64 



THE GENERAL FEATURES OF WINGS 



forward beneath the fore wing thus insuring the synchronous action of the 

 two wings. Examples of this exist in several families of moths, in butter- 

 flies, and in the Ephemerida. 



The clothing of the wings. — With the greater number of insects the 

 wings appear to be naked ; but in many cases they are only apparently so, 

 an examination with a microscope revealing a fine covering of setag or 

 spines. On the other hand, the wings of certain insects are obviously 

 clothed. Thus the wings of the aquatic Trichoptera are densely clothed 

 with long hairs, and those of the Lepidoptera are covered with scales. 



The more conspicuous clothing of wings is composed of setae more or less 

 modified. In the aquatic Trichoptera the setfe are essentially typical in 

 form, being hair-like stitxctures attached to the membrane of the wing by a 



C •^^' Sc, 



Fig. 5I-- 



-Hypothetical tracheation of a wing of the primitive nymph 

 (After C. & N.). 



movable joint. In the Lepidoptera and in certain other insects, the setae 

 are modified into scales. 



In addition to setce the wings of certain insects bear hair-like stmctures 

 which are morphologically different from setae; they lack the joint at the 

 base and are probably merely elongated cuticular nodules. 



A more extended discussion of the clothing of the wings is given later 

 in the discussion of the wings of the Lepidoptera. 



The principal wing-veins. — It has been shown in the previous chapters 

 that in the more generalized insects the principal wing-veins arc developed 

 about preexisting tracheae; and a study of these tracheae in the more 

 generalized members of several orders of insects has enabled us to construct 

 a diagram rejjresenting the hypothetical type of the primitive wing-vena- 

 tion. 



In nymphs and i^upas the tracheae about which the principal veins are 

 later developed extend separate from within the thorax, and their courses 

 are easily traced. But it frecjuently happens that througli the narrowing 



