Part 18

The thorax is composed of three segments; each bears a pair of jointed legs, and in the vast majority of insects the two hindmost bear each a pair of wings. From these three pairs of thoracic legs comes the name--Hexapoda--which distinguishes the class. And the wings, though not always present, are highly characteristic of the Hexapoda, since no other group of the Arthropoda has acquired the power of flight. In the more generalized insects the abdomen evidently consists of ten segments, the hindmost of which often carries a pair of tail-feelers, (cerci or cercopods) and a terminal anal segment. In some cases, however, it can be shown that the cerci really belong to an eleventh abdominal segment which usually becomes fused with the tenth. With very few exceptions the abdomen is without locomotor limbs. Paired processes on the eighth and ninth abdominal segments may be specialized as external organs of reproduction, but these are probably not appendages. The female genital opening usually lies in front of the eighth abdominal segment, the male duct opens on the ninth.

In all main points of their internal structure the Hexapoda agree with other Arthropoda. Specially characteristic of the class, however, is the presence of a complex system of air-tubes (tracheae) for respiration, usually opening to the exterior by a series of paired spiracles on certain of the body segments. The possession of a variable number of excretory tubes (Malpighian tubes), which are developed as outgrowths of the hind-gut and pour their excretion into the intestine, is also a distinctive character of the Hexapoda.

The wings of insects are, in all cases, developed after hatching, the younger stages being wingless, and often unlike the parent in other respects. In such cases the development of wings and the attainment of the adult form depend upon a more or less profound transformation or metamorphosis.

With this brief summary of the essential characters of the Hexapoda, we may pass to a more detailed account of their structure.

EXOSKELETON

The outer cellular layer (ectoderm or "hypodermis") of insects as of other Arthropods, secretes a chitinous cuticle which has to be periodically shed and renewed during the growth of the animal. The regions of this cuticle have a markedly segmental arrangement, and the definite hardened pieces (sclerites) of the exoskeleton are in close contact with one another along linear sutures, or are united by regions of the cuticle which are less chitinous and more membranous, so as to permit freedom of movement.

_Head._--The head-capsule of an insect (figs. 1, 2) is composed of a number of sclerites firmly sutured together, so that the primitive segmentation is masked. Above is the crown (_vertex_ or _epicranium_), on which or on the "front" may be seated three simple eyes (ocelli). Below this comes the front, and then the face or clypeus, to which a very distinct upper lip (_labrum_) is usually jointed. Behind the labrum arises a process--the _epipharynx_--which in some blood-sucking insects becomes a formidable piercing-organ. On either side a variable amount of convex area is occupied by the compound eye; in many insects of acute sense and accurate flight these eyes are very large and sub-globular, almost meeting on the middle line of the head. Below each eye is a cheek area (_gena_), often divided into an anterior and a posterior part, while a distinct chin-sclerite (_gula_) is often developed behind the mouth.

[Illustration: From Miall and Denny, _The Cockroach_, Lovell Reeve & Co.

FIG. 1.--Head and Jaws of Cockroach (_Blatta_). Magnified 10 times. A, Front; B, side; C, back; v, vertex; f, frons; cl, clypeus; lbr, labrum; oc, compound eye; ge, gena; mn, mandible; ca, st, pa, ga, la, cardo, stipes, palp, galea, lacinia of first maxilla; sm, m, pa', pg, sub-mentum, mentum, palp, galea of 2nd maxilla.]

_Feelers._--Most conspicuous among the appendages of the head are the feelers or antennae, which correspond to the anterior feelers (antennules) of Crustacea. In their simpler condition they are long and many-jointed, the segments bearing numerous olfactory and tactile nerve-endings. Elaboration in the form of the feelers, often a secondary sexual character in male insects, may result from a distal broadening of the segments, so that the appendage becomes serrate, or from the development of processes bearing sensory organs, so that the structure is pinnate or feather-like. On the other hand, the number of segments may be reduced, certain of them often becoming highly modified in form.

[Illustration: After Marlatt, _Entom. Bull._ 14, n. s. (U.S. Dept. Agric.).

FIG. 2.--Head of Cicad, front view. Ia, frons; b, clypeus (the pointed labrum beneath it); II, mandible; III, first maxilla; (a, base; b, sheath; c, piercer), III', inner view of sheath; IV, second maxillae forming rostrum (b, mentum; c, ligula).]

_Jaws._--The mandibles of the Hexapoda are usually strong jaws with one or more teeth at the apex (fig. 1, A, B, mn), articulating at their bases with the head-capsule by sub-globular condyles, and provided with abductor and adductor muscles by means of which they can be separated or drawn together so as to bite solid food, or seize objects which have to be carried about. They never bear segmented limbs (palps) and only exceptionally (as in the chafers) is the skeleton composed of more than one sclerite. The mandibles often furnish a good example of "secondary sexual characters," being more strongly developed in the male than in the female of the same species. In most insects that feed by suction the mandibles are modified. In bugs (Heteroptera) and many flies, for example, they are changed into needle-like piercers (fig. 2, II), while in moths and caddis-flies they are reduced to mere vestiges or altogether suppressed.

As previously mentioned, a pair of minute jaws--the _maxillulae_--are present in the lowest order of insects, between the mandibles and the first maxillae. They usually consist of an inner and an outer lobe arising from a basal piece, which bears also in some genera a small palp (see APTERA).

In their typical state of development, the _first maxillae_ offer a striking contrast to the mandibles, being composed of a two-segmented basal piece (_cardo_ and _stipes_, fig. 1, C, ca, st) bearing a distinct inner and outer lobe (_lacinia_ and _galea_, fig. 1, C, la, ga) and externally a jointed limb or palp (fig. 1, C, pa). Such maxillae are found in most biting insects. In insects whose mouths are adapted for sucking and piercing, remarkable modifications may occur. In many blood-sucking flies, for example, the galea is absent, while the lacinia becomes a strong knife-like piercer and the palp is well developed. In bugs and aphids the lacinia is a slender needle-like piercer (fig. 2, III), while the palp is wanting. In butterflies and moths the lacinia is absent while the galea becomes a flexible process, grooved on its inner face, so as to make with its fellow a hollow sucking-trunk, and the palp is usually very small.

The _second pair of maxillae_ are more or less completely fused together to form what is known as the _labium_ or "lower lip." In generalized biting insects, such as cockroaches and locusts (Orthoptera), the parts of a typical maxilla can be easily recognized in the labium. The fused cardines form a broad basal plate (_sub-mentum_) and the stipites a smaller plate (_mentum_)--see fig. 1, C, sm, m--jointed on to the sub-mentum, while the galeae, laciniae and palps remain distinct. In specialized biting insects, such as beetles (Coleoptera), the labium tends to become a hard transverse plate bearing the pair of palps, a median structure--known as the _ligula_--formed of the conjoined laciniae, and a pair of small rounded processes--the reduced galeae--often called the "paraglossae," a term better avoided since it has been applied also to the maxillulae of Aptera, entirely different structures. The long sucking "tongue" of bees is probably a modification of the ligula. In bugs and aphids (Hemiptera), the fused second maxillae form a jointed grooved beak or rostrum (fig. 2, IV) in which the slender piercers (mandibles and first maxillae) work to and fro.

This second pair of maxillae (or labium) form then the hinder or lower boundary of the mouth. In front or above the mouth is bounded by the labrum, while the mandibles and first maxillae lie on either side of it. A median process, known as the _hypopharynx_ or tongue, arises from the floor of the mouth in front of the labium, and becomes most variously developed or specialized in different insects. The salivary duct opens on its hinder surface. It does not appear to represent a pair of appendages, but the maxillulae of the Aptera become closely associated with it. According to the view of R. Heymons, the hypopharynx represents the sterna of all the jaw-bearing somites, but other students consider that it belongs to the mandibular and first maxillary segments, or entirely to the segment of the first maxillae.

_Neck._--The head is usually connected with the thorax by a distinct membranous neck, strengthened in the more generalized orders with small chitinous plates (_cervical sclerites_). These have been interpreted as indicating one or more primitive segments between the head and thorax. Probably, however, as suggested by T. H. Huxley (_Anat. Invert. Animals_, 1877), they really belong to the labial segment which has not become completely fused with the head-capsule. It has been shown by C. Janet (1889), from careful studies of the musculature, that the greater part of the head-capsule is built up of the four anterior head-segments, the hindmost of which has the mandibles for its appendages, and this conclusion is in the main supported by the recent work on the head skeleton of J. H. Comstock and C. Kochi (1902) and W. A. Riley (1904).

_Thorax._--The three segments which make up the thorax or fore-trunk are known as the _prothorax_, _mesothorax_ and _metathorax_ (see fig. 3). The dorsal area of the prothorax is occupied by a single sclerite, the _pronotum_ (fig. 3, d), which is large and conspicuous in those insects, such as cockroaches, bugs (Heteroptera) and beetles, which have the prothorax free--i.e. readily movable on the segment (mesothorax) immediately behind--smaller and of less importance where the prothorax is fixed to the mesothorax, as in bees and flies. The dorsal area of the mesothorax, and also of the metathorax, may be made up of a series of sclerites arranged one behind the other--_prescutum_, _scutum_, _scutellum_ and _post-scutellum_ (fig. 3, e, f, g, h), the scutellum of the mesothorax being often especially conspicuous. Ventrally, each segment of the thorax has a _sternum_ with which a median _pre-sternum_ and paired _episterna_ and _epimera_ are often associated (see figs. 3, 4). The recent suggestion of K. W. Verhoeff (1904) that the hexapodan thorax in reality contains six primitive segments is entirely without embryological support.

_Legs._--Each segment of the thorax carries a pair of legs. In most insects the leg is built up of nine segments: (1) a broad triangular, sub-globular, conical or cylindrical haunch (_coxa_); (2) a small _trochanter_; (3) an elongate stout thigh (_femur_); (4) a more slender shin (_tibia_); and (5-9) a foot consisting of five _tarsal segments_. The fifth (distal) tarsal segment carries a median adhesive pad--the _pulvillus_--on either side of which is a claw. The pulvillus is probably to be regarded as a true terminal (tenth) segment of the leg, while the claws are highly modified bristles. Numerous bristles are usually present on the thighs, shins and feet of insects, some of them so delicate as to be termed "hairs," others so stout and hard that they are named "spines" or "spurs." In the relative development and shape of the various segments of the leg there is almost endless variety, dependent on the order to which the insect belongs, and the special function--walking, running, climbing, digging or swimming--for which the limb is adapted. The walking of insects has been carefully studied by V. Graber (1877) and J. Demoor (1890), who find that the legs are usually moved in two sets of three, the first and third legs of one side moving with the second leg of the other. One tripod thus affords a firm base of support while the legs of the other tripod are brought forward to their new positions.

[Illustration: After Marlat, _Ent. Bull._ 3, n.s. (U.S. Dept. Agr.).

FIG. 3.--Thorax of Saw-Fly (_Pachynematus_).

I, Dorsal view. II, Ventral view. III, Lateral view. IV, Lateral view with segments separated. _Prothorax_: a, Episternum. b, Sternum. c, Coxa of fore-leg. d, Pronotum. _Mesothorax_: e, Prescutum. f, Scutum. g, Scutellum. h, Post-scutellum. i, Mesophragma. j, _Epimeron_. k, _Episternum_. l, Coxa of middle leg. _Metathorax_: m, Scutum. o, Epimeron. p, Coxa of hind leg. n, _First Abdominal Segment_. t, Tegula at base of fore-wing.]

[Illustration: After Miall and Denny, _The Cockroach_, Lovell Reeve & Co.

FIG. 4.--Legs and Ventral Thoracic Sclerites of Female Cockroach

(_Blatta_).

I, Fore-leg and pro-sternum (S) in front of which are the ventral cervical sclerites (c). cx, Coxa. tr, Trochanter. fe, Thigh. tb, Shin. ta, Tarsal segments. II, Middle leg and mesosternum. III, Hind-leg and metasternum. In IIIA, the episternum (a) and epimeron (b) are slightly separated.]

_Wings._--Two pairs of wings are present in the vast majority of insects, borne respectively on the mesothorax and metathorax. At the base of the wing, i.e. its attachment to the trunk, we find a highly complex series of small sclerites adapted for the varied movements necessary for flight. Those of the dragon-flies (Odonata) have been described in detail by R. von Lendenfeld (1881). The long axis of the wings, when at rest, lies parallel to the body axis. In this position the outer margin of the wing is the _costa_, the inner the _dorsum_, and the hind-margin the _termen_. The angle between the costa and termen is the _apex_. When the wing is spread, its long axis is more or less at a right angle to the body axis. A wing is an outgrowth from the dorsal and pleural regions of the thoracic segment that bears it, and microscopic examination shows it to consist of a double layer of cuticularized skin, the two layers being in contact except where they are thickened and folded to form the firm tubular nervures, which serve as a supporting framework for the wing membrane, enclose air-tubes, and convey blood. These nervures consist of a series of trunks radiating from the wing-base and usually branching as they approach the wing-margins, the branches being often connected by short transverse nervures, so that the wing-area is marked off into a number of "cells" or areolets.

[Illustration: After Quail, _Natural Science_, vol. xiii., J. M. Dent & Co.

FIG. 5.--Wing-Neuration in a Cossid Moth. 2, sub-costal; 3, radial; 4, median; 5, cubital; 6, 7, 8, anal nervures.]

The details of the nervuration vary greatly in the different orders, but J. H. Comstock and J. G. Needham have lately (1898-1899) shown that a common arrangement underlies all, six series of longitudinal or radiating nervures being present in the typical wing (see fig. 5). Along the costa runs a costal nervure. This is followed by a sub-costal which sometimes shows two main branches. Then comes the radial--usually the most important nervure of the wing--typically with five branches, and the median with four. These sets arise from a main trunk towards the front region of the wing-base. From another hinder trunk arise the two-branched cubital nervure and three separate anal nervures. In the hind-wing of many insects the number of radial branches becomes reduced, while the anal area is especially well developed and undergoes a fan-like folding when the wings are closed. Great diversity exists in the texture and functions of fore and hind-wings in different insects; these differences are discussed in the descriptions of the various orders. The wings often afford secondary sexual characters, being not infrequently absent or reduced in the female when well developed in the male (see fig. 6). Rarely the male is the wingless sex.

In addition to the wings there are smaller dorsal outgrowths of the thorax in many insects. Paired erectile plates (patagia) are borne on the prothorax in moths, while in moths, sawflies, wasps, bees and other insects there are small plates (tegulae)--see Fig. 3, t--on the mesothorax at the base of the fore-wings.

_Abdomen._--In the abdominal exoskeleton the segmental structure is very clearly marked, a series of sclerites--dorsal terga and abdominal sterna--being connected by pale, feebly chitinized cuticle, so that considerable freedom of movement between the segments is possible. The first and second abdominal sterna are often suppressed or reduced, on account of the strong development of the hind-legs. In many insects ten, and in a few eleven, abdominal segments can be clearly distinguished in addition to a small terminal anal segment. The female genital opening usually lies between the seventh and eighth segments, the male on the ninth. Prominent paired limbs are often borne on the tenth segment, the elongate tail-feelers (cerci) of bristle-tails and may-flies, or the forceps of earwigs, for example. In the Embiidae, a family of Isoptera, it has been shown by G. Enderlein (1901) that these cerci clearly belong to a partially suppressed eleventh segment, and R. Heymons (1895-1896) has proved by embryological study that in all cases they really belong to this eleventh segment, which in the course of development becomes fused with the tenth. Smaller appendages (such as the stylets of male cockroaches) may be carried on the ninth segment. Pairs of processes carried on the eighth and ninth segments often become specialized to form the ovipositor of the female (see fig. 14) and the genital armature of the male. A marked modification of the hinder abdominal segments may be noticed in most insects, the sclerites of the eighth and ninth being frequently hidden by those of the seventh. In the higher orders several of the hinder segments may be altogether suppressed.

[Illustration: From Miall and Denny, _The Cockroach_, Lovell Reeve & Co.

FIG. 6.--Outline of Male ([Male sign]) and Female ([Female sign]) Cockroaches (_Blatta_) from the side, showing Abdominal Segments (numbered 1-10).]

[Illustration: From Miall and Denny (after Newton), _The Cockroach_, Lovell Reeve & Co.

FIG. 7.--Brain of Cockroach from side. oe, Gullet; op, optic nerve; sb, sub-oesophageal ganglion; mn, mx, mx', nerves to jaws; t, tentorium.]

INTERNAL ORGANS

_Nervous System._--The nervous system in the Hexapoda is built up on the typical arthropodan plan of a double ventral nerve-cord with a pair of ganglia in each segment, the cords passing on either side of the gullet and connecting with an anterior nerve-centre or brain (fig. 7) in the head. The brain innervates the eyes and feelers, and must be regarded as a "syncerebrum" representing the ganglia of the three foremost limb-bearing somites united with the primitive cephalic lobes. Behind the gullet lies the sub-oesophageal nerve-centre (fig. 7, sb), composed of the ganglia of the four hinder head-somites and sending nerves to the jaws. A pair of ganglia in each thoracic segment is usual (fig. 8), and as many as eight distinct pairs of abdominal ganglia may often be distinguished, the hindmost of which represents the fused ganglia of the last four segments. But in many highly organized insects a remarkable concentration of the trunk-ganglia takes place, all the nerve-centres of the thorax and abdomen in the chafers and in the Hemiptera, for instance, being represented by a single mass situated in the thorax. The legs, wings and other organs of the trunk receive their nerves from the thoracic and abdominal ganglia, and the fusion of several pairs of these ganglia may be regarded as corresponding to a centralization of individuality. A special "sympathetic" system arises by paired nerves from the oesophageal connectives; these nerves unite, and send back a median recurrent nerve associated with ganglia on the gullet and crop, whence proceed cords to various parts of the digestive system.

In connexion with the central nervous system there are usually numerous organs of special sense. Most insects possess a pair of compound eyes, and many have, in addition, three simple eyes or ocelli on the vertex. The nature of these organs is described in the article ARTHROPODA. The surface of a compound eye is seen to be covered with a large number of hexagonal corneal facets, each of which overlies an ommatidium or series of cell elements (fig. 9, A, B). There are over 25,000 ommatidia in the eye of a hawk moth.

[Illustration: After Miall and Denny, _The Cockroach_, Lovell Reeve & Co.

FIG. 8.--Ventral Muscles and Nerve Cord of Cockroach.]

Auditory organs of a simple type are present in most insects. These consist of fine rods suspended between two points of the cuticle, and connected with nerve-fibres; they are known as chordotonal organs. In many cases a more complex ear is developed, which may be situated in strangely diverse regions of the insect's body. In locusts (_Acridiidae_) a large ovate, tympanic membrane (fig. 9, G) is conspicuous on either side of the first abdominal segment; on the inner surface of this membrane are two horn-like processes in contact with a delicate sac containing fluid, connected with which are the actual nerve-endings. In the nearly-related crickets and long-horned grasshoppers (_Locustidae_) the ears are situated in the shins of the fore-legs (see fig. 9, F). Just below the knee-joint there is a swelling, along which two narrow slits run lengthwise. They lead into chambers, formed by inpushing of the cuticle, whose delicate inner walls are in contact with air-tubes; on the outer surface of these latter are ridges, along which the special nerve-endings are arranged. An ear of another type is found in the swollen second segment of the feeler in many male gnats and midges, the cuticle between this segment and the third forming an annular drum which is connected with numerous nerve-endings, while the fine bristles on the more distal segments vibrate in response to the note produced by the humming of the female.

[Illustration: From Ridley, _Insect Life_, vol. 7 (U.S. Dept. Agr.).

FIG. 9.--Single Ommatidium of Cockroach's Eye (after Grenacher). B, Section through compound eye (after Miall and Denny); C, organs of smell in cockchafer (after Kraepelin); D, a, b, sensory pits on cercopods of golden-eye fly; c, sensory pit on palp of stone-fly (after Packard); E, sensory hair (after Miall and Denny); F, ear of long-horned grasshopper; a, Front shin showing outer opening and air-tube; b, section (after Graber); G, ear of locust from within (after Graber). All highly magnified.]

Many of the numerous hairs (fig. 9, E) that cover the body of an insect have a tactile function. The sense of smell resides chiefly in the feelers, on whose segments occur tiny pits, often guarded by peg-like or tooth-like structures and containing rod-like cells (fig. 9, C) in connexion with large nerve-cells. It is said that 13,000 such olfactory organs are present on the feeler of a wasp, and 40,000 on the complex antennae of a male cockchafer. Organs of similar type on the maxillae and epipharynx appear to exercise the function of taste.

[Illustration: After Miall and Denny, _The Cockroach_, Lovell Reeve & Co.

FIG. 10.--Dorsal Muscles, Heart and Pericardial Tendons of Cockroach.]

_Muscular System._--The muscles in the Hexapoda are striated, as in Arthropods generally, the large fibres being associated in bundles which are attached from point to point of the cuticle, so as to move adjacent sclerites with respect to one another (see figs. 8, 10). For example, the contraction of the tergo-sternal muscles, connecting the dorsal with the ventral sclerites of the abdomen, lessens the capacity of the abdominal region, while the contraction of the powerful muscles arising from the thoracic walls, and inserted into the proximal ends of the thighs, flexes or extends the legs.