THE ANIMAL KINGDOM 



425 



The mouth parts conform to two general types, viz., biting 

 and sucking. In the biting type there is an upper lip or labrum 

 and a lower lip or labium, each of which is a chitinous plate. 

 The labium bears a pair of palps. Inserted laterally between 

 the lips are a pair of maxillae and a pair of mandibles (Fig. 247). 

 Each maxilla consists of a forked lacinia and a spoon-shaped 

 galea, and also bears a single palp. The palps are not only 

 tactile but also contain organs of taste and smell. Typical 

 mandibles are hard resistant plates of irregular outline and 

 edged with sharp points. Both maxillae and mandibles operate 

 with a side-to-side motion. The mandibles are the chewing 

 organs, the maxillae serving with 

 the lips to hold and manipulate the 

 food. In sucking insects such as 

 the mosquito the mouth parts are 

 shaped to form a slender beak 

 (Fig. 247B). The elongate max- 

 illae and mandibles lie in a sheath 

 formed by the labium, the labrum 

 being reduced to a small plate at 

 the upper side of the base of the 

 beak. There are variations of 

 these two types, combining ^Xnter. 

 features of both. 



On the head there are usually two kinds of eyes, simple and 

 compound, and a pair of segmented antennae. The compound 

 eye has been described (p. 192). The simple eye or ocellus is 

 .much smaller and consists of a spheroidal mass of light-sensitive 

 cells covered externally by a lens-shaped thickening of the 

 integument. 



Each leg, beginning at the proximal end, consists of five 

 parts: (1) a short coxa, (2) a short trochanter, (3) a long, broad 

 femur, (4) a slender tibia, and (5) a foot or tarsus, composed of 

 several segments (Fig. 248). The insect wing is without a fore- 

 runner in the preceding groups of animals. Since primitive 

 insects are wingless, the insect wing must be a structure that 

 originated in the insect group. The insect wing is a chitinous 

 plate, usually very thin and flexible, and reinforced by "veins" 

 composed of branches of the tracheal system. In beetles the 

 anterior wings are thickened and serve as a protective covering 



Fig. 248. — Parts of metathora- 

 cic leg of grasshopper, c, coxa; 

 f, femur; ta, tarsus; ti, tibia; tr, 



