54 THE GENERAL FEATURES OF WINGS 



it for that purpose. In the fixed fan-hke type of wing the so-called concave 

 and convex veins are qtiite evenly developed. 



The folding fan-like type of wing is better fitted to function as a para- 

 chute when outspread than as an organ for beating the air; in this type the 

 convex veins are much more prominent than the concave veins. 



The elytra. — 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 hemelytra. — The front wings of the Heteroptera, which are thickened 

 at the base like elytra, are often designated the hemelytra. 



The tegniina. — The thickened fore wings of Orthoptera are termed the 

 tegmina by many writers. 



The halteres. — The hind wings of the Diptera, which are knobbed thread- 

 like organs, are termed the halteres. The hind wings of the males of the 

 family Coccidae are also thread-like. 



The pseudo-halteres . — The re- 

 duced front wings of the Strepsip- 

 tera are known as the pseudo-halteres . 

 The margins of wings. — An 

 insect's wing is more or less tri- 

 angular in outline; it, therefore, 

 presents three margins: the costal 



,w.i margin or casta (Fig. 36, a-b); the 



Fig. 36.-Diagram of a wing showing ; • ^p- ^ ^^ ^^ ^^ 



margins and angles. '^ _ v & o > /> 



inner margin (Fig. 36, c-d). 



The angles of wings. — The angle at the base of the costal margin of a 



wing is the Immeral angle (Fig. 36, a); that between the costal miargin and 



the outer margin is the apex of the wing (Fig. 36,6); and that between the 



outer margin and the inner margin is the anal angle (Fig. 36, c). 



The tegula. — In several orders of insects there is at the base of the costal 

 vein a small, hairy, shghtly chitinized pad; this is the tegida (Fig. 38, Tg). 

 In the more speciaHzed orders, the Lepidoptera, the H\Tnenoptera, and the 

 Diptera, the tegula is largely developed so as to form a scale-like plate 

 overlapping the base of the wing. 



The tegulse of the front wings of Lepidoptera are specially large and are 

 carried by special tegidar plates of the notum. These in turn, are sup- 

 ported by special internal tegidar arms from the bases of the pleural wing 

 processes (Snodgrass, '09). 



The axillary cord. — The posterior margin of the membrane at the base 

 of the wing is sually thickened and corrugated; this cord-like structure is 

 termed the axillary cord. The axillary cord normally arises, on each side, 



