DIP TER A. 
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 
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 
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