CHAPTEE XXII 



INSEOTA (continued) 

 Order VI. : Diptbra, or Two-winged Flies 



General ^HE Diptera are unlike all other Insects in 

 Character- having only the front pair of wings. These wings 

 istics. ^^g usually membranous and transparent. Behind 

 them is a pair of very small, club-shaped structures, known as 

 "halteres" or "balancers," which 

 appear to represent the second pair 

 of wings usually present in in- 

 sects; these halteres are sometimes 

 covered, or partially covered, by a 

 lobe of the front wing called the 



" squama " (Fig. 2-t5, s). *''°- ^^^-.J^ Uipterous Fly 



The three divisions of the thorax „ 



, . , T . . . 4-, Squama. 



which are distinct in many insects, 



are, in Diptera, fused together in one mass, thus giving a 

 firmer support to the muscles of the wings. The head bears 

 a pair of very large compound eyes, usually larger in the 

 male than the female, and also three small simple eyes 

 or ocelli. Placed close together at the top of the head is a 

 pair of antennae which vary considerably in form among the 

 different families of Diptera, and supply one of the character- 

 istics by which they are distinguished. The mouth-parts are 

 modified for piercing and suction, often forming, as in house- 

 flies, a short trunk or proboscis ; the homology of the various 

 structures within it is, however, very uncertain as yet. 



The metamorphosis is complete and very striking ; the 

 larva is usually a small legless grub known as a maggot, and 

 the pupal stage in many cases is remarkable for the dis- 



327 



