DIPTERA. 



433 



it which is crossed by the fine lines is plaited somewhat like 

 a fan and folded over the other portion. By this means the 

 wing, 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 



Fig. 506. — Wing of Blepharocera. 



finally unfolded, it does not become perfectly flat, but 

 slight, alternating elevations and depressions remain, show- 

 ing 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 

 reduced to the minimum (see pp. 189, 190). In the net- 

 winged midges, which also emerge from swiftly-flowing 

 water, the difficulty is met by the wings reaching their full 

 development 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 simple eyes. 

 Each compound eye is divided into two parts: an upper 

 half, in which the ocelli are very large ; and a lower half, in 



