubular expansion of the
body wall (Fig 153).
Muscles of the
body operating at its base manipulated this structure. The legs of Cambrian trilobites were fully
segmented, and fossils of the earliest insects demonstrate that segmentation
occurred very early in their evolution.
Little evidence on their evolution is available from embryological
studies, however. Therefore, it may
be speculated that their probable evolution was from such a theoretical,
tubular evagination. Segmentation of
a limb completely encased in cuticula would increase its efficiency
considerably. It may be assumed that
this segmentation was no more illogical than the segmentation or metamerism
of the body. The first
segment of the leg probably occurred at its base leaving a basal coxapodite and a distal telopodite. Further divisions or segments of these two
principal regions are referred to as coxites and telopodites. In order to accomplish more effective muscle
attachment and articulation, the coxapodite was subdivided into a basal subcoxa and an apical coxa. These divisions are still evident in some of the primitive
Apterygota. In the more highly
evolved forms elements of the subcoxa were elaborated into the pleural plates
for further support of the legs and wings.
In some of the primitive forms, lateral movable lobes have developed
from the coxapodite. Where the lobes
occur on the ental margin of the coxapodite they are identified as endites; those on
the ectal surface are exites. Progressive evolution of a walking leg may
have resulted in further segmentation and the provision of these segments
with muscles for their articulation.
In the present‑day insect leg these segments are identified from
proximal to distal as the trochanter,
femur, tibia, from one to five tarsi, and a pretarsus. It was assumed
that the appendages of the hypothetical gnathocephalon of a prototype evolved
into the mandibulate mouthparts of a chewing insect. A comparison of the segments and
musculature of a typical maxilla with that of a leg provides a reasonably
plausible explanation for this theory.
The cardo is
the articulating segment of the maxilla and appears to be comparable with the
subcoxa of a primitive leg. Articulating with the cardo is the stipes
which seems to be homologous with the coxa. Both coxa and stipes bear a long,
segmented telopodite. The telopodite
of the maxilla is an undifferentiated maxillary palp, while that of the leg
is a specialized series of segments identified as trochanter, femur, tibia
and tarsi. Endites of the stipes
evolved into the specialized lacinia and galea of the food ingesting
mechanism. An examination
of the labium demonstrates that it is a composite of two appendages. Each half of the labium is essentially
composed of the same basic segments as the maxilla. The basal, articulating postmentum is comparable with the
cardo, the prementum bears the same relationship as the stipes since it bears
the 3‑segmented telopodite or labial palp. The endites glossa and paraglossa are comparable with the
lacinea and galea of the maxilla. The
mandibles are probably more highly evolved toward a simplification of
structure. The terminal product of
this specialization is a fusion of the basic subcoxa and coxa, and an
elimination of the telopodite. In
some insects, a distinct sclerite or prostheca
is discernible. This sclerite is
probably an endite and is homologous with the lacinia of the maxilla. Sincere appreciation is
extended to Dr. Robert Dicke and Dr. Dorothy Feir of the Department of
Entomology, University of Wisconsin, Madison, and to Dr. Donald Davis,
Department of Entomology, Utah State University for their suggestions,
instruction and encouragement. |
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1/ Refer to Section IV ‑ Origin of the Principal Body Regions.
2/ Refer to
Section V ‑ Composition of the Cuticula.