THE EXTERNAL ANATOMY OF INSECTS 59 



The loss of wings is often confined to one sex of a species; thus 

 with the canker-worm moths, for example, the females are wingless, 

 while the males have well-developed wings; on the other hand, with 

 the fig-insects, Blastophaga, the female is winged and the male 

 wingless. 



Studies of the development of wings have shown that each wing is 

 a saclike fold of the body-wall; but in the fully developed wing, its 

 saclike nature is not obvious; the upper and lower walls become 

 closely applied throughout the greater part of their extent ; and since 

 they become very thin, they present the appearance of a single delicate 

 membrane. Along certain lines, however, the walls remain separate, 

 and are thickened, forming the firmer framework of the wing. These 

 thickened and hollow lines are termed the veins of the wing ; and their 

 arrangement is described as the venation of the wing. 



The thin spaces of the wings which are bounded by veins are 

 called cells. When a cell is completely surrounded by veins it is said 

 to be closed; and when it extends to the margin of the wing it is said 

 to be open. 



The different types of insect wings. What may be regarded as the 

 typical form of insect wing is a nearly flat, delicate, membranous 

 appendage of the body, which is stiffened by the so-called wing-veins ; 

 but striking modifications of this form exist ; and to certain of them 

 distinctive names have been applied, as follows: 



In the Coleoptera and in the Dermaptera, the front wings are 



thickened and serve chiefly to protect the dorsal wall of the body and 



the membranous hind wings, which are folded beneath them when 



not in use. Front wings of this type are termed wing-covers or elytra. 



The front wings of the Heteroptera, which are thickened at the 



base like elytra, are often desig- 

 nated the hemelytra. 



The thickened fore wings of 

 Orthoptera are termed tegmina by 

 many writers. 



The hind wings of Diptera, 

 which are knobbed, thread-like 

 organs, are termed halteres. The 

 hind wings of the males of the 



family Coccidae are also thread- 

 Fig. 70. Diagram of a wing showing j^g 

 margins and angles. ' 



1 he reduced trout wings of the 



Strepsiptera are known as the pseudo-halteres. 



