Anatomy and General Characteristics of the Class Insecta
The body of an insect is divided by means of two marked narrowings into three parts: the head, the chest, and the abdomen.
Fig. 122.—Yellow Fever Mosquito, showing head, thorax, abdomen.
The head is a freely movable capsule bearing four pairs of appendages. Hence it is regarded as having been formed by the union of four rings, since the ancestor of the insects is believed to have consisted of similar rings, each ring bearing a pair of unspecialized legs.
The typical mouth parts of an insect (Fig. [123]) named in order from above, are (1) an upper lip (labrum, ol), (2) a pair of biting jaws (mandibles, ok), (3) a pair of grasping jaws (maxillæ, A, B), and (4) a lower lip (labium, m, a, b). The grasping jaws bear two pairs of jointed jaw fingers (maxillary palpi, D, C), and the lower lip bears a pair of similar lip fingers (labial palpi, d). The biting jaws move sideways; they usually have several pointed notches which serve as teeth. Why should the grasping jaws be beneath the chewing jaws? Why is it better for the lower lip to have fingers than the upper lip? Why are the fingers (or palpi) jointed? (Watch a grasshopper or beetle eating.) Why does an insect need grasping jaws?
Fig. 123.—Mouth Parts of Beetle.
Fig. 124.—External Parts of a Beetle.