The integument consists of a cellular layer, usually called the hypodermis, situated on a basement membrane. The hypodermis, or layer of chitinogenous cells, excretes a matter which remains attached to the body, forming the hard outer layer of the skin. This layer consists of chitin and has no vitality, but its presence no doubt exerts a very important influence on the physiological processes of the Insect. The chitinous investment varies much in thickness and in other properties; in some Insects it is hard, even glassy, so as to be difficult to pierce with a pin, in others it is pliable, and in some very delicate. Chitin is a substance very difficult to investigate; according to the recent researches of Krawkow[91] it may prove to be of somewhat variable chemical composition. After a time the hypodermis excretes a fresh supply of chitin, and, possibly by the commencement of this process, the older chitinous investment becomes separated and is shed. The details have, however, not been ascertained, though their importance has been suggested by Hatchett Jackson.[92] The newly exposed layer of integument is pallid, but afterwards becomes coloured in a manner varying according to the species, the process being possibly due to some secondary exudation permeating the freshly exposed chitin, or modifying some part of its exterior. Lowne informs us that in the imago of the blowfly the great majority of the hypodermic cells themselves enter into the composition of the chitinous integument; and it is perhaps not a matter for surprise that the cells should die on the completion of their functional activity, and should form a part of the chitinous {163}investment. Some writers say that the chitinous layer may be shown to be covered by a delicate extima or outer coat. 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.