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.