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.
Fig. 12.—Cast skin of older nymph (“pupa”). × 2 1/2.
Prolonged boiling in caustic potash, though it dissolves the viscera, does not disintegrate the exoskeleton. This shows that the segments of the integument are not separate chitinous rings, but thickenings of a continuous chitinous investment. Nevertheless, their constancy in position and their conformity in structure often enable us to trace homologies between different segments and different species as certainly as between corresponding elements of the osseous vertebrate skeleton.
Parts of a Somite.
Audouin’s laborious researches into the exoskeleton of Insects[37] resulted in a nomenclature which has been generally adopted. He divides each somite (segment) into eight pieces, grouped in pairs—viz., terga (dorsal plates), sterna (ventral plates), epimera (adjacent to the terga), and episterna (adjacent to the sterna).
While admitting the usefulness of these terms, we must warn the reader not to suppose that this subdivision is either normal or primitive. The eight-parted segment exists in no single larval or adult Arthropod. Lower forms and younger stages take us further from such a type, instead of nearer to it; and Audouin’s theoretical conception is most fully realised in the thorax of an adult Insect with well-developed legs and wings.
The morphologist would derive all the varieties of Arthropod segments from the very simple and uniform chitinous cuticle found in Annelids and many Insect-larvæ. This becomes differentiated by unequal thickening and folding in, and a series of rings connected by flexible membranes is produced. Locomotive and respiratory activity commonly lead to the definition of terga and sterna, which are similarly attached to each other by flexible membranes. A pair of limbs may next be inserted between the terga and sterna, and the simple segment thus composed occurs so extensively in the less modified regions and in early stages that it may well be considered the typical Arthropod somite.
Special needs may lead to the division of the sterna into lateral halves, but this is purely an adaptive change. The third thoracic sternum of the male Cockroach, and the second and third of the female are thus divided, as is also the hinder part of the seventh abdominal sternum of the female.