Obel

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.

Terg..

10

11

12

13

14

15

16

17

18

19

20

Ans21

After Heymons.

·Ab

A

Thoracic Segments. These are always three in number. The three pairs Mx, of legs appear very early as rudiments. Though the Ths, 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

Abx,

segment.

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.

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 FIG. 17.-Morphology of an Insect: anal orifice, and the mass the embryo of Gryllotalpa, somewhat subsequently becomes the diagrammatic. The longitudinal seg upper and lower laminae mented band along the middle line re- anales. In Hemiptera this presents the early segmentation of the telson is absent, and the nervous system and the subsequent anal orifice is placed quite median field of each sternite; the lateral at the termination of the transverse unshaded bands are the eleventh segment. Morelateral fields of each segment; the over, in this order the abshaded areas indicate the more inter- domen shows at first a nally placed mesoderm layer. The seg division into only nine segments are numbered 1-21; 1-6 will form ments and a terminal mass, the head, 7-9 the thorax, 10-21 the abdo- which last subsequently bemen. A, anus; Abx, Abx11, appendage comes divided into two. of 1st and of 11th abdominal segments; The appendages of the Ans, anal piece=telson or 12th abdo- abdomen are called cerci, minal segment; Ant, antenna; De stylets and gonapophyses. deuterencephalon; Md mandible They differ much according Mx, first maxilla; Mx, second to the kind of insect, and maxilla or labium; O, mouth; Obcl, in the adult according to rudimentary labrum and clypeus; sex. Difference of opinion Pre, protencephalon; Sh. So, stig- as to the nature of the mata 1 and 10: Terg, tergite; Thxi, abdominal appendages preappendage of first thoracic segment; vails. The cerci, when Tre, tritencephalon; Ul, a thickening present, appear in the at hinder margin of the mouth. mature insect to be attached to the tenth segment, but according to Heymons they are really appendages of the eleventh seg. ment, 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 scorpionflies (Mecaptera). In those insects in which a median terminal appendage exists between the two cerci this is considered to be a

جالب

B. After Folsom.

germ

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 símilar, 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 A. After Wheeler, Journ. their development considerably later Morph. vol. viii., and Folsom, than cerci and stylets and only after the Bull. Mus. Harvard, xxxvi. apparently complete disappearance of the FIG. 18.-Embryos of embryonic pseudopods. The fact that Springtail (Anurida marithere are two pairs of gonapophyses on tima). Magnified. A, the ninth abdominal segment would be Head-region of fatal to the view that they are in any way band. B, Section through homologous with legs, were it not that head and thorax. The there is some evidence that the division neuromeres are shown in into two pairs is secondary and incom- Arabic, the appendages plete. But another and apparently in- in Roman numerals. superable objection may be raised-that 1, Ocular segment. the appendages of the ninth segment are 2, Antennal. the stylets, and that the gonapophyses 3, Trito-cerebral. cannot therefore be appendicular. The 4. Mandibular. pseudopods that exist on the abdomen of 5. Maxillular. numerous caterpillars may possibly arise, Maxillary. from the embryonic pseudopods, but this 7, Labial. also is far from being established. 8, Prothoracic. Nervous System.-The nervous system is 9. Mesothoracic. ectodermal in origin, and is developed and 10, Metathoracic. 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.

Mesoderm, Coelom and Blood-System.-From the mesoderm most of the organs of the body-muscular, circulatory, reproductive

Cc.

P.

h

P

y

f

S

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)sp are formed in each segment. Each coelomic pouch-as traced by Heymons in his study -ec on the development of the cockroach (Phyllodromia)-divides into three parts, of which the most dorsal contains the primitive germ-cells, the median the disappears, and ventral loses its boundaries as it becomes filled up with the grow ing fat body (fig. 19). This latter, as well as the heart and the walls of the blood spaces, arises by the modification of

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. Yolk.

h, Heart.

mesodermal cells, and the body cavity is formed by the enlarge

g, Germ-cells surrounded by rudiment-cells ment and coalescence

m, Muscle-rudiment.

of ovarian tubes.

n, Nerve-chain.

f, Fat body.

of the blood channels and by the splitting of the fat body. It is therefore a haemocoel,

s, Inpushing of ectoderm to form air-tubes. the coelom of the de

x, Secondary body-cavity.

veloped 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 cellsegmentation. 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.

44

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. Palmén (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.

FIG. 20.-a, Bed-bug (Cimex lectularius, Linn.); newly hatched young from beneath; b, from above; d, egg, magnified; c, foot with claws; e, serrate spine, more highly magnified.

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 examplewhen 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).

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

After Howard, Insect Life, vol. vii.

generate.

h

The study of the physiology of ecdysis in its simpler forms has unfortunately been somewhat neglected, investigators having directed the transformation of a maggot into a fly, or of a caterpillar into a their attention chiefly to the cases that are most striking, such as butterfly. The changes have been found to be made up of two sets of processes histolysis, by which the whole or part of a structure disappears: and histogenesis, or the formation of the new structure. other portions of it develop into the new structures. The hypoBy histolysis certain parts of the hypodermis are destroyed, while dermis is composed of parts of two different kinds, viz. (1) the larger part of the hypodermis that exists in the maggot or caterpillar and is disthat remain comparatively quiescent solved at the metamorphosis; (2) parts previously, and that grow and develop when the other parts degenerate. These centres of renovation are called imaginal disks or folds. The adult caterpillar may be described as a creature the hypodermis of which is studded with Adapted from Koerschelt and buds that expand and form the butter- Herder, and Lowne. fly, while the parts around them de-. FIG. 23.-Diagram showmaggots of the blowfly, Calliphora in larva of fly. I., II., III., In some insects (e.g. the ing position of imaginal buds vomitoria) the imaginal disks are to all the three thoracic segments appearance completely separated from of the larva; 1, 2, 3, buds the hypodermis, with which they are, of the legs of the imago; h, by strings or pedicels. This connexion feeler; e, of eye; b, brain. however, really organically connected bud of head-lobes; f, of was not at first recognized and the true nature of imaginal disks was not at first perceived, even by Weismann, to whom their discovery in Diptera is due. In other insects the imaginal disks are less completely disconnected from the superficies of the larval hypodermis, and may indeed be merely patches thereof. The number of imaginal disks in an individual is large, upwards of sixty having been discovered to take part in the formation of the outer body of a fly. With regard to the internal organs, we need only say that transformation occurs in an essentially similar manner, by means of a development from centres distributed in the various organs. The imaginal disks for the outer wall of the body, some of them, at any rate, include mesodermal rudiments (from which the muscles are developed) as well as hypodermis. The imaginal disks make their appearance (that is, have been first detected) at very different epochs in the life; their absolute origin has been but little investigated. Pratt has traced them in the sheep-tick (Melophagus) to an early stage of the embryonic life.

rudiments.

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.

Ecdysis. The embryonic ectoderm of an insect consists of a layer of cells forming a continuous structure, the orifices in itmouth, 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 Jatter 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.

the tissues were attacked by phagocytic cells that became enlarged and carried away fragments of the tissue; the cells were subsequently identified as leucocytes or blood-cells. Hence the opinion arose that found that in other kinds of insects the tissues degenerate and break histolysis is a process of phagocytosis. It has, however, since been down without the intervention of phagocytes. It has, moreover, been noticed that even in cases where phagocytosis exists a greater or less extent of degeneration of the tissue may be observed before phagocytosis occurs. This process can therefore only be looked necessary to permit of the accompanying histogenesis. This view on as a secondary one that hastens and perfects the destruction is confirmed by the fate of the phagocytic cells. These do not take a direct part in the formation of the new tissue, but it is believed merely yield their surplus acquisitions, becoming ordinary blood-cells or disappearing altogether. As to the nature of histogenesis, nothing more can be said than that it appears to be a phenomenon similar to embryonic growth, though limited to certain spots. Hence we are inclined to look on the imaginal disks as cellular areas that possess exist in the embryo, powers that only become evident in certain in a latent condition the powers of growth and development that special conditions of the organism. What the more essential of these conditions may be is a question on which very little light has been thrown, though it has been widely discussed.

Much consideration has been given to the nature of metamorphosis in insects, to its value to the creatures and to the mode of its origin. Insect metamorphosis may be briefly described as phenomena of development characterized by abrupt changes of appearance and of structure, occurring during the period subsequent to embryonic development and antecedent to the reproductive state. It is, in short, a peculiar mode of growth and adolescence. The differences in appearance between the caterpillar and the butterfly, striking as they are to the eye, do not sufficiently represent the phenomena of metamorphosis to the intelligence. The changes that take place involve a revolution in the being, and may be summarized under three headings: (1) The food-relations of the individual are profoundly changed, an entirely different set of mouth-organs appears and the kind and

quantity of the food taken is often radically different. (2) A | wingless, sedentary creature is turned into a winged one with superlative powers of aerial movement. (3) An individual in which the reproductive organs and powers are functionally absent becomes one in which these structures and powers are the only reason for existence, for the great majority of insects die after a brief period of reproduction. These changes are in the higher insects so extreme that it is difficult to imagine how they could be increased. In the case of the common drone-fly, Eristalis tenax, the individual, from a sedentary maggot living in filth, without any relations of sex, and with only unimportant organs for the ingestion of its foul nutriment, changes to a creature of extreme alertness, with magnificent powers of flight, living on the products of the flowers it frequents, and endowed with highly complex sexual structures.

Forms of Larva.-The unlikeness of the young insect to its parent is one of the factors that necessitates metamorphosis. It is instructive, further, to trace among metabolic insects an increase in the degree of this dissimilarity. An adult Hexapod is provided with a firm, well-chitinized cuticle and six conspicuous jointed legs. Many larval Hexapods might be defined in similar general terms, unlike as they are to their parents in most points of detail. Examples of such are to be seen in the grubs of may-flies, dragon-flies, lacewing-flies and ground-beetles (fig. 24). This type of active, armoured larvaoften bearing conspicuous feelers on the head and long jointed cercopods on the tenth abdominal segment-was styled campodeiform by F. Brauer (1869), on account of its likeness in shape to the bristle-tail Campodea. As an extreme contrast to this Modern Classification.campodeiform type, we take the maggot FIG. 24.-Cam- of the house-fly (fig. 25)-a vermiform podeiform Larva of larva, with soft, white, feebly-chitinized Ground-Beetle cuticle and without either head-capsule (Aepus marinus). or legs. Between these two extremes, numerous intermediate forms can be traced: the grub (wireworm) of a click-beetle, with narrow elongate well-armoured body, but with the legs very short; the grub of a chafer, with the legs fairly developed, but with the cuticle of all the trunk-segments soft and feebly chitinized; the wellknown caterpillar of a moth (fig. 21, e) or saw-fly, with its long cylindrical body, bearing the six shortened thoracic legs and a variable number of pairs of "pro-legs" on the abdomen (this being the eruciform type of larva); the soft, white, woodd

a

After

TERZI

After Howard, Ent. Bull. 4, D. s. (U.S. Dept. Agr.).

FIG. 25.-Vermiform Larva (maggot) of House-fly (Musca domestica). Magnified; b, spiracle on prothorax; c, protruded head region; d, tail-end with functional spiracles; e, f, head region with mouth hooks protruded; g, hooks retracted; h, eggs. All magnified. boring grub of a longhorn-beetle or of the saw-fly Sirex, with its stumpy vestiges of thoracic legs; the large-headed but entirely legless, fleshy grub of a weevil; and the legless larva, with greatly reduced head, of a bee. The various larvae of the above series, however, have all a distinct head-capsule, which is altogether wanting in the degraded fly maggot. These differences in larval form depend in part on the surroundings

among which the larva finds itself after hatching; the active, armoured grub has to seek food for itself and to fight its own battles, while the soft, defenceless maggot is provided with abundant nourishment. But in general we find that elaboration of imaginal structure is associated with degradation in the nature of the larva, eruciform and vermiform larvae being characteristic of the highest orders of the Hexapoda, so that unlikeness between parent and offspring has increased with the evolution of the class.

Hypermetamorphosis.-Among a few of the beetles or Coleoptera (q.v.), and also in the neuropterous genus Mantispa, are found life-histories in which the earliest instar is campodeiform and the succeeding larval stages eruciform. These later stages, comprising the greater part of the larval history, are adapted for an inquiline or a parasitic life, where shelter is assured and food abundant, while the short-lived, active condition enables the newly-hatched insect to make its way to the spot favourable for its future development, clinging, for example, in the case of an oil-beetle's larva, to the hairs of a bee as she flies towards her nest. The presence of the two successive larval forms in the life-history constitutes what is called hypermetamorphosis. Most significant is the precedence of the eruciform by the campodeiform type. In conjunction with the association mentioned above of the most highly developed imaginal with the most degraded larval structure, it indicates clearly that the active, armoured grub preceded the sluggish soft-skinned caterpillar or maggot in the evolution of the Hexapoda.

Nymph.-The term nymph is applied by many writers on the Hexapoda to all young forms of insects that are not sufficiently unlike their parents to be called larvae. Other writers apply the term to a "free" pupa (see infra). It is in wellnigh universal use for those instars of ametabolous and hemimetabolous insects in which the external wing-rudiments have become conspicuous (fig. 27). The mature dragon-fly nymph, for example, makes its way out of the water in which the early stages have been passed and, clinging to some water-plant, undergoes the final ecdysis that the imago may emerge into the air. Like most ametabolic and hemimetabolic Hexapoda, such nymphs continue to move and feed throughout their lives. But examples are not wanting of a more or less complete resting habit during the latest nymphal instar. In some cicads the mature nymph ceases to feed and remains quiescent within a pillar-shaped earthen chamber. The nymph of a thrips-insect (Thysanoptera) is sluggish, its legs and wings being sheathed by a delicate membrane, while the nymph of the male scaleinsect rests enclosed beneath a waxy covering.

Sub-imago.-Among the Hexapoda generally there is no subsequent ecdysis nor any further growth after the assumption of the winged state. The may-flies, however, offer a remarkable exception to this rule. After a prolonged aquatic larval and nymphal life-history, the winged insect appears as a sub-imago, whence, after the casting of a delicate cuticle, the true imago

emerges.

Pupa. In the metabolic Hexapoda the resting pupal instar shows externally the wings and other characteristic imaginal organs which have been gradually elaborated beneath the larval cuticle. It is usual to distinguish between the free pupae (fig. 26, b)-of Coleoptera and Hymenoptera, for example -in which the wings, legs and other appendages are not fixed to the trunk, and the obtect pupae (fig. 21, d)-such as may be noticed in the majority of the Lepidoptera-whose appendages are closely and immovably pressed to the body by a general hardening and fusion of the cuticle. In the degree of mobility there is great diversity among pupae. A gnat pupa swims through the water by powerful strokes of its abdomen. while the caddis-fly pupa, in preparation for its final ecdysis, bites its way out of its subaqueous protective case and rises through the water, so that the fly may emerge into the air. Some pupae are thus more active than some nymphs; the essential character of a pupa is not therefore its passivity, but that it is the instar in which the wings first become evident externally.