Vocabulary you should learn:
It is worthwhile here to attempt some generalizations on insect anatomy and morphology on which more detailed modifications of each aquatic order can be fitted. The cuticle of an insect, as stated before, forms a more or less hardened exoskeleton and, although perfectly continuous over the whole body, it remains flexible along certain definite, and usually transverse, lines. In the latter positions, the cuticle becomes infolded and is membranous in character. Consequently, the body of an insect presents a joined structure-referred to as segmentation-and is divided into a series of successive rings variously known as segments, somites, or metameres. The flexible infolded portion of the cuticle between adjacent segments is the intersegmental membrane whose function is to allow for free movement of the body.
The cuticle exhibits localized areas of hardening termed sclerites, which meet one another along certain lines of union known as sutures. In the case of movable sclerites, their membranous continuity may be concealed, but if the cuticle of an insect be distended, many of the sclerites will be forced apart and it is then seen that they are connected by membrane along the lines of the sutures. Others of the sclerites may be rigidly fixed, and inseparable in this manner. The sutures in these cases being little more than linear impressions (this situation will be found most often in some of the higher orders, especially Coleoptera). In certain regions, the sclerites may not come into apposition along sutures, and appear instead like islands of cuticle surrounded by membrane. Complete fusion of adjacent sclerites is common, particularly among the higher orders and all traces of sutures may be lost.
In the majority of adults as well as in many of the larvae, the body wall of a typical segment is divisible into four definite sclerotized regions: a dorsal region or tergum, a ventral region or sternum, and a lateral region or pleuron on each side of the body. Each of these regions may be differentiated into separate sclerites. In this case, the sclerites composing the tergum are known as tergites, those of the sternum as sternites, and those constituting each pleuron as pleurites. In certain instances, one may see small detached plates between adjacent segments. Such sclerites are often called intersegmentalia and belong partly to the segment in front and partly to the one behind them. According to their position, they may be termed intertergites, interpleurites, or intersternites.
In the embryonic stage of insects, each body segment may bear a pair of out-growths or appendages which may or may not be retained in post-embryonic life.
Among adult insects, an appendage is normally attached to its segment between the pleuron and the sternum.
Typical appendages are joined tubes invested with a dense cuticle. Between each pair of joints or segments, the cuticle remains membranous and becomes infolded to form the articular membrane. One account of its jointed structure, the whole or part of an appendage is movable by means of its muscles. An insect appendage consists typically of a limb base and a shaft which represents the endopodite of Crustacea. There is no conclusive evidence of a biramous condition among the appendages in any insects.
In addition to true appendages, numerous other outgrowths of the body wall are found in various insects. Unlike true appendages, processes of the body wall are by no means invariably represented by embryonic counterparts. They may or may not be segmentally arranged, they may be originally paired or unpaired, and more than a single pair is sometimes borne on a segment. They differ from cuticular processes in containing a definite extension of the body cavity and in some cases, they are freely movable. The principal types of organs which come under this category are pseudopods which are characteristic of many true fly larvae, but often are referred to as prolegs; gills, which are found in most of the immature stages of aquatic insects; and wings, confined to the meso- and meta-thorax and which attain their full development in adult insects.
The head capsule is a hard, compact case formed of several sclerites fused together. To take a typical generalized head we would find at the top a Y-shaped epicranial suture. The stem of the Y forms a median line and the two arms diverge anteriorly. The frons is the unpaired sclerite which lies between the arms of the epicranial suture. It bears the median ocellus and its distal limit is marked on either side by the invaginations or small pits which form the anterior arms of the tentorium. The clypeus lies just anterior to the frons and is often fused to it owing to a com-plete or partial obliteration of the suture which separates them (clypeo-frontal suture). The labrum of an unpaired sclerite ususally movable articulated with the clypeus by means of the clypeo-labral suture.
In general, the epicranium forms the whole of the upper region of the head from the frons to the neck. That portion of the epicranium which lies immediately behind the frons and between the compound eyes is referred to as the vertex. The vertex usually carries the paired ocellae and the antennae but is not differentiated as a separate sclerite. The occiput is the hinder part of the epicranium between the vertex and the neck. It is rarely present as a distinct sclerite. The gena forms the lateral area below and behind the eyes on each side. At its junction with the clypeus, it bears a facet for articulation of the mandible. The mouthparts of insects consist typically of the labrum or upper lip, the labium, or lower lip, an upper pair of jaws called mandibles, and a lower or posterior pair of jaws called the maxillae. Arising from the floor of the mouth is a median tongue-like structure called the hypopharynx and associated superlinguae. Mouth parts vary widely in form, this variation being correlated with the method of feeding and other uses to which they are put. Examination of the mouth parts may, therefore, give a clue to the method of feeding and frequently the nature of the food of an insect.
We will have to consider the mouth parts separately for each of the Orders as we come to them because of their tremendous modification from Order to Order and because of their significance in taxonomy. The labrum is a simple plate hinged to the clypeus and capable of a limited amount of up and down movement. It forms the roof of the buccal cavity.
The mandibles or true jaws each represent the basal joint or coxopodite of the typical arthropod limb. They undergo tremendous modification in the various groups. The maxillae are composed of: the cardo or hinge, which is the first or proximal piece, and in many insects the only part directly attached to the head; the stripes or footstalk which articulates which(?) the distal border of the cardo and bears an outer scerite called the palpifer (and sometimes an inner sclerite, the subgalea or parastipes). The palpifer carries a maxillary palpus which is the most conspicuous appendage of the maxillae. It if 1- to 7- jointed and sensory in function. Distally, the maxilla is composed of two lobes: an outer one or alea and an inner one, or lacinia. The former is two-jointed, frequently, and it often partially overlaps the lacinia. The lacinia or blade as a rule, is spined or toothed on its inner border.
The labium (or second maxillae), is formed by the fusion of a pair of appendages which are serially homologous with the maxillae. The labium is divided into two primary regions; a proximal postmentum and a distal prementum, the line of division between the two being the labial suture. In some orders, just behind the labial suture is a distal sclerite called the mentum and the proximal area of the original postmental plate will then be termed the submentum. Near the base of the prementum is the palpiger which carries the labial palpus and often resembles a basal joint of the latter. The labial palps are composed of one to four joints and function as sensory organs. Arising from the distal margin of the prementum are two pairs of lobes which collectively form the ligula. These are: an outer pair of paraglossae and an inner pair of glossae.
Sometimes some small sclerites exist in the neck region. These are called, logically enough, cervical sclerites. They are found in most of the orders of insects but are best developed in the more primitive groups, among the aquatic groups, chiefly the Odonata.
The thorax is composed basically of three segments: prothorax, mesothorax, and metathorax. In almost all insects, each segment bears a pair of legs and in the majority of adult insects, both the meso- and the meta-thorax carry a pair of wings. In all cases where the legs are wanting, their absence is due to atrophy. This condition is rare among adults but is the rule among larvae of the Diptera and certain families of Coleoptera. The absence of wings may be an ancestral character in the Apterygota. But among the Pterygota it is always an acquired feature, due to the atrophy of pre-existing organs. The thorax is exhibited in its simplest form in the Thysanura and in the larvae of many Orders. In these instances the segments differ but little in size and proportions, but usually with the acquisition of wings, a correlated specialization of thoracic structure results. The meso- and meta-thorax become more or less intimately welded together, and the union is often so close that the limits of those regions can only be ascertained with difficulty. In Orders where the wings are of about equal area, these two thoracic segments are of equal size. This is true particularly in the Odonata. Where the forewings are markedly larger than the hind pair, there is a correspondingly greater development of the meso-thorax. (A good example is the Diptera where the hind wings are absent.) The prothorax never bears wings and is variable in the degree of its development (it may be noted that some of the fossil ancestors of the stoneflies had a structure on the margins of the pro-thorax which some paleontologists have interpreted as a third pair of wings).
To introduce a few more terms common in the literature - in many immature stages and also in the adults of the more generalized insects, the tergum of each segment is a simple undivided plate or notum. In the wing-bearing segments of most adults, the tergum is composed of large anterior plate or notum, already mentioned, and a narrower posterior plate or post-notum, which has a rise in the inter-segmental membrane, the scutum, and the scutellum. These are illustrated on the blackboard, the example being the thorax of a cranefly.
The side of the thorax is termed the pleuron and is composed of sclerites called pleurites. There is an anterior sclerite called the epimeron, the two separated by the pleural suture. These sclerites may be variously modified, fused, or further subdivided. In many insects, including both Corydalis, Tipula and Tabanus the episternum is divided into upper and lower halves. Similar division of the epimeron may be found in some forms. The sclerites forming the sternum also are subject to various subdivision, fusion, or other modification generally.
Each thoracic segment typically bears one pair of legs. A generalized typical insect leg consists fundamentally of a basal segment, tibia the coxa, which joins the body and is usually surrounded by the epimeron and episternum. Next comes a small segment called the trochanter, then the heaviest (usually) segment, the femur, followed by the tibia, and finally by the tarsus, which may have anywhere from one to five joints.
In various of the aquatic Orders the coxa is subject to some modification, as are most of the other leg segments. The trochanter, for instance, is divided into two subsegments in the Odonata. The femur undergoes wide modification in some of the aquatic insects. The tibia is almost always slender and usually equal to or exceeding the femur in length. It often bears tibial spurs. The tarsus consists primitively of a single element, as seen in the Protura. More usually, however, it is divided into subsegments, typically five in number. It terminates in a claw or claws which are subject to wide variation and often have taxonomic value.
The abdomen is subject to a wide range of variation among the immature and adult stages of the various aquatic Orders. The abdomen frequently bears gills or other appropriate appendages in the immature stages. The abdomen also bears the reproductive organs and so-called accessory genitalia which are of tremendous importance in taxonomic work, especially as regards male specimens. A generalized abdominal tip for a typical male insect may be sketched on the board and you will soon see that it is very close to the form assumed by living mayflies. Almost every morphological feature of the insect body has been employed at one time or another by one worker or another as showing taxonomic significance. At first glance, mouth parts may seem too complicated to "learn" and wing venation itself may present a seemingly bewildering maze. However, just as in learning to recognize makes of automobiles, soon you will be spotting insects not only to higher category but even to the species level on the basis of a quick observation.
Leonard's lecture notes converted to html: 16 January 2001 (EB)
Page last edited: January 24, 2005