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? [Illustration: FIG. 123.—MOUTH PARTS OF BEETLE. ] [Illustration: FIG. 124.—EXTERNAL PARTS OF A BEETLE. ] The chest, or =thorax=, consists of three rings (Fig. 124) called the front thorax (prothorax), middle thorax (mesothorax) and hind thorax (metathorax), or first, second, and third rings. The first ring bears the first pair of legs, the second ring bears the second pair of legs and the upper or front wings, and the third ring bears the third pair of legs and the under or hind wings. [Illustration: FIG. 125.—LEG OF INSECT. ] The =six feet= of insects are characteristic of them, since no other adult animals have that number, the spider having eight, the crayfish and crabs having ten, the centipedes still more, while birds and beasts have less than six. Hence the insects are sometimes called the Six-Footed class (_Hexapoda_). The insects are the only animals that have the =body= in =three divisions=. Man, beasts, and birds have only two divisions (head and trunk). Worms are not divided. =Define= the class _insecta_ by the two facts characteristic of them (_i.e._ possessed by them alone), viz.: Insects are animals with ———— and ————. Why would it be ambiguous to include “hard outer skeleton” in this definition? To include “bilateral symmetry”? “Segmented body”? The definition of a class must _include_ all the individuals of the class, and _exclude_ all the animals that do not belong to the class. [Illustration: FIG. 126.—FOOT OF FLY, with climbing pads. ]