This arrangement of parts holds with the appendages of the body of an insect as well as with the body itself; the legs, mouth-parts and antennæ are all tubular organs, having a firm outer skeleton supporting the inner parts.
Movement of the body and its appendages is provided for by narrow, flexible, zone-like areas in the skeleton which encircle the body and the appendages, at frequent intervals. This segmented condition of the body is easily seen in the hind part or abdomen, which appears to consist of a series of rings.
The microscopic structure of the body-wall is comparatively simple. There is an inner cellular part which consists of a single layer of cells: this is the hypodermis (Plate XXV, 2, h); and the outer or hard part: this is the cuticle (Plate XXV, 2, c).
The hypodermis is the active living part; it produces the cuticle, which receives additions from it constantly during the life of the insect. On this account, when a section of the cuticle is examined with a microscope it presents a layered appearance.
Moulting of the cuticle.—From time to time during the growth of the insect the outer layers of the cuticle are shed; this is known as moulting. After a moult, the inner layers of the cuticle, which have now become the outer layers, but which are still soft, stretch to accommodate the increased size of the body, and then soon become hard. This moulting, or shedding of the skin, takes place about six
