106 STRUCTURAL AND SYSTEMATIC ZOOLOGY 



spines. They have longer antennae than the preceding, 

 and the mouth is armed with two formidable fangs con- 

 nected with poison glands. They*are carnivorous and 

 active. Such is the Centipede (Scolopendra, Fig. 80). 



2. Diplopoda, having a cylindrical body, each segment, 

 except the anterior, being furnished with two pairs of 

 legs. They are slow of locomotion, harmless, and vege- 

 tarian. The thousand-legged worm (Julus) is a common 

 representative. 



CLASS 4. Insecta 



Insects are distinguished by having head, thorax, and 

 abdomen distinct, three pairs of jointed legs, one pair of 

 antennae, and generally two pairs of wings. The number 

 of segments in the body never exceeds twenty. The 

 head, apparently one, is formed by the union of four 

 segments. The thorax consists of three, the prothorax, 

 mesothorax, and metathorax, each bearing a pair of 

 legs; the wings, if present, are carried by the last two 

 segments (Fig. 295). The abdomen is usually composed 

 of ten segments, more or less movable upon one another. 

 The skin is hardened with chitin, and to it, as in all 

 Arthropods, the muscles are attached. All the append- 

 ages are hollow. 



The antennae are inserted between or in front of the 

 eyes. There is a great variety of forms, but all are 

 tubular and jointed. They are supposed to be organs 

 of touch, and seem also to be sensitive to sound and odor 

 (Fig. 344). The eyes are usually compound, composed 

 of a large number of hexagonal corneae, or facets (from 

 fifty in the ant to many thousands in the winged insects) 

 (Fig. 352). They are never placed on movable stalks, as 

 the lobster's. Besides these, there are three simple eyes, 

 called ocelli. The mouth may be fitted for biting (masti- 

 catory), as in beetles, or for sucking (suctorial), as in 



