INSECTS. 237 



shown by the appendages. The antennae, of which there 

 are only a single pair, are sensory in function. In many 

 cases they clearly bear organs of smell, and in some they 

 may also be hearing-organs. In the primitive condition 

 the mouth-parts are fitted for biting and eating hard sub- 

 stances, the mandibles being strong jaws, while the max- 

 illae and labium serve to hold the food in place. These 

 latter bear jointed prolongations the palpi which are 

 sensory. In other insects these mouth-parts are modified 

 and united into a sucking-tube which frequently is a 

 piercing-organ of no mean capabilities. This sucking-tube 

 is variously constituted in the different orders of hexa- 

 pods. Usually the labium forms the chief part of the 

 organ, the other parts (mandibles and maxillae) playing 

 inside the labial tube, but in the butterflies the tube is 

 formed from the maxilloe, the other parts being greatly 

 reduced . 



The thorax is composed of three segments, named, from 

 in front backwards, the prothorax, mesothorax, and meta- 

 thorax. Of these the first is frequently movable upon the 

 next. Each segment bears a pair of legs, made up of 

 several joints, the number varying according to the num- 

 ber in the foot (tarsus), the rest of the member usually 

 consisting of four joints. On the dorsal surface of the 

 meso- and metathorax occur the wings, the characters of 

 which are largely used in the classification of insects. 



/ 



They are entirely lacking in the lowest insects (Thysanu- 

 ra) as well as in individuals of other groups, as worker 

 ants, many parasites, and the females of certain moths. 

 In the flies the posterior wings are greatly reduced, so 

 that they appear like a pair of knobbed hairs, termed 

 ' balancers,' since if they be removed the fly cannot con- 

 trol its motions. Frequently both pairs of wings are 



