DIPTERA 



825 



folds in the wing. The significance of these folds is evident when a 

 net-winged midge is observed in the act of issuing from its pupa-skin 



Fig. 1048. — Wing of Blepharocera tenuipes. 

 When the wing is first pulled out of the wing-sheath of the pupa, that 

 part of it which is crossed by the fine lines is plaited somewhat like a 

 fan and folded over the other portion. By this means the \ying, 

 which is fully developed before the adult emerges, is packed within 

 the wing-sheath of the pupa, which is much shorter and narrower 

 than the wing. When the wing is finally unfolded, it does not become 

 perfectly flat, but slight, alternating elevations and depressions 

 remain, showing the positions of the former folds, a permanent record 

 of the unique history of the wings of these insects. 



Ordinarily the wings of insects, while still in the wing-sheaths of 

 the pupa, are neither longer nor wider than the wing-sheaths, but 

 expand after the adult emerges from the pupa skin. Usually it takes 

 considerable time for the wings to expand and become fit for flight 

 and during this interval the insect is in an almost helpless condition. 

 In certain caddice-flies 

 that emerge from 

 swiftly-flowing water, 

 the time required for 

 the expansion of the 

 wings has been re- 

 duced to the mini- 

 mum. In the net- ' 

 winged midges, which 

 also emerge from 

 swiftly-flowing water 

 the difficulty is met by 

 the wings reaching 

 theirfull develop- 

 ment before the adult 

 leaves the pupa-skin. 

 It is only necessary 



when the adult emerges from the water that it should unfold its 

 wings to be ready for flight. 



The members of this family have three ocelli. The compound 



^1 ~^l 



Fig. 1050 



Fig. 1049. — Section of head through the eyes 

 of Blepharocera tenuipes: 0, ocelH; br, 

 brain; /./, large facets; 5./, smah facets, 

 0. I, optic lobes. (From Kellogg.) 



