414 THE STUDY OF INSECTS. 



these rudimentary wings is not known ; but they doubtless 



have some important use, for they are present in nearly all 



flies, even when the front wings are wanting. 



The mouth-parts of flies are formed for sucking, and 



sometimes also for piercing. Their structure differs greatly 



in different families; and 

 in some cases it is exceed- 

 ingly difficult to determine 

 the correspondence of the 

 different parts. In the 

 more typical forms the 

 mouth-parts consist of six 

 bristle -like or lance -like 

 organs enclosed in a sheath, 

 and a pair of jointed palpi. 

 A difference of opinion 

 exists as to the corre- 



Fig. 48 2 .-a crane-fly, showing whTgs and spondence of these parts ; 

 halteres - but according to the most 



generally accepted view the six bristles represent the 

 upper lip (labrum), the tongue (Jiypopharynx), the two man- 

 dibles, and the two maxillae, and the sheath enclosing these 

 bristles is the lower lip (labium). The palpi which are not 

 enclosed in the sheath are the maxillary palpi. At the tip 

 of the lower lip there is, on each side, a lobe-like appendage ; 

 these are the labial palpi. The labial palpi of certain flies 

 are quite large ; in the House-fly, for example, they are ex- 

 panded into broad plates, which are fitted for rasping. 



In their transformations flies pass through a complete 

 metamorphosis. The larvae are commonly called maggots. 

 These are usually cylindrical and are footless ; some possess 

 a distinct head, others do not ; the form of the mouth-parts 

 varies greatly in the different families; and there are re- 

 markable variations in the form of the respiratory organs, 

 especially as to the number and position of the spiracles. 

 The pupae are usually either naked or enclosed in the last 



