Page:Insects - Their Ways and Means of Living.djvu/42

 head, a thorax bearing the legs, and a terminal abdomen (Fig. 63). On the head is located a pair of long, slender antennae (Ant) and a pair of large eyes (E). Winged insects have usually two pairs of wings attached to the back of the thorax (W2, W3).

The outside of the insect's body, instead of presenting a continuous surface like that of most animals, shows many encircling rings where the hard integument appears to be infolded, as it really is, dividing each body region except the head into a series of short overlapping sections. These body sections are called segments, and all insects and their relatives, including the centipedes, the shrimps, lobsters, and crabs, and the scorpions and spiders, are segmented animals. The insect's thorax consists of three segments, the first of which carries the first pair of legs, the second the middle pair of legs, and the third the hind pair of legs. The abdomen usually consists of ten or eleven segments, but generally has no appendages, except a pair of small peglike organs at the end known as the cerci, and, in the adult female, the prongs of the ovipositor (Fig. 2 B), which belong to the eighth and ninth segments. The head, besides carrying the antennae (Fig. 63, Ant), has three pairs of appendages grouped about the mouth, which serve as feeding organs and are known collectively as the mouth parts. The presence of four pairs of appendages on the head raises the question, then, as to why the head is not segmented like the thorax and the abdomen. At an early stage of embryonic growth the head is segmented, and each pair of its appendages is borne by a single segment, but the head segments are later condensed