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    "source_key": "britannica_1911",
    "source_title": "Encyclopaedia Britannica (1911)",
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    "chunk_id": "1911:exoskeleton:7c1682ace934",
    "title": "EXOSKELETON",
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    "verified_text": "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. _circulatory system._--insects afford an excellent illustration of the remarkable type of blood-system characterizing the arthropoda. the dorsal vessel is an elongate tube, whose abdominal portion is usually chambered, forming a contractile heart (fig. 10). at the constrictions between the chambers are paired slits, through which the blood passes from the surrounding pericardial sinus. the dorsal vessel is prolonged anteriorly into an aorta, through which the blood is propelled into the great body-cavity or haemocoel. after bathing the various tissues and organs, the blood returns dorsalwards into the pericardial sinus through fine perforations of its floor, and so makes its way into the heart again. some water-bugs, e.g. of the families _belostomatidae_, _nepidae_, _corixidae_ and _hydrometridae_ have a pulsating sac at each knee-joint to assist the flow of blood through the legs, while in dragon-flies and locusts (_acridiidae_) there is a ventral pulsating diaphragm, which forms the roof of a sinus enclosing the nerve-cords. [illustration: after miall and denny, _the cockroach_, lovell reeve & co. fig. 11.--ventral portion of air-tubes in cockroach.] _respiratory system._--as mentioned above, respiration by means of air-tubes (tracheae) is a most characteristic feature of the hexapoda. an air-tube consists of an epithelium of large polygonal cells with a thin basement-membrane externally and a chitinous layer internally, the last-named being continuous with the outer cuticle. the chitinous layer is usually strengthened by thread-like thickenings which, in the region close to the outer opening of the tube, form a network enclosing polygonal areas, but which, through most of the tracheal system, are arranged spirally, the strengthening thread not forming a continuous spiral, but being interrupted after a few turns around the tube. the tracheal system in hexapods is very complex, forming a series of longitudinal trunks with transverse anastomosing connexions (fig. 11), and extending by the finest sub-division and by repeated branching into all parts of the body. in insects of active flight the tubes swell out into numerous air-sacs, by which the breathing capacity is much increased. atmospheric air gains access to the air-tubes through paired _spiracles_ or _stigmata_, which usually occur laterally on most of the body-segments. these spiracles have firm chitinous edges, and can be closed by valves moved by special muscles. when the spiracles are open and the body contracts, air is expired. the subsequent expansion of the body causes fresh air to enter the tracheal system, and if the spiracles be then closed and the body again contracted, this air is driven to the finest branches of the air-tubes, where a direct oxygenation of the tissues takes place. the physiology of respiration has been carefully studied by f. plateau (1884). in aquatic insects various devices for obtaining or entangling air are found; these modifications are described in the special articles on the various orders of insects (coleoptera, hemiptera, &c.). many insects have aquatic larvae, some of which take in atmospheric air at intervals, while others breathe dissolved air by means of tracheal gills. these modifications are mentioned below in the section on metamorphosis. [illustration: from miall and denny, _the cockroach_, lovell reeve & co. fig. 12.--food canal of cockroach. s, salivary glands and reservoir. c, crop (the gizzard below it). coe, caecal tubes (below them the stomach). k, kidney tubes. i, intestine. r, rectum.] _digestive system._--a striking feature in the food-canal of the hexapoda, as in other arthropods, is the great extent of the \"fore-gut\" and \"hind-gut,\" lined with a chitinous cuticle, continuous with the exoskeleton. the fore-gut is composed of a tubular gullet, a large sac-like crop (fig. 12, c) and a proventriculus or \"gizzard,\" whose function is to strain the food-substances before they pass on into the tubular stomach, which has no chitinous lining. this organ, usually regarded as a \"mid-gut,\" gives off a number of secretory caecal tubes (fig. 12, coe). at its hinder end it is continuous with the hind-gut, which is usually differentiated into a tubular coiled intestine (fig. 12, i) and a swollen rectum (fig. 12, r). from the fore-end of the hind-gut arise the slender malpighian tubes (fig. 12, k), which have a renal function. on either side of the gullet are from one to ten pairs of salivary glands (fig. 12, s) whose ducts open into the mouth. some of these glands may be modified for special purposes--as silk-producing glands in caterpillars or as poison-glands in blood-sucking flies and bugs. the food passing into the crop is there acted on by the saliva and also by an acid gastric juice which passes forwards from the stomach through the proventriculus. as the various portions of the food undergo digestion, they are allowed to pass through the proventriculus into the stomach, where the nutrient substances are absorbed. _excretory system._--nitrogenous waste-matter is removed from the body by the malpighian tubes which open into the food-canal, usually where the hind-gut joins the stomach. these tubes vary in number from four to over a hundred in different orders of insects. the cells which line them and also the cavities of the tubes contain urates, which are excreted from the blood in the surrounding body-cavity. this cavity contains an irregular mass of whitish tissue, the fat-body, consisting of fat-cells which undergo degradation and become more or less filled with urates. when the worn-out cells are broken down, the urates are carried dissolved in the blood to the malpighian tubes for excretion. the fat-body is therefore the seat of important metabolic processes in the hexapod body. _reproductive system._--all the hexapoda are of separate sexes. the ovaries (fig. 13) in the female are paired, each ovary consisting of a variable number of tubes (one in the bristle-tail _campodea_ and fifteen hundred in a queen termite) in which the eggs are developed. from each ovary an oviduct (fig. 13, od) leads, and in some of the more primitive insects (bristle-tails, earwigs, may-flies) the two oviducts open separately direct to the exterior. usually they open into a median vagina, formed by an ectodermal inpushing and lined with chitin. the vagina usually opens in front of the eighth abdominal sternite. behind it is situated a spermatheca (fig. 14, sp) and the ovipositor previously mentioned, with its three pairs of processes (fig. 14, g, g). [illustration: from miall and denny, _the cockroach_, lovell reeve & co. fig. 13.--ovaries of cockroach, with oviducts od and colleterial glands cg.] [illustration: from miall and denny, _the cockroach_, lovell reeve & co. fig. 14.--hinder abdominal segment and ovipositor of female cockroach. magnified. t^8 &c. tergites. s^7, 7th sternite. s^8, sclerite between 7th and 8th sterna. s^9, 8th sclerite. od, vagina. sp, spermatheca. g, anterior, and g, posterior gonapophyses.] the paired testes of the male consist of a variable number of seminal tubes, those of each testis opening into a _vas deferens_. in some bristle-tails and may-flies, the two _vasa deferentia_ open separately, but usually they lead into a sperm-reservoir, whence issues a median ejaculatory duet. the male opening is on the ninth abdominal segment, to which belong the processes that form the claspers or genital armature. accessory glands are commonly present in connexion both with the male and the female reproductive organs. the poison-glands of the sting in wasps and bees are well-known examples of these. embryology _the egg._--among the hexapoda, as in arthropods generally, the egg is large, containing an accumulation of yolk for the nourishment of the growing embryo. most insect eggs are of an elongate oval shape; some are globular, others flattened, while others again are flask-shaped, and the outer envelope (_chorion_) is often beautifully sculptured (figs. 20, d; 21, a, b). various devices are adopted for the protection of the eggs from mechanical injury or from the attacks of enemies, and for fixing them in appropriate situations. for example, the egg may be raised above the surface on which it is laid by an elongate stalk; the eggs may be protected by a secretion, which in some cases forms a hard protective capsule or \"purse\"; or they may be covered with shed hairs of the mother, while among water-insects a gelatinous envelope, often of rope-like form, is common. in various groups of the hexapoda--aphids and some flesh-flies (_sarcophaga_), for example--the egg undergoes development within the body of the mother, and the young insect is born in an active state; such insects are said to be \"viviparous.\" _parthenogenesis._--a number of cases are known among the hexapoda of the development of young from the eggs of virgin females. in insects so widely separated as bristle-tails and moths this occurs occasionally. in certain gall-flies (_cynipidae_) no males are known to exist at all, and the species seems to be preserved entirely by successive parthenogenetic generations. in other gall-flies and in aphids we find that a sexual generation alternates with one or with many virgin generations. the offspring of the virgin females are in most of these instances females; but among the bees and wasps parthenogenesis occurs normally and always results in the development of males, the \"queen\" insect laying either a fertilized or unfertilized egg at will. _maturation, fertilization and segmentation._--polar bodies were first observed in the eggs of hexapoda by f. blochmann in 1887. the two nuclei are successively divided from the egg nucleus in the usual way, but they frequently become absorbed in the peripheral protoplasm instead of being extruded from the egg-cell altogether. it appears that in parthenogenetic eggs two polar nuclei are formed. according to a. petrunkevich (1901-1903), the second polar nucleus uniting with one daughter-nucleus of the first polar body gives rise to the germ-cells of the parthenogenetically-produced male. there is no reunion of the second polar nucleus with the female pronucleus, but, according to the recent work of l. doncaster (1906-1907) on the eggs of sawflies, the number of chromosomes is not reduced in parthenogenetic egg-nuclei, while, in eggs capable of fertilization, the usual reduction-divisions occur. fertilization takes place as the egg is laid, the spermatozoa being ejected from the spermatheca of the female and making their way to the protoplasm of the egg through openings (micropyles) in its firm envelope. the segmentation of the fertilized nucleus results in the formation of a number of nuclei which arrange themselves around the periphery of the egg and, the protoplasm surrounding them becoming constricted, a blastoderm or layer of cells, enclosing the central yolk, is formed. within the yolk the nuclei of some \"yolk cells\" can be distinguished. [illustration: from nussbaum in miall and denny's, _cockroach_, lovell, reeve & co. fig. 15.--diagram showing formation of germinal layers. e, ectoderm; m, inner layer. magnified.] _germinal layers and food-canal._--the embryo begins to develop as an elongate, thickened, ventral region of the blastoderm which is known as the ventral plate or germ band. along this band a median furrow appears, and a mass of cells sinks within, the one-layered germ band thus becoming transformed into a band of two cell-layers (fig. 15). in some cases the inner layer is formed not by invagination but by proliferation or by delamination. the outer of these two layers (fig. 15, e) is the ectoderm. with regard to the inner layer (_endoblast_ of some authors, fig. 15, m) much difference of opinion has prevailed. it has usually been regarded as representing both endoderm and mesoderm, and the groove which usually leads to its formation has been compared to the abnormally elongated blastopore of a typical gastrula. no doubt can be entertained that the greater part of the inner layer corresponds to the mesoderm of more ordinary embryos, for the coelomic pouches, the germ-cells, the musculature and the vascular system all arise from it. further, there is general agreement that the chitin-lined fore-gut and hind-gut, which form the greater part of the digestive tract, arise from ectodermal invaginations (stomodaeum and proctodaeum respectively) at the positions of the future mouth and anus. the origin of the mid-gut (mesenteron), that has no chitinous lining in the developed insect, is the disputed point. according to the classical researches of a. kowalevsky (1871 and 1887) on the embryology of the water-beetle _hydrophilus_ and of the muscid flies, an anterior and a posterior endoderm-rudiment both derived from the \"endoblast\" become apparent at an early stage, in close association with the stomodaeum and the proctodaeum respectively. these two endoderm-rudiments ultimately grow together and give rise to the epithelium of the mid-gut. these results were confirmed by the observations of k. heider and w. m. wheeler (1889) on the embryos of two beetles--_hydrophilus_ and _doryphora_ respectively. v. graber, however (1889), stated that in the _muscidae_, while the anterior endoderm-rudiment arises as kowalevsky had observed, the posterior part of the \"mid-gut\" has its origin as a direct outgrowth from the proctodaeum. the recent researches of r. heymons (1895) on the orthoptera, and of a. lecaillon (1898) on various leaf beetles, tend to show that the whole of the \"mid-gut\" arises from the proliferation of cells at the extremity of the stomodaeum and of the proctodaeum. on this view the entire food-canal in most hexapoda must be regarded as of ectodermal origin, the \"endoblast\" represents mesoderm only, and the median furrow whence it arises can be no longer compared with the blastopore. according to heymons, the yolk-cells must be regarded as the true endoderm in the hexapod embryo, for he states (1897) that in the bristle-tail _lepisma_ and in dragon-flies they give rise to the mid-gut. these views are not, however, supported by other recent observers. j. carriere's researches (1897) on the embryology of the mason bee (_chalicodoma_) agree entirely with the interpretations of kowalevsky and heider, and so on the whole do those of f. schwangart, who has studied (1904) the embryonic development of lepidoptera. he finds that the endoderm arises from an anterior and a posterior rudiment derived from the \"endoblast,\" that many of the cells of these rudiments wander into the yolk, and that the mesenteric epithelium becomes reinforced by cells that migrate from the yolk. k. escherich (1901), after a new research on the embryology of the muscid diptera, claims that the fore and hind endodermal rudiments arise from the blastoderm by invagination, and are from their origin distinct from the mesoderm. on the whole it seems likely that the endoderm is represented in part by the yolk, and in part by those anterior and posterior rudiments which usually form the mesenteron, but that in some hexapoda the whole digestive tract may be ectodermal. it must be admitted that some or the later work on insect embryology has justified the growing scepticism in the universal applicability of the \"germ-layer theory.\" heider has suggested, however, that the apparent origin of the mid-gut from the stomodaeum and proctodaeum may be explained by the presence of a \"latent endoderm-group\" in those invaginations. [illustration: from nussbaum in miall and denny, _the cockroach_, lovell reeve & co. fig. 16.--cross section of embryo of german cockroach (_phyllodromia_). s, serosa; a, amnion; e, ectoderm; n, rudiment of nerve-cord; m, mesodermal pouches.] _embryonic membranes._--a remarkable feature in the embryonic development of most hexapoda is the formation of a protective membrane analogous to the amnion of higher vertebrates and known by the same term. usually there arises around the edge of the germ band a double fold in the undifferentiated blastoderm, which grows over the surface of the embryo, so that its inner and outer layers become continuous, forming respectively the _amnion_ and the _serosa_ (fig. 16, a, s). the embryo of a moth, a dragon-fly or a bug is invaginated into the yolk at the head end, the portion of the blastoderm necessarily pushed in with it forming the amnion. the embryo thus becomes transferred to the dorsal face of the egg, but at a later stage it undergoes reversion to its original ventral position. in some parasitic hymenoptera there is only a single embryonic membrane formed by delamination from the blastoderm, while in a few insects, including the wingless spring-tails, the embryonic membranes are vestigial or entirely wanting. in the bristle-tails _lepisma_ and _machilis_, an interesting transitional condition of the embryonic membranes has lately been shown by heymons. the embryo is invaginated into the yolk, but the surface edges of the blastoderm do not close over, so that a groove or pore puts the insunken space that represents the amniotic cavity into communication with the outside. heymons believes that the \"dorsal organ\" in the embryos of the lower arthropoda corresponds with the region invaginated to form the serosa of the hexapod embryo. wheeler, however, compares with the \"dorsal organ\" the peculiar extra embryonic membrane or indusium which he has observed between serosa and amnion in the embryo of the grasshopper _xiphidium_. _metameric segmentation._--the segments are perceptible at a very early stage of the development as a number of transverse bands arranged in a linear sequence. the first segmentation of the ventral plate is not, however, very definite, and the segmentation does not make its appearance simultaneously throughout the whole length of the plate; the anterior parts are segmented before the posterior. in orthoptera and thysanura, as well as some others of the lower insects, twenty-one of these divisions--not, however, all similar--may be readily distinguished, six of which subsequently enter into the formation of the head, three going to the thorax and twelve to the abdomen. in hemiptera only eleven and in collembola only six abdominal segments have been detected. the first and last of these twenty-one divisions are so different from the others that they can scarcely be considered true segments. _head segments._--in the adult insect the head is insignificant in size compared with the thorax or abdomen, but in the embryo it forms a much larger portion of the body than it does in the adult. its composition has been the subject of prolonged difference of opinion. formerly it was said that the head consisted of four divisions, viz. three segments and the procephalic or prae-oral lobes. it is now ascertained that the procephalic lobes consist of three divisions, so that the head must certainly be formed from at least six segments. the first of these, according to the nomenclature of heymons (see fig. 17), is the mouth or oral piece; the second, the antennal segment; the third, the intercalary or prae-mandibular segment; while the fourth, fifth, and sixth are respectively the segments of the mandibles and of the first and second maxillae. these six divisions of the head are diverse in kind, and subsequently undergo so much change that the part each of them takes in the formation of the head-capsule is not finally determined. the labrum and clypeus are developed as a single prolongation of the oral piece, not as a pair of appendages. the antennal segment apparently entirely disappears, with the exception of a pair of appendages it bears; these become the antennae; it is possible that the original segment, or some part of it, may even become a portion of the actual antennae. the intercalary segment has no appendages, nor rudiments thereof, except, according to h. uzel (1897), in the thysanuran _campodea_, and probably entirely disappears, though j. h. comstock and c. kochi believe that the labrum belongs to it. the appendages of the posterior three or trophal segments become the parts of the mouth. the appendages of the two maxillary segments arise as treble instead of single projections, thus differing from other appendages. from these facts it appears that the anterior three divisions of the head differ strongly from the posterior three, which greatly resemble thoracic segments; hence it has been thought possible that the anterior divisions may represent a primitive head, to which three segments and their leg-like appendages were subsequently added to form the head as it now exists. this is, however, very doubtful, and an entirely different inference is possible. besides the five limb-bearing somites just enumerated, two others must now be recognized in the head. one of these is the ocular segment, in front of the antennal, and behind the primitive pre-oral segment. the other is the segment of the maxillulae (see above, under _jaws_), behind the mandibular somite; the presence of this in the embryo of the collembolan _anurida_ has been lately shown (1900) by j. w. folsom (fig. 18, v. 5), who terms the maxillulae \"superlinguae\" on account of their close association with the hypopharynx or lingua. in reference to the structure of the head-capsule in the imago, it appears that the clypeus and labrum represent, as already said, an unpaired median outgrowth of the oral piece. according to w. a. riley (1904) the epicranium or \"vertex,\" the compound eyes and the front divisions of the genae are formed by the cephalic lobes of the embryo (belonging to the ocular segment), while the mandibular and maxillary segments form the hinder parts of the genae and the hypopharynx. [illustration: after heymons. fig. 17.--morphology of an insect: the embryo of _gryllotalpa_, somewhat diagrammatic. the longitudinal segmented band along the middle line represents the early segmentation of the nervous system and the subsequent median field of each sternite; the lateral transverse unshaded bands are the lateral fields of each segment; the shaded areas indicate the more internally placed mesoderm layer. the segments are numbered 1-21; 1-6 will form the head, 7-9 the thorax, 10-21 the abdomen. a, anus; abx1 abx11, appendage of 1st and of 11th abdominal segments; ans, anal piece = telson or 12th abdominal segment; ant, antenna; de, deuterencephalon; md, mandible; mx1, first maxilla; mx2, second maxilla or labium; o, mouth; obcl, rudimentary labrum and clypeus; pre, protencephalon; st1 st10, stigmata 1 and 10; terg, tergite; thx1, appendage of first thoracic segment; tre, tritencephalon; ul, a thickening at hinder margin of the mouth.] great difference of opinion exists as to the hypopharynx, which has even been thought to represent a distinct segment, or the pair of appendages of a distinct segment. heymons considers that it represents the sternites of the three trophal segments, and that the gula is merely a secondary development. folsom looks on the hypopharynx as a secondary development. riley holds that the hypopharynx belongs to the mandibular and maxillary segments, while the cervical sclerites or gula represent the sternum of the labial segment. the ganglia of the nervous system offer some important evidence as to the morphology of the head, and are alluded to below. _thoracic segments._--these are always three in number. the three pairs of legs appear very early as rudiments. though the thoracic segments bear the wings, no trace of these appendages exists till the close of the embryonic life, nor even, in many cases, till much later. the thoracic segments, as seen in an early stage of the ventral plate, display in a well-marked manner the essential elements of the insect segment. these elements are a central piece or sternite, and a lateral field on each side bearing the leg-rudiment. the external part of the lateral field subsequently grows up, and by coalescence with its fellow forms the tergite or dorsal part of the segment. _abdominal segments and appendages._--we have already seen that in numerous lower insects the abdomen is formed from twelve divisions placed in linear fashion. eleven of these may perhaps be considered as true segments, but the twelfth or terminal one is different, and is called by heymons a telson; in it is placed the anal orifice, and the mass subsequently becomes the upper and lower laminae anales. in hemiptera this telson is absent, and the anal orifice is placed quite at the termination of the eleventh segment. moreover, in this order the abdomen shows at first a division into only nine segments and a terminal mass, which last subsequently becomes divided into two. the appendages of the abdomen are called cerci, stylets and gonapophyses. they differ much according to the kind of insect, and in the adult according to sex. difference of opinion as to the nature of the abdominal appendages prevails. the cerci, when present, appear in the mature insect to be attached to the tenth segment, but according to heymons they are really appendages of the eleventh segment, their connexion with the tenth being secondary and the result of considerable changes that take place in the terminal segments. it has been disputed whether any true cerci exist in the higher insects, but they are probably represented in the diptera and in the scorpion-flies (mecaptera). in those insects in which a median terminal appendage exists between the two cerci this is considered to be a prolongation of the eleventh tergite. the stylets, when present, are placed on the ninth segment, and in some thysanura exist also on the eighth segment; their development takes place later in life than that of the cerci. the gonapophyses are the projections near the extremity of the body that surround the sexual orifices, and vary extremely according to the kind of insect. they have chiefly been studied in the female, and form the sting and ovipositor, organs peculiar to this sex. they are developed on the ventral surface of the body and are six in number, one pair arising from the eighth ventral plate and two pairs from the ninth. this has been found to be the case in insects so widely different as orthoptera and aculeate hymenoptera. the genital armature of the male is formed to a considerable extent by modifications of the segments themselves. the development of the armature has been little studied, and the question whether there may be present gonapophyses homologous with those of the female is open. [illustration: a. after wheeler, _journ. morph._ vol. viii., and folsom, _bull. mus. harvard_, xxxvi. b. after folsom. fig. 18.--embryos of springtail (_anuridamaritima_). magnified. a, head-region of germ band. b, section through head and thorax. the neuromeres are shown in arabic, the appendages in roman numerals. 1, ocular segment. 2, antennal. 3, trito-cerebral. 4, mandibular. 5, maxillular. 6, maxillary. 7, labial. 8, prothoracic. 9, mesothoracic. 10, metathoracic.] in the adult state no insect possesses more than six legs, and they are always attached to the thorax; in many thysanura there are, however, processes on the abdomen that, as to their position, are similar to legs. in the embryos of many insects there are projections from the segments of the abdomen similar, to a considerable extent, to the rudimentary thoracic legs. the question whether these projections can be considered an indication of former polypody in insects has been raised. they do not long persist in the embryo, but disappear, and the area each one occupied becomes part of the sternite. in some embryos there is but a single pair of these rudiments (or vestiges) situate on the first abdominal segment, and in some cases they become invaginations of a glandular nature. whether cerci, stylets and gonapophyses are developed from these rudiments has been much debated. it appears that it is possible to accept cerci and stylets as modifications of the temporary pseudopods, but it is more difficult to believe that this is the case with the gonapophyses, for they apparently commence their development considerably later than cerci and stylets and only after the apparently complete disappearance of the embryonic pseudopods. the fact that there are two pairs of gonapophyses on the ninth abdominal segment would be fatal to the view that they are in any way homologous with legs, were it not that there is some evidence that the division into two pairs is secondary and incomplete. but another and apparently insuperable objection may be raised--that the appendages of the ninth segment are the stylets, and that the gonapophyses cannot therefore be appendicular. the pseudopods that exist on the abdomen of numerous caterpillars may possibly arise from the embryonic pseudopods, but this also is far from being established. _nervous system._--the nervous system is ectodermal in origin, and is developed and segmented to a large extent in connexion with the outer part of the body, so that it affords important evidence as to the segmentation thereof. the continuous layer of cells from which the nervous system is developed undergoes a segmentation analogous with that we have described as occurring in the ventral plate; there is thus formed a pair of contiguous ganglia for each segment of the body, but there is no ganglion for the telson. the ganglia become greatly changed in position during the later life, and it is usually said that there are only ten pairs of abdominal ganglia even in the embryo. in orthoptera, heymons has demonstrated the existence of eleven pairs, the terminal pair becoming, however, soon united with the tenth. the nervous system of the embryonic head exhibits three ganglionic masses, anterior to the thoracic ganglionic masses; these three masses subsequently amalgamate and form the sub-oesophageal ganglion, which supplies the trophal segments. in front of the three masses that will form the sub-oesophageal ganglion the mass of cells that is to form the nervous system is very large, and projects on each side; this anterior or \"brain\" mass consists of three lobes (the prot-, deut-, and tritencephalon of viallanes and others), each of which might be thought to represent a segmental ganglion. but the protocerebrum contains the ganglia of the ocular segment in addition to those of the procephalic lobes. these three divisions subsequently form the supra-oesophageal ganglion or brain proper. there are other ganglia in addition to those of the ventral chain, and janet supposes that the ganglia of the sympathetic system indicate the existence of three anterior head-segments; the remains of the segments themselves are, in accordance with this view, to be sought in the stomodaeum. folsom has detected in the embryo of _anurida_ a pair of ganglia (fig. 18, 5) belonging to the maxillular (or superlingual) segment, thus establishing seven sets of cephalic ganglia, and supporting his view as to the composition of the head. _air-tubes._--the air-tubes, like the food-canal, are formed by invaginations of the ectoderm, which arise close to the developing appendages, the rudimentary spiracles appearing soon after the budding limbs. the pits leading from these lengthen into tubes, and undergo repeated branching as development proceeds. _dorsal closure._--the germ band evidently marks the ventral aspect of the developing insect, whose body must be completed by the extension of the embryo so as to enclose the yolk dorsally. the method of this dorsal closure varies in different insects. in the colorado beetle (_doryphora_), whose development has been studied by w. m. wheeler, the amnion is ruptured and turned back from covering the germ band, enclosing the yolk dorsally and becoming finally absorbed, as the ectoderm of the germ band itself spreads to form the dorsal wall. in some midges and in caddis-flies the serosa becomes ruptured and absorbed, while the germ band, still clothed with the amnion, grows around the yolk. in moths and certain saw-flies there is no rupture of the membranes; the russian zoologists tichomirov and kovalevsky have described the growth of both amnion and embryonic ectoderm around the yolk, the embryo being thus completely enclosed until hatching time by both amnion and serosa. v. graber has described a similar method of dorsal closure in the saw-fly _hylotoma_. [illustration: after heymons, _zeit. wiss. zoolog._ vol. 53. fig. 19.--cross sections through abdomen of german cockroach embryo. a (later than fig. 16) magnified. b (still more advanced, dorsal closure complete) magnified. ec, ectoderm. en, endoderm. sp, splanchnic layer of mesoderm. y, yolk. h, heart. p, pericardial septum. c, coelom. g, germ-cells surrounded by rudiment-cells of ovarian tubes. m, muscle-rudiment. n, nerve-chain. f, fat body. s, inpushing of ectoderm to form air-tubes. x, secondary body-cavity.] _mesoderm, coelom and blood-system._--from the mesoderm most of the organs of the body--muscular, circulatory, reproductive--take their origin. the mass of cells undergoes segmentation corresponding with the outer segmentation of the embryo, and a pair of cavities--the coelomic pouches (fig. 16, m)--are formed in each segment. each coelomic pouch--as traced by heymons in his study on the development of the cockroach (_phyllodromia_)--divides into three parts, of which the most dorsal contains the primitive germ-cells, the median disappears, and the ventral loses its boundaries as it becomes filled up with the growing fat body (fig. 19). this latter, as well as the heart and the walls of the blood spaces, arises by the modification of mesodermal cells, and the body cavity is formed by the enlargement and coalescence of the blood channels and by the splitting of the fat body. it is therefore a haemocoel, the coelom of the developed insect being represented only by the cavities of the genital glands and their ducts. _reproductive organs._--in the cockroach embryo, before the segmentation of the germ-band has begun, the primitive germ-cells can be recognized at the hinder end of the mesoderm, from whose ordinary cells they can be distinguished by their larger size. at a later stage further germ-cells arise from the epithelium of the coelomic pouches from the second to the seventh abdominal segments, and become surrounded by other mesoderm cells which form the ovarian or testicular tubes and ducts (fig. 19, g). in the male of _phyllodromia_ the rudiment of a vestigial ovary becomes separated from the developing testis, indicating perhaps an originally hermaphrodite condition. an exceedingly early differentiation of the primitive germ-cells occurs in certain diptera. e. metchnikoff observed (1866) in the development of the parthenogenetic eggs produced by the precocious larva of the gall-midge _cecidomyia_ that a large \"polar-cell\" appeared at one extremity during the primitive cell-segmentation. this by successive divisions forms a group of four to eight cells, which subsequently pass through the blastoderm, and dividing into two groups become symmetrically arranged and surrounded by the rudiments of the ovarian tubes. e. g. balbiani and r. ritter (1890) have since observed a similar early origin for the germ-cells in the midge _chironomus_ and in the _aphidae_. the paired oviducts and vasa deferentia are, as we have seen, mesodermal in origin. the median vagina, spermatheca and ejaculatory duct are, on the other hand, formed by ectodermal inpushings. the classical researches of j. a. palmen (1884) on these ducts have shown that in may-flies and in female earwigs the paired mesodermal ducts open directly to the exterior, while in male earwigs there is a single mesodermal duct, due either to the coalescence of the two or to the suppression of one. in the absence of the external ectodermal ducts usual in winged insects, these two groups resemble therefore the primitive aptera. the presence of rudiments of the genital ducts of both sexes in the embryo of either sex is interesting and suggestive. the ejaculatory duct which opens on the ninth abdominal sternum in the adult male arises in the tenth abdominal embryonic segment and subsequently moves forward. growth and metamorphosis [illustration: after marlatt, _ent. bull._ 4, n. s. (u.s. dept. agr.). fig. 20.--a, bed-bug (_cimex lectularis_, linn.); newly hatched young from beneath; b, from above; d, egg, magnified; c, foot with claws; e, serrate spine, more highly magnified.] [illustration: from mally, _ent. bull._ 24 (u.s. dept. agr.). fig. 21.--e, f, owl moth (_heliothis armigera_); a, b, egg, highly magnified; c, larva or caterpillar; d, pupa in earthen cell.] after hatching or birth an insect undergoes a process of growth and change until the adult condition is reached. the varied details of this post-embryonic development furnish some of the most interesting facts and problems to the students of the hexapoda. wingless insects, such as spring-tails and lice, make their appearance in the form of miniature adults. some winged insects--cockroaches, bugs (fig. 20) and earwigs, for example--when young closely resemble their parents, except for the absence of wings. on the other hand, we find in the vast majority of the hexapoda a very marked difference between the perfect insect (imago) and the young animal when newly hatched and for some time after hatching. from the moth's egg comes a crawling caterpillar (fig. 21, c), from the fly's a legless maggot (fig. 25, a). such a young insect is a _larva_--a term used by zoologists for young animals generally that are decidedly unlike their parents. it is obvious that the hatching of the young as a larva necessitates a more or less profound transformation or metamorphosis before the perfect state is attained. usually this transformation comes with apparent suddenness, at the penultimate stage of the insect's life-history, when the passive pupa (fig. 21, d) is revealed, exhibiting the wings and other imaginal structures, which have been developed unseen beneath the cuticle of the larva. hexapoda with this resting pupal stage in their life-history are said to undergo \"a complete transformation,\" to be metabolic, or holometabolic, whereas those insects in which the young form resembles the parent are said to be ametabolic. such insects as dragon-flies and may-flies, whose young, though unlike the parent, develop into the adult form without a resting pupal stage are said to undergo an \"incomplete transformation\" or to be hemimetabolic. the absence of the pupal stage depends upon the fact that in the ametabolic and hemimetabolic hexapoda the wing-rudiments appear as lateral outgrowths (fig. 22) of the two hinder thoracic segments and are visible externally throughout the life-history, becoming larger after each moult or casting of the cuticle. hence, as has been pointed out by d. sharp (1898), the marked divergence among the hexapoda, as regards life-history, is between insects whose wings develop outside the cuticle (exopterygota) and those whose wings develop inside the cuticle (endopterygota), becoming visible only when the casting of the last larval cuticle reveals the pupa. metamorphosis among the hexapoda depends upon the universal acquisition of wings during post-embryonic development--no insect being hatched with the smallest external rudiments of those organs--and on the necessity for successive castings or \"moults\" (ecdyses) of the cuticle. [illustration: after howard, _insect life_, vol. vii. fig. 22.--nymph of locust (_schistocera americana_), showing wing-rudiments.] _ecdysis._--the embryonic ectoderm of an insect consists of a layer of cells forming a continuous structure, the orifices in it--mouth, spiracles, anus and terminal portions of the genital ducts--being invaginations of the outer wall. this cellular layer is called the hypodermis; it is protected externally by a cuticle, a layer of matter it itself excretes, or in the excretion of which it plays, at any rate, an important part. the cuticle is a dead substance, and is composed in large part of chitin. the cuticle contrasts strongly in its nature with the hypodermis it protects. it is different in its details in different insects and in different stages of the life of the same insect. the \"sclerites\" that make up the skeleton of the insect (which skeleton, it should be remembered, is entirely external) are composed of this chitinous excretion. the growth of an insect is usually rapid, and as the cuticle does not share therein, it is from time to time cast off by moulting or ecdysis. before a moult actually occurs the cuticle becomes separated from its connexion with the underlying hypodermis. concomitant with this separation there is commencement of the formation of a new cuticle within the old one, so that when the latter is cast off the insect appears with a partly completed new cuticle. the new instar--or temporary form--is often very different from the old one, and this is the essential fact of metamorphosis. metamorphosis is, from this point of view, the sum of the changes that take place under the cuticle of an insect between the ecdyses, which changes only become externally displayed when the cuticle is cast off. the hypodermis is the immediate agent in effecting the external changes. [illustration: adapted from koerschelt and herder and lowne. fig. 23.--diagram showing position of imaginal buds in larva of fly. i.,",
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