214 ELEMENTS OF COMPARATIVE ZOOLOGY. 



into a sucking- tube which frequently is a piercing- organ 

 of no mean capabilities. 



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. Each 

 segment bears a pair of legs, made up of several joints, the 

 number varying according to the number 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 (Thysanures) as well as in individuals 

 of other groups, as 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 control its motions. Frequently both pairs of 

 wings are used in flight, but in certain groups the front 

 pair are much thickened and hardened, so that they are con- 

 verted into wing-covers (elytra) which protect the hinder 

 wings when at rest. 



The abdomen is normally composed of ten segments, but 

 this number may be reduced. In some insects the ab- 

 domen joins the thorax by its whole width, while in others 

 it is contracted in front to a slender stalk as in the wasps. 

 The appendages of the abdomen, in the adult, are never 

 locomotor in function. In the lowest insects rudimentary 

 appendages may occur on all segments of the abdomen, but 

 in the higher groups only three pairs, at most, occur, and 

 two of these are modified into an organ (ovipositor) for 

 laying the eggs. In the bees, wasps, etc., the ovipositor is 

 at the same time an offensive weapon, the sting. 



The alimentary canal has few convolutions. Into the 



