THE GENERAL FEATURES OF WINGS 53 



When the wings are all present, there are two pairs: one pair borne 

 by the mesothorax; and one pair, by the metathorax; prothoracic wings 

 are unknown among living insects. 



In certain cases one pair of wings may be either wanting or so modified in 

 form as to be not fitted for organs of flight. Thus the flies, or Diptera, have 

 only the first pair of wings fitted for flight, the second pair being repre- 

 sented by a pair of knobbed threads, the halteres, which are probably organs 

 of some special sense ; and with the earwigs and the beetles, the first pair 

 of wings are greatly thickened and serve as wing-covers or elytra, which 

 protect the hind wings when they are not in use. In these cases the hind 

 wings are the chief, if not the only organs of flight. 



The fundamental structure of the wings of insects. — 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 frame- 

 work 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. 

 Many of the older writers termed the wing-veins nerviires and their arrange- 

 ment the netiration 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; 

 but 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 foim 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. 



The fan-like vuings. — While the typical form of insect wing presents a 

 nearly flat surface, in many insects the wings are more or less corrugated, 

 and in some the corrugating of the wings has proceded so far that a fan-like 

 form of wing has resulted. 



Among the more perfectly fan-like wings two quite distinct types can 

 be recognized; these may be designated as the fixed fan-like type and the 

 folding fan-like type respectively. The most perfect examples of the former 

 are the wings of the Ephemerida; of the latter, the anal area of the hind 

 wings of the Acrididse. 



In the fixed fan-like type the corrugating of the wing serves to strengthen 

 it, and thus fit it to withstand better the strain brought upon it by beating 

 the air in flight. This can be easily illustrated by first attempting to use a 

 flat sheet of paper as a fan and then after folding the paper in plaits using 



