DIPTERA 



111 



pair being greatly reduced in size. 

 The second pair of wings are 

 known as the halteres, they are 

 thread-Hke, enlarged at the end, 

 and bear organs of special sense, 

 the function of which has not 

 been definitely determined. They 

 are present in nearly all mem- 

 bers of the order, even when the 

 front wings are wanting; they 

 can be easily seen in a crane-fly 



(Fig- 993)- 



The fore wings are thin, mem- 

 branous, and usually either naked 

 or clothed with microscopic setee; 

 but with mosquitoes the wings 

 bear a fringe of scale-like setse on 

 the margin and usually also on 

 each of the wing-veins, and in 



the moth-like flies (Psychodidae) and in some others the clothing of 

 sets is very conspicuous. 



In the more generalized members of this order the venation of 

 the mesothoracic wings corresponds quite closely to the h^^pothetical 

 primitive type. Neither accessory nor intercalary veins are ever de- 

 veloped, and only the principal cross-veins are present. The most 



Fig. 993. — ^A crane-fly, 

 and halteres. 



showing wings 



/?, R., 



idA 

 Fig. 994. — Wing of A Jiisopus. 



striking divergence from this type is the fact that vein M is only 

 three-branched. The wing of Anisopus (Fig. 994) is a good example 

 of a generalized dipterous wing except that the branches of radius 

 have been reduced. In the more specialized forms the typical arrange- 

 ment of the veins has been greatly modified by the approaching and 

 coalescing of the tips of adjacent veins, as shown later. 



