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.