DEVELOPMENT OF THE IMAGO IN THE DIPTERA
The flies, particularly the Muscidæ and their allies (Brachycera), are the most highly modified of insects, their larvæ having undergone the greatest amount of reduction and loss of limbs, this atrophy involving even most of the head. The following account has been prepared in part from the works of Weismann, Ganin, Miall, and Pratt, but mostly from the excellent general summarized account given by Korschelt and Heider.
In the holometabolic orders of insects, with their resting pupal stage, during which no food is taken, the entire activity of life seems to be turned to deep-seated and complicated internal developmental processes. These inner changes involve an almost complete destruction of many organs of the larva, and their renewal from certain germs (the imaginal buds) already present in the larva, as will be seen in the highly modified Muscidæ. Only a few larval organs become directly transferred into the body of the pupa and imago. Such are the rudiments of the genital system. The heart also, and the central portion of the nervous system, suffer only slight and unimportant, almost trivial, internal changes. On the other hand, most of the other organs of the larva become completely destroyed: the hypodermis, most of the muscles, the entire digestive canal with the salivary glands; while their cells, under the influence of the blood corpuscles (leucocytes), which here act as phagocytes, fall into pieces, which are taken up by them and become digested. Simultaneously with this destructive, histolytic process, the new formation of the organs by the imaginal buds, already indicated in the embryo, is accomplished in such a way that the continuity of the organs in most cases remains unimpaired. This process of transformation can only be understood by considering that of the embryonal germs of the organs, (1) only a part is destined for the use of the larva in growth, and for the performance of certain functions which exhaust themselves during larval life, so that it is no more capable of farther transformation, and finally becomes destroyed; while (2) a second part of the embryonal germs or rudiments persists first in an undeveloped condition, as imaginal buds, in order to undertake during the pupa stage the regeneration of the organs.
Though Swammerdam knew that the rudiments of the wings were already present under the skin of the larvæ, we are indebted, for our present knowledge, to the thorough and profound investigations of Weismann on the metamorphosis of the Diptera, and also to the researches of Ganin and others who have worked on the pupæ of Muscidæ, in which the development is most complicated and modified. In the more generalized and primitive Diptera, such as Corethra, the processes of formation of the pupa and imago are much simpler than in the muscids and Pupipara. These processes are still simpler in the Lepidoptera and Hymenoptera, and for this reason we have given a summary of what has been done on these organs by Newport, Dewitz, and especially by Bugnion.
Our knowledge of this subject is still very imperfect, only the more salient points having been worked out, and, as Korschelt and Heider state, there is still lacking certain proof as to how far the relations of the internal changes known to exist in the Muscidæ also apply to other orders of insects, though it must be considered that in the pupa of Lepidoptera, Hymenoptera, perhaps also the Coleoptera, and we would add in the Neuroptera as well as the male Coccidæ, very similar metamorphic processes take place.
a. Development of the outer body-form
The form of the imago is completely marked out in the pupa, so that the transition from the pupa to the imago is comparatively slight and only depends on the modification and development of the parts already present.
Fig. 622.—Anterior part of young larva of Simulium sericea, showing the thoracic imaginal buds: p, prothoracic bud (only one not embryonic); w, w′, fore and hind wing-buds; l, l′, l″, leg-buds; n, nervous system; br, brain; e, eye; sd, salivary duct; p, prothoracic foot.—After Weismann.
In most cases the modification in question consists of the changes occurring during the passage from the larval form to the imago, the reformation of parts already present being most marked, while the new rudiments only participate in a limited way in the process. Thus, for example, the head of the caterpillar together with the antennæ and mouth-parts, also the thoracic limbs, pass directly and unchanged from the larva into the pupa. The compound eyes and the wings are, however, new formations, the latter arising from imaginal buds. The same is the case with many other Heterometabola, where the passage of the larva into the pupa in general is due to a transformation of parts already present. The changes in the brain, the fusion of certain ganglia of the ventral nervous cord, the changes in the abdomen, involving the reduction in the number of segments and the remodelling of the end of the body, and the formation of the ovipositor or sting, and in the higher Hymenoptera the transfer of the 1st abdominal segment to the thorax, and the origin of the genital armature,—all these should here be taken into account.
It should be observed that in every case where the larvæ are footless, as in Diptera, all the Hymenoptera except the phytophagous ones and certain coleopterous larvæ, the limbs of the imago stage are, in the earliest stages, indicated as new structures in the form of imaginal buds.
Formation of the imago in Corethra.—Corethra may serve as an example of such a relatively simple metamorphosis. Its larva belongs to the group of eucephalous dipterous larvæ. The head of the perfect insect is already indicated in the larva, and its parts, with certain modifications, pass directly into the pupa. The compound eyes, and this is a rare exception among the Holometabola, are present in the larva. On the other hand, the thoracic legs, the wings, and halteres are developed out of new rudiments which are present in the last larval stage, before pupation. Each thoracic segment has four of them, two ventral and two dorsal (Fig. 622); the ventral buds becoming the legs. Of the dorsal pairs, that of the mesothorax develops into wings, that of the metathorax into halteres, while from the corresponding rudiments of the prothorax in Corethra arise the stigma-bearing dorsal or respiratory processes of the pupa, and in Simulium a tuft of tracheal gills (Fig. 623, ra; see also Fig. 582).
Fig. 623.—Late larva of Simulium, showing the rudiments of the pupal structures within the larval skin: l1, l2, l3, fore, middle, and hind legs of the fly; ra, respiratory appendages of pupa; w, wing of fly; h, halter of fly.—After Miall.
Fig. 624.—Imaginal buds in larva of Corethra (diagrammatic cross-section of thorax): invaginations (fe and be) of the larval hypodermis (lhy) in whose bases the rudiments of wings (fa) and legs (ba) arise; lh, chitinous integument of the larva.—After Lang.
These imaginal buds may be regarded as evaginations of the outer surface of the body. The only difference is that the buds of the appendages as a whole seem sunken below the level of the surface of the body, being situated at the bottom of an evagination, as in the buds of the head and trunk in the Pilidium larva of nemertean worms, and in the rudiments of the lower surface of the body of Echinus present in the pluteus larva.
The lumen of the invagination in which the appendages of Corethra (and other Holometabola) are situated is called by Van Rees the peripodal cavity, and the external sheath bordering it, which is naturally continuous with the hypodermis of the body, the peripodal membrane (Fig. 636, p).
We must adopt the view that the rudiments of the appendages (imaginal buds) are from the first divided into ectodermal and mesodermal portions, which are derived from the corresponding germ-layers of the larva. The ectoderm of the rudiments of the appendages is continuous with the peripodal membrane, and through it with the hypodermis. Weismann was inclined to derive the organs (tracheæ, muscles, etc.) developing within the germs of the appendages from a hypertrophy of the neurilemma of a nerve passing down from within into the imaginal bud, and held that nerves and tracheal branches soon after passed into the inner surface of the imaginal bud. (Korschelt and Heider.)
When the imaginal buds of the appendages enlarge, then the peripodal membranes become correspondingly distended, and the limbs within assume a more or less crumpled position, and in Corethra are spirally twisted, while the rudiments of the wings are folded. The completion of the rudimentary limbs is accomplished simply by their passing out of the invagination in which they originated. The limbs thus gradually become free, the peripodal membrane is seen to reach the level of the rest of the hypodermis and become a part of it, and the base of the extremity is no longer situated in a cavity.
The internal organs of Corethra undergo but to a slight degree the destruction (histolysis) which is so thoroughgoing in the Muscidæ. Kowalevsky states that in the mid-intestine of Corethra a histolysis of the larval and reconstruction of the imaginal epithelium goes on in the same way as has been described in Musca. Most of the larval organs pass without histolytic changes directly over into those of the pupal and imaginal stages; the muscles in general are also unchanged, but those of the appendages and wings are made over anew. The last arise, according to Weismann, in the last larval stage from strings of cells which are already present in the embryo.
When we consider how insignificant the internal transformations are during the metamorphosis of the Tipulidæ, of which Corethra serves as an example, we can scarcely doubt that we here have before us conditions which illustrate the passage between an incomplete and a complete metamorphosis. Thus, among other things, should be mentioned the short duration of the pupa stage and the activity of the pupa, as also the early appearance of the germs of the compound eyes, a character which Corethra has in common with the Hemimetabola. (Korschelt and Heider.)
Formation of the imago in Culex.—In respect to the formation of the imaginal head, Culex is still more primitive than Corethra. Miall and Hammond find from Hurst’s partly unpublished descriptions and preparations that there are no deep invaginations for the compound eyes or antennæ of the imago.
“The compound eye forms beneath the larval eye-spots, and is at first relatively simple and composed of few facets. The number increases by the gradual formation of partial and marginal invaginations, each of which forms a new element. The imaginal antenna grows to a much greater length than that of the larval antenna, and its base is accordingly telescoped into the head, while the shaft becomes irregularly folded.[[115]] Culex, though more modified than Chironomus in many respects, e.g. in the mouth-parts, is relatively primitive with respect to the formation of the imaginal head, and shows a mode of development of the eye and antenna which we may suppose to have characterized a remote and comparatively unspecialized progenitor of Chironomus.”
Formation of the imago in Chironomus.—The development of the head of the imago of Chironomus dorsalis has been discussed by Miall and Hammond. The invaginations which give rise to the head of the fly could not be discovered even in a rudimentary state until after the last larval moult.
“Weismann has given reasons for supposing that invaginated imaginal rudiments could not come into existence before the last larval moult in an insect whose life-history resembles that of Corethra or Chironomus. If the epidermis were invaginated in any stage before the ante-pupal one, the new cuticle, moulded closely upon the epidermis, would become invaginated also, and would appear at the next moult with projecting appendages like those of a pupa or imago. This is actually the way in which the wings are developed in some larval insects with incomplete metamorphosis. In Muscidæ the invaginations for the head of the imago have been traced back to the embryo within the egg,[[116]] but the almost total subsequent separation of the disks from the epidermis renders their development independent of the growth of the larval cuticle and of the moults that probably take place therein.”
The pupal and imaginal cuticles do not follow at all closely the larval skin, but, says Miall, become at particular places folded far into the interior. “The folds which give rise to the head of the fly are two in number and paired. They begin at the larval antenna on each side of the head, and gradually extend further and further backwards. The object of the folds is to provide an extended surface which can be moulded, without pressure from surrounding objects, into the form of the future head. On one part of each fold the facets of the large compound eyes are developed; another part gives rise to the future antenna, a large and elaborate organ, which springs from the bottom of the fold, and whose tip just enters the very short antenna of the larva. The folds for the head ultimately become so large that the larval head cannot contain them, and they extend far into the prothorax. Here a difficulty occurs. If the generating cuticle of the prothorax were also to be folded inwards, the future prothorax would take a corresponding shape. But the prothorax of the fly has a form dictated to it by the limbs which it bears and by the muscles to which it gives attachment. These call for a great reduction in its length, and a peculiar shape, which it is not here necessary to describe. It will be enough to realize that the epidermis of the future prothorax cannot be sacrificed to the folds which are to give rise to the head of the fly. All interference between the two developing structures is obviated by the provision of a transverse fold, which pushes into the prothorax from the neck, and forms a sort of internal pocket. The floor of the pocket forms two longitudinal folds, which prolong the folds originating in the larval head. The roof of the pocket shrinks up and forms the connection between the head and thorax of the fly. Ultimately the head-part is drawn out, leaving the prothoracic structures unaffected.”[[117]]
Fig. 625.—Process of formation of the parts of the head of the fly in the larva of Chironomus (male): A, the new epidermis thrown into complicated folds which have been cut away in places to show the parts within. B, the same parts in horizontal section; lc, larval cuticle; tf, transverse fold; tf′, upper wall of the same; m, cut edge of new epidermis; ant, larval antenna; an, nerve to the same; ant′, antenna of fly; lf, longitudinal fold; o, eye of fly; on, optic nerve; an′, root of antennary nerve; br, brain; œs, œsophagus; b, bulb of antenna of fly; s, s, s′, blood-spaces.—After Miall.
The development of the head of the fly of Chironomus appears, as Miall and Hammond state, to be intermediate between the groups Adiscota and Discota of Weismann; i.e. “between the types in which the parts of the head of the fly are developed in close relation to those of the larva, and the types in which deep invaginations lead apparently to the formation of similar new parts far within the body, the seeming independence of the new parts being intensified by thoroughgoing histolysis,” and they suggest that possibly types may be discovered intermediate between Chironomus and Muscidæ.
Fig. 626.—A, B, C, D, diagrams of transverse sections showing the development of the wings, legs, and the imaginal hypodermis of muscid flies from the imaginal buds of the larva during metamorphosis: lh, chitinous integument of larva from which the underlying hypodermis (lhy) has withdrawn; iid, imaginal buds of wings, iiv, of legs; is, the cords connecting them with the hypodermis; fl, wing-germs; b, leg-germs; ihy, imaginal hypodermis spreading out in D from the imaginal buds. The imaginal rudiments of the hypodermis are indicated by thick, black outlines, the larval hypodermis by two thin, parallel lines.—After Lang.
We are now prepared to consider the extremely complicated changes, in the Muscidæ, leaving out of consideration the origin of the wings from imaginal buds, which has already been discussed on pp. 126–137.
Formation of the imago in Muscidæ.[[118]]—In the flesh, and undoubtedly the house, and allied flies the germs or imaginal buds of the legs and wings arise in the same way as in Corethra. But in the Muscidæ, the buds are situated far within the interior of the body, the peripodal cavities appear closed, and the peripodal membrane stands in connection with the hypodermis merely by means of a delicate thread-like stalk. This connecting cord, which was first detected by Dewitz, and whose interpretation was entirely right, shows in its interior, as Van Rees proved, a narrow cavity.
Though the earliest stages in the development of imaginal buds in the embryo of the Muscidæ are still unknown, yet we shall not go far astray if we refer them, like the imaginal buds of Corethra, to hypodermal invaginations. We must, then, regard the stalk-like connection just mentioned as the long drawn-out neck of this invagination.
In general, the development of the appendages (Figs. 626, 627) goes on as described in Corethra. The rudiments of the legs enlarge and show at an early date the first traces of the later joints. They are so packed in the peripodal cavities that the single joints of the extremities appear as if pushed in “like the joints of a travelling cup.” (Van Rees.) The evagination of the completely formed buds of the limbs, which occurs on the first day after the beginning of pupation, goes on in such a way that the stalk of the imaginal bud (Figs. 626, B; 627, B) shortens, while its cavity widens so that the limbs finally, as in Corethra, pass out through the widely opened mouth of the peripodal invagination, which at the same time gradually completely disappears. The peripodal membrane is converted into a thickened part of the hypodermis in the region adjoining the base of the leg, and from this thickened hypodermal portion, the formation of the hypodermis of the entire imaginal thorax goes on, as the larval hypodermis is gradually destroyed.
Fig. 627.—Imaginal buds of muscid flies in process of development: A, brain (c) and ventral ganglion (v) of a larva, 7 mm. long, of C. vomitoria; h, head-rudiment; rc, portion of ventral cord; pd, prothoracic rudiment; vc3, third nerve; md, mesothoracic rudiment. B, mesothoracic rudiment more advanced, in a pupa, just formed, of Sarcophaga carnaria, showing the base of the sternum and folds of the forming leg, the central part (f) representing the foot. C, the rudimentary leg of the same more advanced; f, femur; t, tibia; f1, f5, tarsal joints. D, two buds from a larva, 20 mm. long, of Sarcophaga, attached to tracheæ; msw, mesonotal and wing-germ; mt, metathoracic rudiment. E, r, mesothoracic germ of a 7 mm. long larva attached to a tracheal twig.—After Weismann and Graber, from Sharp.
We must here settle the question as to the first origin of the mesodermal portions of the rudiments of the appendages. We can already distinguish in the imaginal buds of the fully grown muscid larva a clear separation between an ectodermal and an inner mesodermal part. Ganin derived the mesodermal part through a sort of differentiation and separation of the innermost layer of the ectodermal part, and Van Rees has, in general, confirmed this view. Kowalevsky, on the other hand, is inclined to the view that the mesodermal part of the imaginal bud is derived from the embryonal cells of the mesoderm. He finds scattered throughout the mesoderm, under the hypodermis of the larva, so-called wandering cells (Fig. 632, A, w), which are different in appearance from the leucocytes and from the elements from which the formation of the mesodermal parts of the imaginal rudiments proceed. Kowalevsky is inclined to believe that there are in each segment rudiments of the imaginal mesoderm, but which are so delicate and indifferent that we cannot find them in the first stages of their origin. From these mesodermal imaginal rudiments the above-mentioned wandering cells of the mesoderm are derived, which afterwards come into connection with the ectodermal portion of the imaginal buds.
Still more complicated and difficult to understand is the development of the head-section of muscids. We must remember that in muscid larvæ the head-section exists in its most rudimentary form, being the result of extreme modification and degeneration. The small size of the head is also due to the fact that it is more or less retracted within the thoracic region. Then, as shown by the researches of Weismann, in the last embryonal stages, the forehead, mandibles, and the whole region of the head around the mouth invaginate and form a sunken cavity (Fig. 628, p), in which the chitinous supports of the hooks characteristic of muscid larvæ are soon developed. This sunken part of the head, at whose inner end is the œsophagus, is called by the not entirely appropriate name of “pharynx,” and it must at present be remembered that the hollow space thus named is not a part of the digestive canal. It is an invaginated section of the head, and the formation of the head of the imago mainly depends on the evagination of this region.
The first rudiments of the most important parts of the head (eyes, antennæ, and forehead), occur in the youngest larvæ as paired masses of cells which lie in the thorax next to the two halves of the brain (for this reason called by Weismann “brain-appendages”), which are from their first origin connected with the pharynx, and may be regarded as the imaginal buds of the head. These appear very soon in later stages in the shape of elongated sacs widening at the hinder end (Fig. 628, A and B, h), which from their origin are to be regarded as evaginations of the pharynx. Very soon epithelial thickenings appear in the wall of this sac-shaped brain-appendage, in which the rudiments of the parts of the future head may be recognized.
Disk-shaped thickenings in the hinder widened part of the brain-appendage form the rudiments of the compound eyes, which therefore may be called the eye-buds. On the basal surface of the eye-buds is situated a nervous expansion which is connected by a nerve with the supraœsophageal ganglion. This nerve becomes the optic nerve of the perfect animal, while the optic ganglion is clearly separated from the brain.
In the anterior, more cylindrical or tube-like part of the brain-appendage we find the “frontal buds” (ss), on which the antennal rudiments (at) soon bud out, in exactly the same way as the rudiments of the limbs arise from the imaginal buds.
Fig. 628.—Diagrammatic representation of the position of the imaginal buds in the larva (A) and pupa (B) of Musca (the wing rudiments omitted): as, eye-buds; at, antennal germs; b1-b3, germs of the legs; bg, central ganglionic cord: g, brain; h, so-called frontal appendage (Hirnanhang): m, peripodal membrane; o, opening of the frontal appendage into the pharynx; oe, œsophagus; p, so-called “pharynx”; r, rudiment of the proboscis; ss, frontal bud; st, stalk-like connection of the peripodal membrane with the hypodermis; I-III, 1st, 2d, and 3d thoracic segments.—Adapted from Van Rees, by Korschelt and Heider.
Originally (Fig. 628, A) the brain-appendages lie tolerably far behind in the thorax of the larva, so that they connect the hindermost part of the wall of the pharynx with the foremost section of the brain, which they surround in the form of a mushroom. Afterwards, however, subsequent to pupation, they move, together with the central nervous system, farther forward (B), whereby they (if we have correctly understood the descriptions of Weismann and Van Rees) laterally surround the pharynx with their anterior end, which is somewhat ventrally bent. At the same time, there becomes established a gradually widening communication (B, o) between the brain-appendage and the pharynx, which soon extends in the form of a lateral pharyngeal fissure along the entire length of the brain-appendage. As a result, the cavity of the brain-appendage and the pharynx so completely unite that the two soon form a single sac, the head-sac or vesicle (Fig. 6–9, k). The walls of this head-vesicle are the later head-wall, the most important parts of which can now be recognized (the antennæ, eyes, rudiments of the beak). It is now necessary that the head-vesicle (Fig. 629, +, +) be, by the eversion of the pharynx, turned outward in order that the head of the pupa may be completed. By this eversion of invaginated parts, the former mouth-opening of the pharynx becomes a neck-section (Fig. 629, +, +) by which head and thorax are now united. (Korschelt and Heider.)
Fig. 629.—Diagram of the changes to pupa of Musca before imago appears; the wing-germs not drawn: as, eye-buds; at, antennal germs; b1-b3, leg-germs; bg, ventral nerve-cord; g, brain; k, head-vesicle (originating from the union of the pharynx with the hypophysis, Hirnanhängen); oe, œsophagus; r, germ of the proboscis; ss, germ of the forehead; I, II, III, 1st, 2d, and 3d thoracic segments.—Based on Kowalevsky and Van Rees, with changes, after Korschelt and Heider.
The cause of the eversion of the head-vesicle, which Weismann directly observed, appears to be due to an increase of the inner pressure through a contraction of the hinder parts of the body. The anterior end of the œsophagus now becomes turned down ventrally corresponding to the conformation of the head of the imago.
It has been shown that the so-called pharynx is only an invaginated part of the outer surface of the larval head. The brain-appendage Korschelt and Heider consider to be the diverticulum of this invagination, in which the single parts of the body lie in an invaginated state. They may throughout be compared to the rudiments of the thoracic limbs. All these imaginal buds have been traced back to the invaginated parts of the outer surface of the body, i.e. the ectoderm.
It should be borne in mind that the process of development of the head of the highly-modified Muscidæ is much more complex than in the more primitive Diptera.
In their essay on the development of the head of the imago of Chironomus, Miall and Hammond arrange the dipterous types thus far examined, in the order of complexity of the invaginations which give rise to the head of the imago, in the following order:—
1. Culex. Relatively simple. Invaginations of the imaginal buds, shallow. 2. Corethra, Simulium. } Intermediate. 3. Chironomus, Ceratopogon. } 4. Muscidæ. Relatively complex. Invaginations deep, and apparently, but not really, unconnected with the epidermis.
b. Development of the internal organs of the imago
Fig. 630.—Median longitudinal section through larva of blow-fly during the process of histolysis: an, antenna; between an and w, rudiments of eye; w wings; h, halteres; b1-b3, legs; f, fat-body; d, middle of intestine; n, ganglia; st, stigma; 6, 7, 6th and 7th body-segments.—After Graber, from Sharp.
It has already been observed that most of the organs of muscid larvæ (and this applies to most Diptera, Lepidoptera, Coleoptera, and Hymenoptera) are destroyed through the action of leucocytes, and that their reformation is accomplished by definite groups of embryonal cells, the imaginal buds or folds. Destruction and rebuilding occur during the pupa stage in such a way that in many cases while this process is going on the continuity of the organs does not seem to be disturbed. These transformations especially concern the hypodermis, the digestive canal, the muscles, the fat-body, and the salivary glands.
The transformation of the tracheal system is only partial, being in part a simple process of regeneration through cell-division. Slighter changes affect the heart, the central nervous system, and the reproductive system (Fig. 630).
Fig. 631.—Diagram of the formation of the imaginal hypodermis on the abdomen of Muscidæ: hi, imaginal buds of the hypodermis; lh, larval hypodermis.—After Lang.
Fig. 632.—Section through the abdominal bud of the hypodermis of Musca: A, of the larva; B and C, of the pupa; h, larval hypodermis; h′ separated portion of the same attacked by phagocytes; i, imaginal bud; k, phagocytes with what are called cell-wrecks or fragments (so-called granulated cells); k′, phagocytes enclosing hypodermal nuclei; m, mesoderm-germ of the imaginal bud; w, wandering cells.—After Kowalevsky, from Korschelt and Heider.
The hypodermis.—The hypodermis of the imago arises through an extension of the ectodermal part of the imaginal buds. We have already mentioned this for the thorax. As the appendages of the thorax in the pupa gradually attain perfection, the hypodermis layer spreads from the place of their insertion, the layer consisting of numerous small cells whose origin we must refer to the peripodal membrane. This layer continues to spread over the surface of the pupal thorax, while at the same time the area of the larval hypodermis, consisting of large cells, is seen to diminish. Hence the thin edge of the newly-formed hypodermis (Fig. 631, hi) slowly grows into the space between the superficial cuticula and the larval hypodermis (Fig. 632, h), so that at this place the old hypodermis undergoing destruction eventually lies on the inner side of the newly-formed epithelial layer (B). We therefore see from this that, during the replacement of the old hypodermis by the new, the continuity of the superficial epithelium is never interrupted. Since the edges of the two kinds of hypodermis overlap, the surface of the body is nowhere bare of epithelium. The dissolution of the larval hypodermis is accomplished under the influence of the leucocytes (Fig. 632, k), which attack the larval hypodermis-cells and absorb their contents piece by piece, and so fill themselves with bits of the hypodermis-cells and their nuclei; since these fragments have the shape of roundish granules, they were called by Weismann granule-balls. These granule-balls, which fill the body-cavity of the later pupal stage, are nothing else than the leucocytes (blood corpuscles) which have absorbed the fragments of tissue of the larval body.
It should here be said that the destruction of the larval tissues is not to be attributed to the previous death of the cells, but is the result of the action of the leucocytes on tissues which, though weakened in their vital power, are still living. While the completely healthy, active tissues, i.e. those of the imaginal buds, withstand the attacks of the leucocytes, the less healthy larval tissues are by the attacks of the leucocytes divided into fragments and eaten and digested by them. This process is most marked in the histolysis of the larval muscles. The destruction of most of the larval organs depends, therefore, on the capacity of the amœboid blood-corpuscles for taking food and on intracellular digestion, as was first shown by Metschnikoff, who has given to these leucocytes the name of “phagocytes.”
This process of histolysis goes on in the same way in the head and abdomen as in the thorax. In the abdomen, as Ganin first proved, there are in each of the eight segments of which it consists in the larva four small cellular islets or imaginal buds (Figs. 631, hi, 632, i), from which originate the new hypodermis.
Van Rees has lately found in the abdominal segments another pair of smaller imaginal buds. The four imaginal buds occurring in the last segment are situated close to each other, encircling the anal opening (Fig. 633, ims), and take part in the formation of the hind-intestine, the rectal pouches and rectal papillæ. To this segment also belong the two pairs of imaginal genital buds (rudiments of the external sexual organs) which were first found by Künckel d’Herculais in Volucella.
The newly formed hypodermis spreads rapidly over the outer surface of the body, so that hypodermal areas corresponding to the separate imaginal buds soon unite. Simultaneously with this completion of the definite epithelial layer the larval hypodermis becomes completely destroyed by the phagocytes.
The muscles.—A similar process of destruction by phagocytes affects the greater number of the larval muscles, except the three pairs of thoracic muscles employed in respiration, and which pass intact from the larva to the imago. Indeed, the dissolution of the muscles is the first process which occurs in the metamorphosis. The destruction of the larval muscles is accomplished in such a way that, a great number of leucocytes which have collected on the surface of the muscular fibres, press through the sarcolemma and enter within the muscular tissue, filling the spaces formed between them, By this means the muscles break up into a number of rounded particles which are taken into the interior of the leucocytes. Thus a collection of granule-balls arise from the muscles, which finally separate from each other and become scattered throughout the body-cavity of the pupa. In the same way as the muscular substance, the muscle-nuclei are taken up and digested by the phagocytes.
The imaginal muscles develop from the definitive mesoderm which has originated from the mesoderm of the imaginal buds (Fig. 632, C, m).
The digestive canal.—As in the hypodermis and muscles, the histolysis of the larval digestive tract and its new formation from separate imaginal buds go on simultaneously, so that the continuity of the process is not interrupted.
Fig. 633.—Digestive tract of a Musca larva with the imaginal germs: bd, cœca; s, food-reservoir; is, imaginal ring of the salivary gland (sp); f, fat-cells at the end of the salivary gland; pr, proventriculus; r, its ring; ie, imaginal cells of the mid-intestinal epithelium; ch, chyle-stomach; ma, urinary tubes; im, imaginal cells of the mid-intestinal, muscular layer; ims, binder, abdominal, imaginal buds; h, hind intestinal, imaginal bud; ht, hind-intestine.—After Kowalevsky, from Korschelt and Heider.
The imaginal buds of the much-shortened pupal digestive canal occur in the mid-intestine (stomach) in the form of numerous scattered groups of cells (Fig. 633, ie), and in the fore- and hind-intestine in the form of rings (v and h) of imaginal tissue. The imaginal ring of the fore-intestine (v) lies in the region of the proventriculus (pr, compare Fig. 635, im), while that of the hind-intestine is situated directly behind the base of the urinary tubes. The regeneration of these two parts of the digestive canal is not entirely accomplished by these two rings, but the imaginal rudiments of the neighboring parts of the outer surface of the body also have a share in it. Thus it appears that the foremost part of the œsophagus is built up from the imaginal buds in the region of the mouth, while the imaginal buds surrounding the anus in the 8th abdominal segment (Fig. 633. ims) produce by invagination the rectal pouches, together with the rectal papillæ.
Fig. 634.—Cross-section through the mid-intestine of pupa of Musca: e, rejected and degenerate epithelium of the larval stomach; f, cellular layer newly formed around the same; m, muscular layer; m′, imaginal cell of m; o, imaginal bud of the mid-intestinal epithelium; t, tracheal stem.—After Kowalevsky, from Korschelt and Heider.
Fig. 635.—Longitudinal section through the proventriculus of a muscid larva: im, fore-intestinal, imaginal ring; oe, œsophagus; pr, proventriculus.—After Kowalevsky, from Korschelt and Heider.
The formation of the mid-intestine (stomach) takes place in such a way that the island-like imaginal buds spread out by cell-multiplication over the outer or basal surface of the larval mid-intestinal epithelium (Fig. 634, o), until they finally unite, so as to form the wall of the imaginal mid-intestine (stomach). At the same time the entire larval epithelium (e) is cast in the interior and forms the so-called yellow body, which becomes surrounded by a layer of small cells and a jelly-like mass, and remains until its destruction in the pupal stomach. The larval muscular layer (m) remains intact as long as the imaginal mid-intestine is not fully developed, when it is attacked and destroyed by phagocytes. The final muscular layer arises from single cells lying on the outer surface of the imaginal buds (Figs. 633, im, 634, m′), which should be regarded as special imaginal cells of the mid-intestinal muscular layer.
The transformation of the fore-intestine is introduced by a degeneration of the proventriculus and sucking stomach. The proventriculus (Fig. 635, pr), which had been formed from a circular fold of the fore-intestine, disappears by the smoothing out of this folded structure. The sucking stomach also similarly degenerates by withdrawing gradually into the œsophagus, so that instead of the original diverticulum there remains only an enlargement of the œsophageal cavity. At the same time this part of the canal is attacked and destroyed by phagocytes, while the destroyed portions become replaced by the gradually extending imaginal parts of the wall. The imaginal ring of the fore-intestine (Fig. 635, im), which, according to Kowalevsky, is concerned in the formation of a great part of the definitive œsophagus, becomes closed at its hinder end so that the communication with the mid-intestine appears to be interrupted.
The hind-intestine of the imago is rebuilt in exactly the same manner. Here also the imaginal ring widens and forms a tube, which while it grows around the openings into the urinary tubes, closes itself against the mid-intestine, while behind it remains in connection with the larval hind-intestine. In a similar way the larval hind-intestine is attacked by the growth from behind of an imaginal ring, which proceeds from imaginal buds near the anus, until finally, when the entire larval hind-intestine is reduced to granule-balls, the two imaginal sections of the tube are brought into contact with each other. (Kowalevsky in Korschelt and Heider.)
The larval salivary glands (Fig. 633, sp) are completely destroyed by phagocytes. Then succeeds the new formation of these glands from imaginal buds, which, according to Kowalevsky, form rings situated at their anterior ends.
The nature of the transformation undergone by the urinary tubes is not yet well ascertained. According to Van Rees, there is in this case perhaps a regeneration of the larval cells by division, but on the other side there may be a histolysis of these elements.
The above-described method of transformation of the digestive canal seems, according to Korschelt and Heider, to be very common among the holometabolic insects. It has not only been observed in the Diptera, but also in the Lepidoptera (Kowalevsky, Frenzel), Coleoptera (Ganin), and Hymenoptera (Ganin). The stripping off of the epithelium of the mid-intestine was found by Kowalevsky to occur also in Corethra, Culex, and Chironomus.
The tracheal system.—As we have seen (p. 448), the tracheal system of caterpillars just before pupation undergoes disintegration, accompanied by a reformation of the peritoneal membrane and tænidia. The larval ectotrachea undergoes histolysis, that of the imago being meanwhile formed; the larval tænidia also break up, dissolve, and are replaced by new tænidia which arise from the nuclei of the peritoneal membrane. That the tracheal system in the Muscidæ during metamorphosis undergoes a transformation is shown, as Korschelt and Heider claim, by the entirely different shape of the system in the maggot, the pupa, and the fly. The air is admitted to the tracheal system of the maggot, not by lateral openings, but through the two stigmata at the end of the body. On the other hand, the pupa breathes by prothoracic spiracles, while the fly has six pairs of lateral stigmata of the normal structure. There may be in the larva and pupa vestigial closed stigmata, as there are in the thorax of caterpillars, with tracheal branches leading to where were once functional stigmata. These stigmatal branches, as well as some other portions of the tracheal system already observed by Weismann, seem, according to Van Rees, to function as imaginal buds for the regeneration of the tracheal matrix, while frequently also a regeneration of this epithelium, by a simple repeated division of cells, may be recognized. The disintegration of the tracheal system is accomplished by means of phagocytes in the manner already described.
The nervous system.—The central nervous system passes directly from the larval into the imaginal stage, since it must continue to exercise most of its functions throughout metamorphosis, though it undergoes important changes of form and position. At the same time, certain histological transformations occur which may be regarded as a histolysis. Such is the destruction and rebuilding in the interior of the organs, which, however, preserve their continuity. Every case of destruction of tissues in the pupa has come to be regarded as a histolysis.
The problem of the transformation of the peripheral nervous system is not yet well understood. Although during the destruction of the larval muscles the motor nerves also in part degenerate, in the case of the nerves distributed to the appendages the conditions are different, as these may be recognized in the larva in the form of the nerve-threads which place the imaginal buds in connection with the central nervous system. These threads, according to Van Rees, pass from the larva into the pupa and imago, so that with the farther development of the rudiments of the extremities, only the distal part of the nerves belonging to them are to be regarded as new formations. (Korschelt and Heider.)
The fat-body.—The larval fat-body is also destroyed through the activity of the leucocytes in the same way as the other tissues. The reformation of the fat-body seems to begin in the mesoderm of the imaginal buds. Possibly, also, the masses or collections of embryonic cells which are regarded by Schaeffer as “blood-forming cells,” may serve to regenerate the fat-body. At all events, they have been derived from the mesodermal tissues. Though Wielowiejsky saw the fat-body of Corethra arising from a cell-layer situated under the hypodermis, yet it is not necessary to regard this observation as favorable to the view of Schaeffer that in Musca the larval fat-body is derived in part from the hypodermis, and in part from the tracheal matrix, thus from the ectodermal tissues. (Korschelt and Heider.)
Definitive fate of the leucocytes.—We have seen that the formation of the organs of the imago originates in the imaginal buds, in all cases where these do not pass directly from the larva into the pupa. The leucocytes, whose numbers in the pupa are greatly increased, take no direct part in the formation of the tissues. Their importance seems to lie in this, that they destroy those larval organs doomed to destruction, the parts of which they take in and digest, and possibly, by their powers of locomotion, convey particles of food to the developing organs.
What, on the other hand, is the fate of the leucocytes after the developmental processes in the pupa have ceased? There can be no doubt that a part of the so-called granule-cells are again transformed into normal blood-corpuscles. Another, and, as it seems, more considerable, share suffer degeneration. Finally, the leucocytes themselves serve as nourishment for the newly formed tissues. Of interest in this direction is the observation of Van Rees, that numerous leucocytes finally pass into the newly-formed hypodermis and then degenerate in crevices between the hypodermis-cells. (Korschelt and Heider.)
It has been suggested by Van Rees that the phagocytes attack all the larval organs indiscriminately, but that the active metabolism of the imaginal buds preserves them from these attacks. He also thinks that Kowalevsky is probably right in supposing that the buds render themselves immune by some poisonous secretion.
Pratt, however, thinks that the supposition of a protecting or poisonous secretion is scarcely necessary to account for the phenomenon, and suggests that the larval tissues are a prey to the phagocytes, because at the end of larval life they become functionless and inactive, so as to become an easy prey to phagocytes or disintegrating influences of any sort. On the other hand, the imaginal buds “in which there is an exceedingly active metabolism, and all the larval organs which remain functional during the metamorphosis are immune from the attacks of the phagocytes. The heart in the muscids continues to beat, as Künckel d’Herculais has observed, during the entire period of the metamorphosis, with the exception of a day or two in the latter half of it. The nervous system must continue functional during the entire time. The three pairs of thoracic muscles which pass intact from the larva to the imago are probably employed in respiration during the metamorphosis. The reproductive glands are, like the imaginal disks, rapidly growing organs.” He adds that among the other holometabolic insects many or most of the larval organs remain functional during metamorphosis, hence there is but little histolysis. “But the larval intestine would always necessarily become unfunctional, and, as we have seen, Kowalevsky is of the opinion that the larval mid-gut in all holometabolic insects contains imaginal disks, and undergoes degeneration during the metamorphosis.”
The post-embryonic changes and imaginal buds in the Pupipara (Melophagus).—The sheep-tick (Melophagus) is still more modified than the Muscidæ; the larva is apodous and acephalous, but, as Pratt observes, much less highly specialized than those of muscids, and in respect to the position of the thoracic buds it closely resembles Corethra. They lie just beneath the hypodermis in two very regular rows, and not in the centre of the body, as in Musca (Figs. 628, 636, C). While, however, in Corethra all the thoracic buds are of larval origin, arising after the last larval moult, in Melophagus, on the other hand, each of these buds, except the dorsal prothoracic, arises in the embryo, as is also the case in Musca.
In the cephalic buds the conditions are similar to those in Musca, but still more complicated. Instead of a single pair of head-buds, there are two pairs, one dorsal and one ventral. “The dorsal pair corresponds to the muscidian head-disks in every respect; they are destined to form the dorsal and lateral portions of the imaginal head, together with the compound eyes. The ventral head-disks have no counterpart in Musca. The fate of these disks or buds is to form the ventral portion of the head, the paired projections forming the rudiments of the proboscis.
“The formation of the head-vesicle proceeds in a way similar to that in Musca. The ventral disk fuses early at its lateral edges with the dorsal pair. The communications between both ventral and dorsal disks and the pharynx rapidly widen (in the old larva they have already become very large), and soon the disks and pharynx form together a single vesicle, which is the head-vesicle.” The imaginal buds of the abdomen Pratt finds to be exactly as in the corresponding ones of Musca.
In the embryo of Melophagus the cephalic and thoracic imaginal buds first appear as local thickenings, followed by the invagination of the ectoderm; the cephalic buds first appear very early in the ontogeny of the insect (Fig. 636, C), just as the germs of the digestive canal, nervous system, and tracheæ are appearing. The single median thickening (v) is destined to form the ventral cephalic bud, while the pair of thickenings behind (d) become the dorsal buds, those homologous with the cephalic buds of Musca.
Fig. 636.—Imaginal buds in Musca,—A, in Corethra,—B, in Melophagus,—C, in embryo of Melophagus: dorsal view of head; b, bud; p, peripodal membrane; c, cord; hy, hypodermis; cl, cuticula; st, stomodæum; v, ventral cephalic bud, behind are the two dorsal cephalic buds (d).—After Pratt.
The thoracic buds, which arise as hypodermic thickenings, do not appear until late in embryonic life, until the time of the involution of the head.
Pratt did not observe in the embryo the buds of the internal organs and of the abdominal hypodermis, and thinks it probable that they appear first in the larva.
c. General summary
We have seen that in Coleoptera, Lepidoptera, Diptera, and Hymenoptera, and with little doubt in all the holometabolous insects, the parts of the imago originate in single formative cellular masses (imaginal buds) already present in the larva, and often even in the later embryonic stages. There are such imaginal buds for each part of the body,—for the appendages of the head, for the legs and wings, for the ovipositor, and probably for the cercopods, for the hypodermis of the abdomen, and for the different sections of the digestive canal. We have seen, as Korschelt and Heider state, that the formation of the mesodermal organs of the imago (muscles, connective tissue, fat-body) begins in the mesodermal part of the imaginal buds, whose first origin is still obscure. Simultaneously with the formation of the imaginal organs, there goes on under the influence of the leucocytes the destruction of the larval organs. Both processes (destruction and regeneration) therefore go on hand in hand, so that the continuity of the organs in question in most cases remains perfect, inasmuch as the complete destruction only ensues after the formation of the final organs. The only exceptions are most of the muscles of the larva, which are destroyed at a very early period.
Moreover, it is evident that the sharp division into larval, pupal, and imaginal stages only applies to the external surface of the body, since they follow one another after successive moults. The processes of the internal development, on the other hand, form an entirely continuous series of transformations between which is no sharp line of demarcation. Yet as a whole the form of the larva, pupa, and imago are kept distinct in adaptation to their separate environments and habits.
Finally, as Pratt very truly remarks, the epigenetic period in insects, when new organs are forming, does not end with the birth of the larva from the egg, but extends through the larval, and even through the pupal period. “The principal significance of the pupal period and the metamorphosis is that it is the time when the larval characters which were adapted for use during a period of free life in the midst of the development, and which would be valueless to the imago, are corrected or abandoned.”