[Illustration: Fig. 10.--Diagram of sensory hair of Insect. _Cc_, chitinous cuticle; _h_, hair; _c_, its generating cell; _g_, ganglion cell; _bm_, basement-membrane.] The chitinous cuticle is often folded in so as to form a deep pit, which, looked at from the inside of the body, resembles a lever, or a hook. Such inward-directed processes serve chiefly for the attachment of muscles, and are termed _apodemes_ (_apodemata_). A simple example is afforded by the two glove-tips which lie in the middle line of the under-surface of the thorax (p. 58, and fig. 27). In other cases the pit is closed from the first, and the apodeme is formed in the midst of a group of chitinogenous cells distant from the superficial layer, though continuous therewith. Many tendons of insertion are formed in this way. The two forked processes in the floor of the thorax (p. 58, and fig. 27) are unusually large and complex structures of the same kind. In the tentorium of the head (p. 39, and fig. 17) a pair of apodemes are supposed to unite and form an extensive platform which supports the brain and gullet. [Illustration: Fig. 11.--Nymph (in last larval stage) escaping from old skin. × 2-1/2.] Like other Arthropoda, Insects shed their chitinous cuticle from time to time. A new cuticle, at first soft and colourless, is previously secreted, and from it the old one gradually becomes detached. The setæ probably serve the same purpose as the “casting-hairs” described by Braun in the crayfish, and by Cartier in certain reptiles,[35] that is, they mechanically loosen the old skin by pushing beneath it. In many soft-bodied nymphs the new skin can be gathered up into a multitude of fine wrinkles, which facilitate separation, but we have not found such wrinkles in the Cockroach, except in the wings. The integument about to be shed splits along the back of the Cockroach, from the head to the end of the thorax,[36] and the animal draws its limbs out of their discarded sheaths with much effort. It is remarkable that the long, tapering, and many-jointed antennæ are drawn out from an entire sheath. At the same time the chitinous lining of the tracheal tubes is cast, while that of the alimentary canal is broken up and passed through the body. [35] A condensed and popular account of these researches will be found in Semper’s Animal Life, p. 20. [36] Prof. Huxley (Anat. Invert. Animals, p. 419) states that the integument splits along the abdomen also, but this is a mistake. [Illustration: Fig. 12.--Cast skin of older nymph (“pupa”). × 2-1/2.]