(With Emphasis on Natural Enemy Identification)
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Insect Morphology is presented for the purpose of instructing those interested in the identification of insects, particularly species with predatory or parasitic behavior. The evolutionary format used is to ease the means by which the various insect structures may be learned.
The text is produced or paraphrased from cited references. It was developed by the author while at the University of Wisconsin and Utah State University, 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.
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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."
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
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 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 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 Pooled Referencessclerotized. 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.
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.
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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 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.
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.
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).
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 (\). 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).
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
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.
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.
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 & \). 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.
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 \, 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).
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.
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.
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 (\). 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.
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.
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.
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 (\). 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.
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.
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.
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.
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.
1/ Refer to Section IV ‑ Origin of the Principal Body Regions.
2/ Refer to Section V ‑ Composition of the Cuticula.