A simplified version of Insect Morphology is presented for the purpose of quickly instructing those interested in the identification of insects, particularly those with predatory or parasitic behavior. The evolutionary format used is to ease the means by which the various insect structures may be learned. Admittedly, some of the trends hypothesized may not be universally accepted as valid.

The text is produced or paraphrased from cited references. It was developed during courses taken by the author at the University of Wisconsin, Utah State University and from various instructors at Wilson College in Chicago, Texas A. & I University in Kingsville and at the University of Illinois. The diagrams were derived and modified from those provided in courses taken by the author from Dr. Robert Dicke at the University of Wisconsin, Madison and Dr. Donald Davis, Utah State University. The terminology of Snodgrass (1952) was 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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Introduction

Insect identification to the specific level requires a substantial knowledge of morphology. The following is an introduction to the gross, comparative morphology of insects. The term, morphology as developed in this work is a study of the functional form of an insect, although details of anatomy or the specific parts of an insect must be described before the functional whole can be grasped. It is a comparative morphology restricted to seven representative species that were chosen to broadly represent the complex spectrum of insect forms. These are in ascending evolutionary sophistication, Silverfish, Thermobia domestica (Packard) ‑ Thysanura; Madeira roach, Leucophaea maderae (Fabricius) ‑ Orthoptera; Milkweed bug, Oncopeltus fasciatus (Dallas) ‑ Hemiptera; June beetle, Phyllophaga rugosa (Melsheimer) ‑ Coleoptera; Noctuid moth, Heliothis zea (Boddie) ‑ Lepidoptera; House fly, Musca domestica (Linnaeus) ‑ Diptera; and the Honey bee, Apis mellifera (Linnaeus) ‑ Hymenoptera

The general plan of this study establishes a typical insect form for comparative purposes which basically represents most insects as we know them today. The cockroach, Leucophaea maderae, was arbitrarily selected by Dr. Robert Dicke as such a "typical" form. This selection was based on concepts of the evolutionary changes that probably occurred from a hypothetical worm‑like ancestor through the primitive silverfish, to the very highly evolved or specialized house fly and honey bee. A primitive structure or system is one that has occurred early in the evolutionary history of insects, while a specialized structure is a more recent elaboration of a primitive form. The establishment of a concept of Aprimitive 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 mouth parts. Phyllophaga rugosa, Heliothis zea, Musca domestica and Apis mellifera are representatives of the four major orders of insects. These illustrate many specializations, especially in the metamorphic forms or larvae that precede the adult stage.

The detailed drawings in the text are useful during dissections and study of preserved and living insects in the manner that an artisan would employ a set of blueprints in his construction of a building or machine. The descriptive text should be studied, the structures identified, and the concepts verified by examination of the drawings. However, all this effort is incomplete at best until one has personally dissected, manipulated and identified the animal's structures and systems. Theoretical concepts are alluded to and then thoroughly discussed in Section IV. All technical terms are in bold faced type and specifically described in Section VII, Morphological Terminology.

Dr. Robert Dicke in his course "Insect Morphology" at the University of Wisconsin, concluded with the following introductory comments, "Proceed carefully and diligently with your study and dissection of these insects. You will be rewarded by a fascinating display of an ingenious and beautifully created machinery that can sense and adapt itself to a complex environment, that can ingest and synthesize a wide range of organic matter, and that comprises a vast group of animals which probably will reproduce and survive in spite of the intentional or incidental efforts of man to exterminate them."

EXTERNAL MORPHOLOGY

SECTION I ‑ THE BODY WALL

Metamerism and the Principal Body Regions

A major characteristic of an Arthropod is the division of its body into segments. This trunk segmentation is usually referred to as metamerism. Each body segment may then be identified as a metamere. Considerable evidence exists that all Arthropods including insects probably evolved from a segmented, worm‑like ancestor or prototype comprising about 20 distinct but undifferentiated metameres.^/1 Each metamere probably was cylindrical or ring‑like in form, and in a series coextensive with the gut or intestinal tract was joined together by transverse invaginations of the body wall. The anterior opening to the gut or mouth was probably situated ventrally between the first metamere or prostomium and second metamere, while the posterior opening to the gut or anus was borne by the last metamere or periproct. With the exception of the periproct, each metamere acquired a pair of ambulatory appendages by means of lateral expansions of the body wall. It is then believed that this prototype evolved into the present day insect form through a series of specializations in which distinct functions of the organism became the responsibility of certain body regions. These body regions or tagmata are the head (region of ingestion and principal sensory perception), the thorax (region of locomotion), and the abdomen (region of visceral function and reproduction). The prostomium and first four metameres are thought to have coalesced into the head region. The locomotory appendages of the prostomium probably evolved into sensory structures or antennae and the three appendages of the posterior metameres of the head complex became modified into organs of ingestion or, the mouth parts. Fusion of the metameres of the head region has been so complete that no evidence of their separate entities exist in present day forms. The 6th, 7th and 8th metameres comprise the thoracic region. In most insect forms, lateral appendages of this region were retained and further specialized to become the principal organs of locomotion. Wings, as additional expansions of the body wall, provided highly specialized and unique forms of locomotory structures. Complex external and internal modifications of the thoracic metameres were required to support and propel the leg and wing mechanisms. The remaining metameres of the hypothetical prototype were evolved into the abdominal tagma. With few exceptions, the ambulatory functions of the lateral appendages of the abdominal metameres were lost or modified into specialized appendages, especially for the reproductive function. The abdominal region, devoted primarily to housing the principal visceral systems, retained many of the features of the undifferentiated primitive metamere. A preliminary examination of the body form of the representative insect species included here will demonstrate that the three body tagmata are distinct even in the caterpillar of Heliothis zea. However, extreme modifications are quite apparent in the illustrated sagittal sections of Leucophaea maderae (Fig 1 ), Apis mellifera (Fig 2 ) and Phyllophaga rugosa (Fig 3 ). The body of Leucophaea maderae is flattened, or dorso‑ventrally compressed, and an outline of the thoracic and at least the first eight abdominal metameres are comparable in size and form. In contrast, the abdomen of Apis mellifera is cylindrical, and the number of abdominal metameres is reduced. An extreme modification of the first abdominal metamere has occurred (fusion with the thorax, e.g., propodeum, and narrow petiolated constriction). A disproportionate development of the 2nd thoracic metamere has evolved along with wing development at the expense of the first and 3rd (prothorax and metathorax).

The Exoskeleton

The body wall or integument (Figs 1, 2 & 3) is the external covering of an organism which maintains its characteristic form and contains the body fluids and tissue systems. 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

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1/ Refer to Section IV ‑ Origin of the Principal Body Regions.

2/ Refer to Section V ‑ Composition of the Cuticula.

hardening and inflexibility is known as sclerotization. A sagittal section of an insect's body demonstrates that the integument serves as its skeletal structure. Compared with the internal bony skeleton of a vertebrate, this structural mechanism is the exoskeleton. Thickness of cuticula and the degree of hardening or sclerotization varies considerably. In Phyllophaga rugosa, the cuticula of the head and protergum is much thicker than similar areas in Leucophaea maderae.

The skeletal structure of a metamere is not a simple inflexible ring of cuticula. Although the abdominal metameres are the least modified from the hypothetical form, at least two divisions of the metamere are apparent as shown in the cross sectional illustrations of Leucophaea maderae (Fig 1) and Apis mellifera (Fig 2 ). A dorsal plate or tergum is separated by a longitudinal infolding of the body wall from a ventral plate or sternum. This comparatively thin and flexible infolding of the body wall is termed a suture. Each of these plates or other areas of the body wall defined or separated by a suture are collectively termed sclerites. The metameres of the thoracic region are further subdivided into sclerites to make up the complex ambulatory and flight mechanism. A thoracic metamere is almost box‑shaped, and besides a tergum and sternum there is a side area or pleura. The tergum, sternum and pleura are rarely simple plates, but are further subdivided into sclerites especially on the wing bearing metameres.

The Endoskeleton

The cuticula is more than an outer skin or protective armor. The body wall may be invaginated to form cuticular ridges or rods wherever additional rigidity of the skeletal structure is advantageous or where supplementary points for muscle attachment are required. These cuticular invaginations are usually hardened or heavily sclerotized. They are called apodemes and collectively comprise the endoskeleton. Apodemes may be simple internal ridges such as the dorsal invaginations between the thoracic metameres of Leucophaea maderae (Fig 1). These dorsal thoracic invaginations may be greatly expanded into a broad plate‑like structure or phragma for muscle attachment as illustrated for Apis mellifera (Fig 2) or Phyllophaga rugosa (Fig 3). Rod‑shaped apodemes may combine to form an effective brace or strut bridging the anterior head cavity. This structure is the tentorium situated at the base of the mouth parts 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. Articulation or expansion between the tergal and sternal sclerites of the abdomen is accomplished by a longitudinal suture.

Protuberances of the Body Wall

The external surface of the cuticula is rarely smooth. In addition to the more apparent protuberances, the cuticula may be variously sculptured with minute depressions, corrugations and striations, or by irregularly alternating concave and convex surfaces. The cuticula may be produced into heavily sclerotized spines such as in the caterpillar of Heliothis zea (Figs 4 & 5). The spines may be sharply pointed or they may be blunt and irregularly shaped knobs. Spines often resemble minute hairs and are referred to as microtrichia (Fig 6 ). The veins and wing membrane of Musca domestica have a scattered covering of microtrichia (Fig 10 ). Although spines usually occur in an irregular pattern, they may be arranged in well defined lines such as on the tibial spurs of Leucophaea maderae (Fig 8 ) or on the ental surface of the labrum in the grub of Phyllophaga rugosa (Fig 108 ). All of these structures are collectively referred to as noncellular processes since the protuberance is composed entirely of heavily sclerotized cuticula and are fixed to and confluent with the exoskeleton.

Frequently, the epidermal cells of the body wall may become modified for the specialized function of secreting single hollow protuberances or unicellular processes. These may exhibit a variety of forms and are referred to by many descriptive terms. The hairlike 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 mouth parts 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 are 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. A broken seta may be identified by the socket in which it was previously seated. 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, hairlike 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. The flat plane of the scale is usually marked by parallel ridges which form minute striations. This sculpturing of the scale may produce a physical coloration due to an interference of reflected light. Protrusions of the entire body wall including the formative epidermis comprise the relatively conspicuous multicellular processes. Such a process may be a simple elevation of the integument bearing a unicellular seta at its apex. The illustration of seta ALPHA in Heliothis zea (Fig 6 ) is an example of a simple multicellular structure termed a chalaza by descriptive entomologists. Common examples of the more conspicuous multicellular processes are the heavily sclerotized, spiny structures termed spurs that are encountered on the legs of many insects. These spurs may be fixed and confluent with the cuticula. Others may be set in a membranous ring and are therefore movable as illustrated by the tibial spurs of Leucophaea maderae (Fig 8). Multicellular processes may bear fixed spines as the microtrichia on the spurs of Leucophaea maderae (Fig 8) as well as single or numerous unicellular setae.

SECTION II ‑ THE HEAD

Evolution of the Insect Head

The principal regions of the insect body are thought to have evolved as composites of cylindrical metameres, each of which in the primitive form bore a pair of ambulatory appendages.^/1 (See Figs. 148, 149 , 150 & 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 mouth parts and the nerve centers associated with them, it may be concluded that these organs of ingestion probably evolved from ambulatory appendages. Since three pairs of structures make up the generalized feeding mechanism, it may be assumed that three metameres were involved in the formation of a second primitive head complex or gnathocephalon. In the present‑day insect, the sensory protocephalon and the ingestive gnathocephalon have coalesced and have become completely fused into a composite structure. Unlike the thorax and abdomen, segmentation of the head is obscure and the sutures as we know them today have little correlation with the metameres that were involved in its formation.

The Typical or Generalized Insect Head

The head of Leucophaea maderae may be used to illustrate a typical, generalized form of head capsule (Figs 12 , 13 , 14 , 15 , 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 which is also depressed to form a pocket or oral cavity to accommodate the operation of the mouth parts.

Internally, the head capsule is braced before the oral cavity by an A‑shaped, composite apodeme formed by invaginations of the integument. 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 mouth parts 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 photo receptors 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 which 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

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1/ Refer to Section IV ‑ Origin of the Principal Body Regions.

additional suture may occur anteriorly to the postocciput and margins the flat posterior aspect of the head. In Leucophaea maderae this suture is more of a marginal ridge, but it may be referred to as the occipital suture and the area enclosed by it as the occiput. Usually, the term occiput is used only to describe the posterior area immediately behind the vertex. The lateral, ventral portion of this sclerite is then referred to as the postgena. However, technically the entire sclerite may be correctly referred to as the occiput.

THERMOBIA DOMESTICA (Figs 17 , 18 , 19 & 20 ).

The head of Leucophaea maderae was described as the "typical form.” But this does not imply that the head of Leucophaea maderae is primitive in the sense of being but little elaborated in comparison with a hypothetical prototype. Thermobia domestica is a relatively primitive insect compared with Leucophaea maderae. The conspicuous epistomal sulcus of Thermobia domestica will readily distinguish the facial areas (Figs 17 & 18 ). Note that the frons and clypeus are large, well-defined sclerites. The gena, however, is a small area immediately before the antenna and below the eyes. All of the other head sclerites described for Leucophaea maderae are absent. The postocciput as a sclerite is inconspicuous, but the invagination of the postoccipital suture forms a large apodeme or postoccipital ridge (Figs 19 & 20 ). The tentorium of Thermobia domestica is of special interest to the morphologist. Previously, this was defined as a cranial brace formed by the fusion of two anterior and two posterior invaginations of the exoskeleton forming the head capsule. In Leucophaea maderae, the tentorium forms an A-shaped structure comprising a posterior tentorial bridge and two anterior arms. However, the posterior tentorial bridge of Thermobia domestica has not fused with the anterior arms although a large central plate has been formed by the posterior fusion of the anterior arms. If the theory on the formation of the tentorium is correct, it may also be assumed that in Thermobia domestica this is a relatively primitive structure.

Specializations in the Adult Head Structure

Further modifications of the insect head from the typical form may cccur 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 mouth parts, 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 relative areas may be determined by the tentorial pits 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 mouth parts may project forward. In the generalized form, the facial area is directed forward and is anterior and vertical in position. The mouth parts are pendant or hang ventrally in position, and the labium which 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 mouth parts forward is advantageous to many species. The head is rotated upward with the mouth par