INSECTS 



{AD, the first four pairs being on the third, fourth, fifth 

 and sixth segments, and the last on the tenth segment. 

 The region ot the three segments in the caterpillar bear- 

 ing the jointed legs corresponds with the thorax ot an 

 adult insect (Fig. 63, 77?), and that following corresponds 

 with the abdomen {Ab). The thorax of the adult insect 

 constitutes the locomotor center of the body, but the 

 wormlike caterpillar has no special locomotor region, and 

 hence its body is not separated into thorax and abdomen. 

 The thoracic legs of the caterpillar terminate each in a 

 single claw, but the foot of each of the abdominal legs 

 has a broad sole provided with a series or circlet of claws 

 and with a central vacuum cup. The abdominal legs 

 of the caterpillar, therefore, are important organs of pro- 

 gression, and are the chief organs of grasping or of cling- 

 ing to hard or flat surfaces. 



The jaws of the caterpillar consist of a pair of large, 

 strong mandibles (Fig. 152, Md) concealed, when closed, 

 behind the labrum. Each jaw is hinged to the lower 

 edge of the cranium at the side of the mouth by two ball- 

 and-socket hinges in such a manner that, when in action, 

 it swings outward and inward on a lengthwise axis. The 

 cutting edges are provided with a number of strong teeth 

 (Fig. 153), the points of which come together or slide 

 past each other when the jaws are closed. 



The large complex organ that projects behind or below 

 the mouth like a thick under lip (Fig. 152 C) is a com- 

 bination of three parts that are separate in other insects. 

 These are the second pair of soft jaw appendages, called 

 maxillae (B, C, Mx), and the true under lip, or labium 

 {Lb). The most important part of this composite struc- 

 ture in the caterpillar, however, is a hollow spine (A, 

 B, C, Spt) pointed downward and backward from the 

 end of the labium. This is the spinneret. From it issues 

 the silk thread with which the caterpillar weaves its tent 

 and its cocoon. 



[286] 



