THE ANATOMY OF INSECTS. 



arrangement is described as the venation or neuration of the wing. The terms 

 veins and nerves are both in general use ; and when applied to the wings of 

 insects, have the same signification. Neither of them is good in this connec- 

 tion ; but they are so firmly established that it would not be well to try to 

 change them. The former, however; is the better. For in very many insects a 

 groove extends along the ental surface of the thickenings of each wall ; and the 

 groove of the dorsal and ventral thickenings being exactly opposed, form a 

 tube in the centre of each so-called vein or nerve, within which the fluids of" 

 the body circulate. In many insects these tubes, or veins as I shall call them, 

 are also traversed by the air-vessels or tracheae. 



The thin spaces circumscribed by the veins are called cells. 



An insect's wing is more or less triangular in outline ; it therefore presents 

 three margins (Fig. 35). To these special names have been applied ; there is 

 however, a lack of uniformity among 

 entomologists in the terms which they 

 use. The cephalic margin, Fig. 35, 

 1-2, is termed the front margin, costal 

 margin, or simply the costa. The distal 

 margin, Fig. 35, 2-3, is known as the 

 outer margin, or dpical margin. And 

 to the caudal margin, Fig. 35, 3-4, are 

 applied the terms inner margin and 

 anal margin. 



The angle of the wing at the union 

 of the cephalic and distal margins, 

 Fig. 35, 2, is the apex of the wing ; and 

 the angle between the distal and cau- 

 dal margins, Fig. 35, 3, is the inner 

 angle. The proximal end of the wing is referred to as the base. 



With certain insects (Hymenoptera and some Homoptera) the cephalic mar- 

 gin of the hind wings bears a row of hooks, which fasten into a corresponding 

 fold on the caudal margin of the front wings. These hooks are named the 

 hdmiili, and serve to hold the two wings of the same side together, thus insur- 

 ing their action as a unit. 



In the moths the wings are united in a somewhat different way. On the 

 lower surface of the front wing near its base there is a hook formed of either a 

 portion of membrane or a tuft of hairs ; into this hook there fits a bristle, the 

 frenulum, which springs from the hind wing near its base. The frenulum is 

 simple in the males ; but it consists of several bristles in the other sex. 



The wings present many characters which are much used in classification. 

 These are variations in texture, form, clothing and venation. 



The most striking variations in texture are presented by the first pair of 

 wings. Special names have been applied to the wings exhibiting the more 

 important of these variations. These are elytra, hemelytrc, and tegmina. 



Th Elytra. The term elytra is applied to the first pair of wings of beetles 



;. These wings are thick, horny or leathery, without veins or with 



* mer-ji^ ceo of them, and when not in use they are horizontal, and meet 



FIG. 35. Wing of Hepialus. 



