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 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 "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 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.
Fig. 87.—Imaginal discs of Muscidae in process of development: A, Brain and ventral ganglion of a larva 7 mm. long of M. vomitoria; v, ventral ganglion; c, cephalic ganglion; h, head rudiment; vc, portion of ventral chain; 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 discs from a larva 20 mm. long of Sarcophaga, attached to tracheae; msw, mesonotal and wing-rudiment; mt, metathoracic rudiment: E, r, mesothoracic rudiment of a 7 mm. long larva attached to a tracheal twig. (After Weismann and Graber.)
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.
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.