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 {164}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.