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.