Chapter XI: Introduction: Habits–classification–structure–chilognatha–chilopoda (5)

The ovaries of the female are replaced in the male by a pair {140}of testes, organs exhibiting much variety of form. The structure may consist of an extremely long and fine convoluted tube, packed into a small space and covered with a capsule; or there may be several shorter tubes. As another extreme may be mentioned the existence of a number of small follicles opening into a common tube, several of these small bodies forming together a testis. As a rule each testis has its own capsule, but cases occur—very frequently in the Lepidoptera—in which the two testes are enclosed in a common capsule; so that there then appears to be only one testis. The secretion of each testis is conveyed outwards by means of a slender tube, the vas deferens, and there are always two such tubes, even when the two testes are placed in one capsule. The vasa deferentia differ greatly in their length in different Insects, and are in some cases many times the length of the body; they open into a common duct, the ductus ejaculatorius. Usually at some part of the vas deferens there exists a reservoir in the form of a sac or dilatation, called the vesicula seminalis. There are in the male, as well as in the female, frequently diverticula, or glands, in connexion with the sexual passages; these sometimes exhibit very remarkable forms, as in the common cockroach, but their functions are quite obscure. There is, as we have already remarked, extreme variety in the details of the structure of the internal reproductive apparatus in the male, and there are a few cases in which the vasa deferentia do not unite behind, but terminate in a pair of separate orifices. The genus _Machilis_ is as remarkable in the form of the sexual glands and ducts of the male as we have already mentioned it to be in the corresponding parts of the female.

Although the internal sexual organs are only fully developed in the imago or terminal stage of the individual life, yet in reality their rudiments appear very early, and may be detected from the embryo state onwards through the other preparatory stages.

The spermatozoa of a considerable number of Insects, especially of Coleoptera, have been examined by {141}Ballowitz;[65] they exhibit great variety; usually they are of extremely elongate form, thread-like, with curious sagittate or simply pointed heads, and are of a fibrillar structure, breaking up at various parts into finer threads.

EXTERNAL SEXUAL ORGANS.—The terminal segments of the body are usually very highly modified in connexion with the external sexual organs, and this modification occurs in such a great variety of forms as to render it impossible to give any general account thereof, or of the organs themselves. Some of these segments—or parts of the segments, for it may be dorsal plates or ventral plates, or both—may be withdrawn into the interior, and changed in shape, or may be doubled over, so that the true termination of the body may be concealed. The comparative anatomy of all these parts is especially complex in the males, and has been as yet but little elucidated, and as the various terms made use of by descriptive entomologists are of an unsatisfactory nature we may be excused from enumerating them. We may, however, mention that when a terminal chamber is found, with which both the alimentary canal and the sexual organs are connected, it is called a cloaca, as in other animals.

PARTHENOGENESIS.

There are undoubted cases in Insects of the occurrence of parthenogenesis, that is, the production of young by a female without concurrence of a male. This phenomenon is usually limited to a small number of generations, as in the case of the Aphididae, or even to a single generation, as occurs in the alternation of generations of many Cynipidae, a parthenogenetic alternating with a sexual generation. There are, however, a few species of Insects of which no male is known (in Tenthredinidae, Cynipidae, Coccidae), and these must be looked on as perpetually parthenogenetic. It is a curious fact that the result of parthenogenesis in some species is the production of only one sex, which in some Insects is female, in others male; the phenomenon in the former case is called by Taschenberg[66] Thelyotoky, in the latter case Arrhenotoky; Deuterotoky being applied to the cases in which two sexes are produced. In some forms of {142}parthenogenesis the young are produced alive instead of in the form of eggs. A very rare kind of parthenogenesis, called paedogenesis, has been found to exist in two or three species of Diptera, young being produced by the immature Insect, either larva or pupa.

GLANDS.

Insects are provided with a variety of glands, some of which we have alluded to in describing the alimentary canal and the organs of sex; but in addition to these there are others in connexion with the outer integument; they may be either single cells, as described by Miall in _Dicranota_ larva,[67] or groups of cells, isolated in tubes, or pouches. The minute structure of Insect glands has been to some extent described by Leydig;[68] they appear to be essentially of a simple nature, but their special functions are very problematic, it being difficult to obtain sufficient of their products for satisfactory examination.

{143}CHAPTER V

DEVELOPMENT

EMBRYOLOGY–EGGS–MICROPYLES–FORMATION OF EMBRYO–VENTRAL PLATE–ECTODERM AND ENDODERM–SEGMENTATION–LATER STAGES–DIRECT OBSERVATION OF EMBRYO– METAMORPHOSIS–COMPLETE AND INCOMPLETE–INSTAR–HYPERMETAMORPHOSIS– METAMORPHOSIS OF INTERNAL ORGANS–INTEGUMENT–METAMORPHOSIS OF BLOWFLY– HISTOLYSIS–IMAGINAL DISCS–PHYSIOLOGY OF METAMORPHOSIS–ECDYSIS.

The processes for the maintenance of the life of the individual are in Insects of less proportional importance in comparison with those for the maintenance of the species than they are in Vertebrates. The generations of Insects are numerous, and the individuals produced in each generation are still more profuse. The individuals have as a rule only a short life; several successive generations may indeed make their appearances and disappear in the course of a single year.

Although eggs are laid by the great majority of Insects, a few species nevertheless increase their numbers by the production of living young, in a shape more or less closely similar to that of the parent. This is well known to take place in the Aphididae or green-fly Insects, whose rapid increase in numbers is such a plague to the farmer and gardener. These and some other cases are, however, exceptional, and only emphasise the fact that Insects are pre-eminently oviparous. Leydig, indeed, has found in the same _Aphis_, and even in the same ovary, an egg-tube producing eggs while a neighbouring tube is producing viviparous individuals.[69] In the Diptera pupipara the young are {144}produced one at a time, and are born in the pupal stage of their development, the earlier larval state being undergone in the body of the parent: thus a single large egg is laid, which is really a pupa.

The eggs are usually of rather large size in comparison with the parent, and are produced in numbers varying according to the species from a few—15 or even less in some fossorial Hymenoptera—to many thousands in the social Insects: somewhere between 50 and 100 may perhaps be taken as an average number for one female to produce. The whole number is frequently deposited with rapidity, and the parent then dies at once. Some of the migratory locusts are known to deposit batches of eggs after considerable intervals of time and change of locality. The social Insects present extraordinary anomalies as to the production of the eggs and the prolongation of the life of the female parent, who is in such cases called a queen.

The living matter contained in the egg of an Insect is protected by three external coats: (1) a delicate interior oolemm; (2) a stronger, usually shell-like, covering called the chorion; (3) a layer of material added to the exterior of the egg from glands, at or near the time when it is deposited, and of very various character, sometimes forming a coat on each egg and sometimes a common covering or capsule for a number of eggs. The egg-shell proper, or chorion, is frequently covered in whole or part with a complex minute sculpture, of a symmetrical character, and in some cases this is very highly developed, forming an ornamentation of much delicacy; hence some Insects' eggs are objects of admirable appearance, though the microscope is of course necessary to reveal their charms. One of the families of butterflies, the Lycaenidae, is remarkable for the complex forms displayed by the ornamentation of the chorion (see Fig. 78, B).

The egg-shell at one pole of the egg is perforated by one or more minute orifices for the admission to the interior of the spermatozoon, and it is the rule that the shell hereabouts is symmetrically sculptured (see Fig. 77), even when it is {145}unornamented elsewhere: the apertures in question are called micropyles. They are sometimes protected by a micropyle apparatus, consisting of raised processes, or porches: these are developed to an extraordinary extent in some eggs, especially in those of Hemiptera-Heteroptera (see Fig. 78, C). Some of these peculiar structures have been described and figured by Leuckart.[70] The purpose they serve is quite obscure.

FORMATION OF EMBRYO.

The mature, but unfertilised, egg is filled with matter that should ultimately become the future individual, and in the process of attaining this end is the seat of a most remarkable series of changes, which in some Insects are passed through with extreme rapidity. The egg-contents consist of a comparatively structureless matrix of a protoplasmic nature and of yolk, both of which are distributed throughout the egg in an approximately even manner. The yolk, however, is by no means of a simple nature, but consists, even in a single egg, of two or three kinds of spherular or granular constituents; and these vary much in their appearance and arrangement in the early stages of the development of an egg, the yolk of the same egg being either of a homogeneously granular nature, or consisting of granules and larger masses, as well as of particles of fatty matter; these latter when seen through the microscope looking sometimes like shining, nearly colourless, globules.

{146}

The nature of the matrix—which term we may apply to both the protoplasm and yolk as distinguished from the minute formative portions of the egg—and the changes that take place in it have been to some extent studied, and Kowalewsky, Dohrn,[71] Woodworth,[72] and others have given some particulars about them. The early changes in the formative parts of the mature egg have been observed by Henking in several Insects, and particularly in _Pyrrhocoris_, his observations being of considerable interest. When the egg is in the ovary and before it is quite mature,—at the time, in fact, when it is receiving nutriment from ovarian cells,—it contains a germinal vesicle including a germinal spot, but when the egg is mature the germinal vesicle has disappeared, and there exists in its place at one portion of the periphery of the egg-contents a cluster of minute bodies called chromosomes by Henking, whom we shall follow in briefly describing their changes. The group divides into two, each of which is arranged in a rod or spindle-like manner, and may then be called a directive rod or spindle. The outer of these two groups travels quite to the periphery of the egg, and there with some adjacent matter is extruded quite outside the egg-contents (not outside the egg-coverings), being in its augmented form called a polar or directive body. While this is going on the second directive spindle itself divides into two groups, the outer of which is then extruded in the manner we have already described in the case of the first polar body, thus completing the extrusion of two directive bodies. The essential parts of the bodies that are successively formed during these processes are the aggregates, called chromosomes; the number of these chromosomes appears to be constant in each species; their movements and dispositions are of a very interesting character, the systems they form in {147}the course of their development having polar and equatorial arrangements. These we cannot further allude to, but may mention that the extrusion of the directive bodies is only temporary, they being again included within the periphery of the egg by the growth and extension of adjacent parts which meet over and thus enclose the bodies.

The arrangements and movements we have briefly alluded to have been limited to the unfertilised condition of the egg (we should rather say, the fertilising element has taken no part in them), and have as their result the union of the chromosomes existing after the extrusion of the two polar bodies, into a small body called the female pronucleus or egg-nucleus (Eikern), while the position of the movements has been an extremely minute portion of the egg near to its outer surface or periphery. The introduction of a sperm, or male, element to the egg through the micropyle gives rise to the formation of another minute body placed more in the interior of the egg, and called the sperm-nucleus. The egg-nucleus, travelling more into the interior of the egg, meets the sperm-nucleus; the two amalgamate, forming a nucleus or body that goes through a series of changes resulting in its division into two daughter-bodies. These two again divide, and by repetitions of such division a large number of nuclei are formed which become arranged in a continuous manner so as to form an envelope enclosing a considerable part (if not quite the whole) of the egg-mass. This envelope is called the blastoderm, and together with its contents will form the embryo. We must merely allude to the fact that it has been considered that some of the nuclei forming the blastoderm arise directly from the egg-mass by a process of amalgamation, and if this prove to be correct it may be admitted that some portions of the embryo are not entirely the result of division or segmentation of combined germ and sperm-nuclei. Wheeler states[73] that some of the nuclei formed by the first differentiation go to form the vitellophags scattered throughout the yolk. We should also remark that, according to Henking, the blastoderm when completed shows at one part a thickening, immediately under which (_i.e._ included in the area the blastoderm encloses) are the two polar bodies, which, as we have seen, were formed by the germinating body at an earlier stage of its activity. Fig. 79 {148}represents a stage in the development of _Pyrrhocoris_, showing the interior of the egg after a body has been formed by the union of the sperm and egg-nuclei; this body is about to undergo division or segmentation, and the equatorial arrangement where this will take place is seen. The two polar bodies P_{1}, P_{2}, after having been excluded, are nearly reincluded in the egg.

THE VENTRAL PLATE.

The next important change after the formation of the blastoderm is the partial detachment of a part of its periphery to become placed in the interior of the other and larger portion. The way in which this takes place will be gathered from the accompanying diagrammatic figures taken from Graber: a thickened portion (_a_ _b_) of the blastoderm becomes indrawn so as to leave a fold (_c_ _d_) at each point of its withdrawal, and these folds afterwards grow and meet so as to enclose the thickened portion. The outer envelope, formed in part by the original blastoderm and in part by the new growth, is called the serosa (_e_ _f_), the inner layer (_g_) of the conjoined new folds being termed the amnion: the part withdrawn to the interior and covered by the serosa and amnion is called the ventral plate, or germinal band (_Keimstreif_), and becomes developed into the future animal. The details of the withdrawal of the ventral plate to the interior are very different in the various Insects that have been investigated.

One of the earliest stages in the development is a differentiation of a portion of the ventral plate into layers from which the future parts of the organisation will be derived. This separation of endoderm from ectoderm takes place by a sort of invagination, analogous with that by which the ventral plate itself is formed. A longitudinal depression running along the middle of the ventral plate appears, and forms a groove or channel, which becomes obliterated as to its outer face by the meeting together of the two margins of the groove (except on the {149}anterior part, which remains open). The more internal layer of the periphery of this closed canal is the origin of the endoderm and its derivatives. Subsequently the ventral plate and its derivatives grow so as to form the ventral part and the internal organs of the Insect, the dorsal part being completed much later by growths that differ much in different Insects; Graber, who has specially investigated this matter, informing us[74] that an astonishing multifariousness is displayed. It would appear that the various modes of this development do not coincide with the divisions into Orders and Families adopted by any systematists.

We should observe that the terms ectoderm, mesoderm, and endoderm will probably be no longer applied to the layers of the embryo when embryologists shall have decided as to the nature of the derived layers, and shall have agreed as to names for them. According to the nomenclature of Graber[75] the blastoderm differentiates into Ectoblast and Endoblast; this latter undergoing a further differentiation into Coeloblast and Myoblast. This talented embryologist gives the following table of the relations of the embryonic layers and their nomenclature, the first term of each group being the one he proposed to use:—

Periblast-------Ectoblast Part of yolk cells.
(Epiblast, \ (Ectoderm, outer /
/ blastoderm).\ layer). /
/ \ /
Protoblast. \ / Coeloblast
\ Centroblast Endoblast or--------(Endoderm in narrower
\(Yolk-cells, Hypoblast, \ sense).
hypoblast, inner layer. \ _Darmdrüsenblatt_.
endoderm (Mesoblast of \
in part of Balfour.) \
Balfour.) Mesoderm and Myoblast (Mesoderm
endoderm. of most authors).
_Darmmuskelblatt_

Nussbaum considers[76] that "there are four layers in the cockroach-embryo, viz. (1) _epiblast_, from which the integument and nervous system are developed; (2) _somatic layer of mesoblast_, mainly converted into the muscles of the body-wall; (3) _splanchnic layer of mesoblast_, yielding the muscular coat of the alimentary canal; and (4) _hypoblast_, yielding the epithelium of the mesenteron."

{150}

Turning our attention to the origin of the segmentation, that is so marked a feature of Insect structure, we find that evidence of division or arrangement of the body into segments appears very early, as shown in our Figure of some of the early stages of development of _Lina_ (a beetle), Fig. 81. In A the segmentation of the ectoderm has not commenced, but the procephalic lobes (_P C_) are seen; in B the three head segments are distinct, while in C the thoracic segmentation has occurred, and that of the abdomen has commenced. Graber considers that in this species the abdomen consists of ten segmental lobes, and a terminal piece or telson. According to Graber[77] this is not a primitive condition, but is preceded by a division into three or four parts, corresponding with the divisions that will afterwards be head, thorax, and abdomen. This primary segmentation, he says, takes place in the Hypoblast (Endoderm) layer of the ventral plate; this layer being, in an early stage of the development of a common grasshopper (_Stenobothrus variabilis_), divided into four sections, two of which go to form the head, while the others become thorax and abdomen respectively. In _Lina_ the primary segmentation is, Graber says, into three instead of four parts. Graber's opinion on the primary segmentation does not appear to be generally accepted, and Wheeler, who has studied[78] the {151}embryology of another Orthopteron, considers it will prove to be incorrect. When the secondary segmentation occurs the anterior of the two cephalic divisions remains intact, while the second divides into the three parts that afterwards bear the mouth parts as appendages. The thoracic mass subsequently segments into three parts, and still later the hind part of the ventral plate undergoes a similar differentiation so as to form the abdominal segments; what the exact number of these may be is, however, by no means easy to decide, the division being but vague, especially posteriorly, and not occurring all at once, but progressing from before backwards.

The investigations that have been made in reference to the segmentation of the ventral plate do not at present justify us in asserting that all Insects are formed from the same number of embryonic segments. The matter is summarised by Lowne, to the effect that posterior to the procephalic lobes there are three head segments and three thoracic segments, and a number of abdominal segments, "rarely less than nine or more than eleven." It will be seen by referring to Figure 81 that the segmentation appears, not simultaneously, but progressively from the head backwards; this of course greatly increases the difficulty of determining by means of a section the real number of segments.

The later stages in the development of Insects are already proved to be so various that it would be impossible to attempt to follow them in detail; but in Fig. 82 we represent a median section of the embryo of _Zygaena filipendula_ at the fifth day. It shows well some of the more important of the general features of the development at a stage subsequent to those represented in Fig. 81, A, B, C. The very distinct stomodaeum (_st_) and proctodaeum (_pr_) are seen as inflexions of the external wall of the body; the segmentation and the development of the {152}ventral parts of the embryo are well advanced, while the dorsal part of the embryo is still quite incomplete.

The method of investigation by which embryologists chiefly carry on their researches is that of dividing the egg after proper preparation, into a large number of thin sections, which are afterwards examined in detail, so as to allow the arrangement to be completely inferred and described. Valuable as this method is, it is nevertheless clear that it should, if possible, be supplemented by direct observation of the processes as they take place in the living egg: this method was formerly used, and by its aid we may still hope to obtain exact knowledge as to the arrangements and rearrangements of particles by which the structures develop. Such questions as whether the whole formative power in the egg is absolutely confined to one or two small centres to which the whole of the other egg contents are merely, as it were, passive accessories, or whether an egg is a combination in which some portion of the powers of rearrangement is possessed by other particles, as well as the chromosomes, in virtue of their own nature or of their position at an early period in the whole, can scarcely be settled without the aid of direct observation of the processes during life.

The importance of the yolk is recognised by most of the recent writers. Nussbaum states (_loc. cit._) that "scattered yolk-cells associate themselves with the mesoblast cells, so that the constituents of the mesoblast have a twofold origin." Wheeler finds[79] that amoeboid cells—he styles them vitellophags—traverse the yolk and assist in its rearrangement; he insists on the importance both as regards quantity and quality of the yolk.

The eggs of some insects are fairly transparent, and the process of development in them can, to a certain extent, be observed by simple inspection with the microscope; a method that was used by Weismann in his observations on the embryology of _Chironomus_. There is a moth (_Limacodes testudo_), that has no objection to depositing its eggs on glass microscope-slides. These eggs are about a millimetre long, somewhat more than half that width, are very flat, and the egg-shell or chorion is very thin and perfectly transparent. When first laid the contents of this egg appear nearly homogeneous and evenly distributed, a finely granular appearance being presented throughout; but in {153}twenty-four hours a great change is found to have taken place. The whole superficial contents of the egg are at that time arranged in groups, having the appearance of separate rounded or oval masses, pressed together so as to destroy much of their globular symmetry. The egg contents are also divided into very distinct forms, a granular matter, and a large number of transparent globules, these latter being the fatty portion of the yolk; these are present everywhere, though in the centre there is a space where they are very scanty, and they also do not extend quite to the circumference. But the most remarkable change that has taken place is the appearance in the middle of the field of an area different from the rest in several particulars; it occupies about one-third of the width and one-third of the length; it has a whiter and more opaque appearance, and the fat globules in it are fewer in number and more indistinct. This area is afterwards seen to be occupied by the developing embryo, the outlines of which become gradually more distinct. Fig. 83 gives an idea of the appearance of the egg about the middle period of the development. In warm weather the larva emerges from this egg ten or eleven days after it has been deposited.

The period occupied by the development of the embryo is very different in the various kinds of Insects; the blowfly embryo is fully developed in less than twenty-four hours, while in some of the Orthoptera the embryonic stage may be prolonged through several months. According to Woodworth the blastoderm in _Vanessa antiopa_ is complete in twenty-four hours after the deposition of the egg, and the involution of the ventral plate is accomplished within three days of deposition.

METAMORPHOSIS.

The ontogeny, or life history of the individual, of Insects is peculiar, inasmuch as a very large part of the development takes {154}place only late in life and after growth has been completed. Insects leave the egg in a certain form, and in that condition they continue—with, however, a greater or less amount of change according to kind—till growth is completed, when, in many cases, a very great change of form takes place. Post-embryonic development, or change of form of this kind, is called metamorphosis. It is not a phenomenon peculiar to Insects, but exists to a greater or less extent in other groups of the Metazoa; while simpler post-embryonic development occurs in nearly all, as in scarcely any complex animals are all the organs completely formed at the time the individual becomes possessed of a separate existence. In many animals other than Insects the post-embryonic development assumes most remarkable and complex forms, though there are perhaps none in which the phenomenon is very similar to the metamorphosis of Insects. The essential features of metamorphosis, as exhibited in the great class we are writing of, appear to be the separation in time of growth and development, and the limitation of the reproductive processes to a short period at the end of the individual life. The peculiar phenomena of the post-embryonic development of the white ants show that there exists some remarkable correlation between the condition of the reproductive organs and the development of the other parts of the organisation. If we take it that the post-embryonic physiological processes of any individual Insect are of three kinds,—growth, development, and reproduction,—then we may say that in the higher Insects these three processes are almost completely separated, and go on consecutively, the order being,—first, growth; second, development; third, reproduction. While, if we complete the view by including the processes comprised in the formation of the egg and the development therein, the series will be—(1) oogenesis, or egg-growth; (2) development (embryonic); (3) growth (post-embryonic); (4) development (post-embryonic); (5) reproduction.

The metamorphosis of Insects is one of the most interesting parts of entomology. It is, however, as yet very little known from a scientific point of view, although the simpler of its external characters have for many ages past attracted the attention and elicited the admiration of lovers of nature. It may seem incorrect to say that little is yet known scientifically of a phenomenon concerning which references almost {155}innumerable are to be found in literature: nevertheless the observations that have been made as to metamorphosis, and the analysis that has been commenced of the facts are at present little more than sufficient to show us how vast and complex is the subject, and how great are the difficulties it presents.

There are three great fields of inquiry in regard to metamorphosis, viz. (1) the external form at the different stages; (2) the internal organs and their changes; (3) the physiological processes. Of these only the first has yet received any extensive attention, though it is the third that precedes or underlies the other two, and is the most important. We will say a few words about each of these departments of the inquiry. Taking first the external form—the instar. But before turning to this we must point out that in limiting the inquiry to the post-embryonic development, we are making one of those limitations that give rise to much misconception, though they are necessary for the acquisition of knowledge as to any complex set of phenomena. If we assume five well-marked stages as constituting the life of an Insect with extreme metamorphosis, viz. (1) the formation and growth of the egg; (2) the changes in the egg culminating in its hatching after fertilisation; (3) the period of growth; (4) the pupal changes; (5) the life of the perfect Insect; and if we limit our inquiry about development to the latter three, we are then shutting out of view a great preliminary question, viz. whether some Insects leave the egg in a different stage of development to others, and we are consequently exposing ourselves to the risk of forgetting that some of the distinctions we observe in the subsequent metamorphosis may be consequential on differences in the embryonic development.

INSTAR AND STADIUM.

Figs. 84 and 85 represent corresponding stages in the life of two different Insects, Fig. 84 showing a locust (_Acridium_), and Fig. 85 a white butterfly. In each A represents the newly-hatched individual; B, the insect just before its perfect state; C, the perfect or imago stage. On comparing the two sets of figures we see that the C stages correspond pretty well as regards the most important features (the position of the wings being unimportant), that the A stages are moderately different, {156}while the B states are not to be recognised as equivalent conditions.

Every Insect after leaving the egg undergoes during the process of growth castings of the skin, each of which is called a moult or ecdysis. Taking for our present purpose five as the number of ecdyses undergone by both the locust and butterfly, we may express the differences in the successions of change we portray in Figs. 84 and 85 by saying that previous to the first ecdysis the two Insects are moderately dissimilar, that the locust undergoes a moderate change before reaching the fifth ecdysis, and undergoes another moderate change at this moult, thus reaching its perfect condition by a slight, rather gradual series of {157}alterations of form. On the other hand, the butterfly undergoes but little modification, remaining much in the condition shown by A, Fig. 85, till the fourth, or penultimate, ecdysis, but then suffers a complete change of form and condition, which apparently is only inferior to another astonishing change that takes place at the fifth or final moult. The chief, though by no means the only, difference between the two series consists in the fact that the butterfly has interposed between the penultimate and the final ecdyses a completely quiescent helpless condition, in which it is deprived of external organs of sense, locomotion, and nutrition; while in the locust there is no loss of these organs, and such quiescent period as exists is confined to a short period just at the fifth ecdysis. The changes exhibited by the butterfly are called "complete metamorphosis," while this phenomenon in the locust is said to be "incomplete." The Insect with complete metamorphosis is in its early stage called a larva, and in the quiescent state a pupa. The adult state in both butterfly and locust is known as imago or perfect Insect.

The most conspicuous of the differences between Insects with complete and those with incomplete metamorphosis is, as we have remarked, the existence in the former of a pupa. The pupal state is by no means similar in all the Insects that possess it. The most anomalous conditions in regard to it occur in the Order Neuroptera. In some members of that Order—the Caddis-flies for instance—the pupa is at first quiescent, but becomes active before the last ecdysis; while in another division—the May-flies—the last ecdysis is not preceded by a formed pupa, nor is there even a distinct pupal period, but the penultimate ecdysis is accompanied by a change of form to the winged condition, the final ecdysis being merely a casting of the skin after the winged state has been assumed. In the _Odonata_ or Dragon-flies there is no pupal stage, but the change of form occurring at the last ecdysis is very great. In those Insects where the interval between the last two moults is not accompanied by the creature's passing into a definite, quiescent pupa, the individual is frequently called then a nymph; but the term nymph has merely a distinctive meaning, and is not capable of accurate definition, owing to the variety of different conditions covered by the word. Eaton, in describing this term as it is used for _Ephemeridae_, says, "Nymphs are young which lead an {158}active life, quitting the egg at a tolerably advanced stage of morphological development, and having the mouth-parts formed after the same main type of construction as those of the adult insect."[80]

The intervals between the ecdyses are called stadia, the first stadium being the period between hatching and the first ecdysis. Unfortunately no term is in general use to express the form of the Insect at the various stadia; entomologists say, "the form assumed at the first moult," and so on. To avoid this circumlocution it may be well to adopt a term suggested by Fischer,[81] and call the Insect as it appears at hatching the first instar, what it is as it emerges from the first ecdysis the second instar, and so on; in that case the pupa of a Lepidopteron that assumed that condition at the fifth ecdysis would be the sixth instar, and the butterfly itself would be the seventh instar.

Various terms are used to express the differences that exist in the metamorphoses of Insects, and as these terms refer chiefly to the changes in the outer form, we will here mention them. As already stated, the locust is, in our own language, said to have an incomplete metamorphosis, the butterfly a complete one. The term Holometabola has been proposed for Insects with complete metamorphosis, while the appellations Ametabola, Hemimetabola, Heterometabola, and Paurometabola have been invented for the various forms of incomplete, or rather less complex, metamorphosis. Some writers use the term Ametabola for Insects that are supposed to exhibit no change of external form after quitting the egg, the contrasted series of all other Insects being then called Metabola. Westwood and others use the word Homomorpha for Insects in which the condition on hatching more or less resembles that attained at the close of the development, and Heteromorpha for those in which the form on emergence from the egg differs much from what it ultimately becomes.

HYPERMETAMORPHOSIS.

There are certain unusual changes to which the term hypermetamorphosis has been applied; these we can here only briefly allude to.

{159}Insects that have complete metamorphoses, and are not supplied with food by their parents or guardians, are provided during their larval life with special modifications of extremely various kinds to fit them for the period of life during which they are obtaining food and growing. Thus caterpillars possess numerous adaptations to fit them for the period during which they live on leaves, while maggots have modifications enabling them to live amongst decomposing flesh. Some larvae are greatly modified in this adaptive way, and when the adaptations change greatly during the life of the larva, hypermetamorphosis is said to exist. As an instance we may mention some beetle larvae that are born with legs by whose aid they can cling to a bee, and so get carried to its nest, where they will in future live on the stores of food the bee provides for its own young. In order that they may be accommodated to their totally different second circumstances, they change their first form, losing their legs, and becoming almost bladder-like creatures, fitted for floating on the honey without being injured by it. Such an occurrence has been described by Fabre[82] in the case of _Sitaris humeralis_, and his figures have been reproduced in Sir John Lubbock's book on the metamorphoses of Insects,[83] as well as in other works, yet they are of so much interest that we give them again, especially as the subject is still only in its infancy; we at present see no sufficient reason for the later of these larval states. Little is, we believe, known as to the internal anatomy of the various instars in these curious cases.

{160}There are certain minute Hymenoptera that deposit their eggs inside the eggs of other Insects, where the beings hatched from the parasitic eggs subsequently undergo their development and growth, finding their sustenance in the yolk or embryo contained in the host-egg. It is evident that such a life is very anomalous as regards both food and the conditions for respiration, and we consequently find that these tiny egg-parasites go through a series of changes of form of a most remarkable character.[84] It would appear that in these cases the embryonic and post-embryonic developments are not separated in the same way as they are in other Insects. We are not aware that any term has yet been proposed for this very curious kind of Insect development, which, as pointed out by Brauer,[85] is doubtless of a different nature from the hypermetamorphosis of _Sitaris_.

CHANGES IN INTERNAL ORGANS.

In relation to the post-embryonic development of the internal organs of the body there is but little exact generalisation to be made, the anatomical condition of these organs at the time of emergence from the egg having been ascertained in but few Insects. We know that in Holometabolous Insects the internal anatomy differs profoundly in the larval and imaginal instars. As to Insects with more imperfect metamorphosis very little information exists, but it appears probable that in many no extensive distinctions exist between the newly-hatched and the adult forms, except in the condition of the reproductive organs. Differences of minor importance doubtless exist, but there is almost no information as to their extent, or as to the periods at which the changes occur; so that we do not know to what extent they may be concentrated at the final ecdysis. In Insects with perfect metamorphosis the structures of the internal organs are, as we have said, in many cases totally different in the larval and imaginal periods of the life; but these changes are far from being uniform in all Holometabola. The nervous system in some cases undergoes a great concentration of the ganglia, in others does not, and important distinctions exist in this respect even within the limits of a single Order, such as the Coleoptera. {161}Some Insects take the same kind of food throughout their lives, but many others change totally in this respect, and their organs for the prehension and digestion of food undergo a corresponding change. Butterflies suck food in the form of liquid juices from flowers by means of a delicate and long proboscis, while the young butterfly—the caterpillar—disdains sweets, and consumes, by the assistance of powerful mandibles, a great bulk of leaves. Other Holometabola undergo no such total change of habits; the tiger-beetle, for instance, is as ferocious a consumer of the juices of Insects in its young stage as it is in the adult condition. Hence Brauer[86] divides Insects, as regards this point, into three categories. The forms in which both the young and adult take food by suction he calls Menorhyncha; those in which both the imago and immature forms feed by mandibles he calls Menognatha; while his Metagnatha consists of those insects that take food by jaws when young, but by suction with tubular mouths when mature. Besides these main divisions there are some exceptional cases to which we need not here allude, our present object being to indicate that in the Metagnatha the digestive organs are of a very different nature in the young and in the adult states of existence.

The internal organs for the continuance of the species are known to be present in a rudimentary stage in the embryo, and it is a rule that they do not attain their full development until growth has been completed; to this rule there may possibly be an exception in the case of the Aptera. But little information of a comparative character exists as to the dorsal vessel and the changes it undergoes during metamorphosis. There is considerable difficulty in connexion with the examination of this structure, but it appears probable that it is one of the organs that changes the least during the process of metamorphosis.

The exact nature of the internal changes that occur during metamorphosis is almost a modern subject. It is of course a matter of great difficulty to observe and record changes that go on in the interior of such small creatures as Insects, and when the phenomena occur with great rapidity, as is frequently the case in Insect metamorphosis, the difficulty is much increased. Nevertheless the subject is of such great interest that it has been investigated with a skill and perseverance that call for the {162}highest admiration. The greater part of the information obtained refers to a single Insect, the blowfly; and amongst those who have made important contributions to it we may mention Weismann,[87] Viallanes,[88] Ganin,[89] and Van Rees,[90] and it is at present under investigation by Lowne. A good deal, too, is becoming known about the processes in the case of the silkworm.

INTEGUMENT AND ECDYSIS.

The integument consists of a cellular layer, usually called the hypodermis, situated on a basement membrane. The hypodermis, or layer of chitinogenous cells, excretes a matter which remains attached to the body, forming the hard outer layer of the skin. This layer consists of chitin and has no vitality, but its presence no doubt exerts a very important influence on the physiological processes of the Insect. The chitinous investment varies much in thickness and in other properties; in some Insects it is hard, even glassy, so as to be difficult to pierce with a pin, in others it is pliable, and in some very delicate. Chitin is a substance very difficult to investigate; according to the recent researches of Krawkow[91] it may prove to be of somewhat variable chemical composition.

After a time the hypodermis excretes a fresh supply of chitin, and, possibly by the commencement of this process, the older chitinous investment becomes separated and is shed. The details have, however, not been ascertained, though their importance has been suggested by Hatchett Jackson.[92] The newly exposed layer of integument is pallid, but afterwards becomes coloured in a manner varying according to the species, the process being possibly due to some secondary exudation permeating the freshly exposed chitin, or modifying some part of its exterior.

Lowne informs us that in the imago of the blowfly the great majority of the hypodermic cells themselves enter into the composition of the chitinous integument; and it is perhaps not a matter for surprise that the cells should die on the completion of their functional activity, and should form a part of the chitinous {163}investment. Some writers say that the chitinous layer may be shown to be covered by a delicate extima or outer coat.

The number of ecdyses varies greatly in Insects, but has been definitely ascertained in only a few forms outside the Order Lepidoptera. In _Campodea_ Grassi says there is a single fragmentary moult, and in many Hymenoptera the skin that is cast is extremely delicate, and the process perhaps only occurs twice or three times previous to the pupal stage. In most Insects, however, ecdysis is a much more important affair, and the whole of the chitinous integument is cast off entire, even the linings of the tracheae, and of the alimentary canal and its adjuncts being parted with. Sir John Lubbock observed twenty-three moults in a May-fly of the genus _Cloëon_,[93] this being the maximum yet recorded, though Sommer states[94] that in _Macrotoma plumbea_ moulting goes on as long as life lasts, even after the Insect has attained its full size.

Some Insects get quit of a considerable quantity of matter by their ecdyses, while in others the amount is comparatively slight. It has been thought that the moulting is effected in order to permit of increase of size of the Insect, but there are facts which point to the conclusion that this is only a factor of secondary importance in the matter. One of these is that many Insects make their first ecdysis almost immediately after they leave the egg; this is the case with the young larva of the blowfly, which, according to Lowne, moults within two hours of its emergence from the egg. We have already referred to the important suggestion made by Eisig[95] that, since chitin is a nitrogenous substance, the ecdyses may be a means of getting rid of waste nitrogenous matter; to which we have added that as chitin also consists largely of carbon, its excretion may be of importance in separating carbonaceous products from the blood.

METAMORPHOSIS OF BLOWFLY.

The phenomena of metamorphosis are displayed to their greatest extent in the transformations and physiological processes of the _Muscid Diptera_, of which the common blowfly is an {164}example. We will briefly consider the information that has been obtained on this subject.

The development of the embryo in the egg of the blowfly is unusually rapid, occupying only a period of twenty to twenty-four hours. After its first moult the blowfly larva grows rapidly during a period of about ten to fourteen days, during which it undergoes moults, the number of which appears not to be definitely ascertained. After becoming full-fed the larva loses its active state, and passes for a period into a condition of comparative quiescence, being spoken of in this state as a resting larva. This quiet period occurs in most full-grown larvae, and is remarkable for the great variation that may occur in its duration, it being in many Insects subject to prolongation for months, in some cases possibly even for years, though in favourable circumstances it may be very short. Lowne informs us that in the blowfly this period of the life is occupied by very great changes in the internal organs, which are undergoing very extensive processes of destruction and rebuilding. After some days the outer skin of the resting larva shrivels, and is detached from the internal living substances, round which it hardens and forms the sort of cocoon or capsule that is so well known. This using of the cast larval skin as a cocoon is, however, limited to certain of the two-winged flies, and perhaps a few other Insects, and so must be considered an exceptional condition. The capsule conceals from view a most remarkable state, known to the old naturalist Réaumur as the "spheroidal condition," but called by more recent writers the pronymph. The pronymphal state may be looked on as being to a great extent a return of the animal to the condition of an egg, the creature becoming an accumulation of soft creamy matter enclosed in a delicate skin. This spheroidal condition, however, really begins in the resting larva, and Van Rees and others think that the delicate membrane enclosing the substance of the pronymph is really the hypodermis of the integument of the larva. Although this seems probable, from the resemblance this condition would in that case present to the phenomena usual in ecdysis, it is not generally admitted, and there is much difficulty in settling the point. Lowne is of a contrary opinion, looking on the limiting membrane as a subsequent formation; he calls it the paraderm. The process of forming the various organs goes on in the pronymph, till the {165}"nymph" has completed its development, the creature having then again taken on a definite form which apparently corresponds to the pupa of Hymenoptera. Great doubt, however, exists as to this equivalence, and indeed as to any exact correspondence between the metamorphic stadia of different Insects, a view which long since was expressed by Sir John Lubbock[96] and Packard. The term nymph is used in this case not because there is any resemblance to the condition similarly named in Insects with less complete metamorphosis, but because the term pupa is applied to the outer case together with the contained nymph. The transformation of the nymph into the perfect blowfly occupies a period very variable according to the temperature.

HISTOLYSIS.—The processes by which the internal organs of the maggot are converted into those of the fly are of two kinds,—histolysis or breaking down, histogenesis or building up, of tissue. The intermediary agents in histolysis are phagocytes, cells similar to the leucocytes or white corpuscles of the blood: the intermediary agents in histogenesis are portions of tissue existing in the larval state incorporated with the different organs, or preserving a connexion therewith even when they are to a great extent separated therefrom. In this latter case they are called imaginal discs, though Professor Miall prefers to term them imaginal folds.[97] The two processes of histolysis and histogenesis, though to some extent mutually dependent (for the material to be built up has to be largely obtained by previous destruction), do not go on _pari passu_, though they are to a great extent contemporaneous. In the resting larva histolysis is predominant, while in the nymph histogenesis is more extensive. Microscopic observation shows that the phenomena connected with the histolysis of the muscular tissue are scarcely distinguishable from those of an inflammatory process, and Viallanes[98] dilates on this fact in an instructive manner. The phagocytes attach themselves to, or enter, the tissues which are to be disintegrated, and becoming distended, assume a granular appearance. By this pseudo-inflammatory process the larval structures are broken down into a creamy substance; the buds, or germs, from which the new organs are to be developed being exempt from the destruction. These buds, of which about sixty or upwards have already been detected, undergo {166}growth as they are liberated, and so the new creature is formed, the process of growth in certain parts going on while destruction is being accomplished in others. Considerable discrepancy prevails as to the extent to which the disintegration of some of the tissues is carried.

According to Kowalevsky[99] it would appear that after the phagocytes have become loaded with granules they serve as nutriment for the growing tissues, and he thinks they become blood-cells in the imago. The process of histolysis has been chiefly studied in the blowfly, and not much is known of it in other Insects, yet it occurs to a considerable extent, according to Bugnion[100] and others, in the metamorphosis of Lepidoptera. Indeed it would almost seem that the processes of histolysis and histogenesis may be looked on as exaggerated forms of the phenomena of the ordinary life of tissues, due to greater rapidity and discontinuity of tissue nutrition.

{167}IMAGINAL DISCS.—The imaginal discs are portions of the larval hypoderm, detached from continuity with the main body of the integument, but connected therewith by strings or pedicels which may be looked on as portions of the basement membrane. Whether these discs, or histoblasts as they are called by Künckel d'Herculais,[101] are distinguished by any important character from other buds or portions of regenerative tissue that, according to Kowalevsky,[102] Korschelt and Heider,[103] and others, exist in other parts of the body, does not appear to be at present ascertained.

We give some figures, taken from Weismann and Graber, of the imaginal rudiments existing in the larvae of _Muscidae_. Although by no means good, they are the best for our purpose we can offer to the reader. Other figures will be found in Lowne's work on the blowfly now in course of publication. Weismann's paper[104] is now thirty years old, and, when it was written, he was not aware of the intimate connexion the rudiments have with the integument; this has, however, now been demonstrated by several observers. Pratt states[105] that the formation of the imaginal discs in _Melophagus ovinus_ takes place in the later stages of the embryonic development, and after the manner formerly suggested by Balfour, viz. invagination of the ectoderm.

Both the regenerative buds and the rudimentary sexual glands are known to be derived directly from the embryo; neither of them undergoes any histolysis, so that we have in them embryonic structures which exist in a quiescent condition during the period in which the larva is growing with great rapidity, and which when the larva has attained its full growth and is disintegrating, then {168}appropriate the products of the disintegration so as to produce the perfect fly.