456 COMPARATIVE ANATOMY CHAP. 



may be reduced to 1 pair, or may be entirely wanting (in both sexes 

 or only in the female), we here have to do with a derived con- 

 dition and with animals which have lost the wings once possessed by 

 their ancestors. In such insects the rudiments of wings or of organs 

 belonging to wings can often still be pointed out. 



The wings are thin lamellate un jointed folds of the body wall, speci- 

 ally of the integument. The 2 lamellae of a wing fold lie close to each 

 other. The wings are veined like the leaf of a plant. The veins 

 for the most part are thickenings of the chitinous cuticle. Within 

 the narrow interior space of the wing, nerves and especially tracheae 

 enter, branching like the veins. Blood-vessels also accompany the 

 veins. The arrangement of the veins is very important for classifica- 

 tion. The exact investigation of the courses of the veins and their 



FIG. 317. A, Larva. B, female imago of Capnia nigra (Perlid) (after Pictet). 



development, and especially the observation of rudimentary veins or 

 veins in the act of disappearing, have led to the result that the wings of 

 the various Hexapodan orders must be traced back not from one to the 

 other, but to a common form of wing. Thus the examination of wings 

 confirms the assumption that all orders of winged insects are derived 

 from a common winged racial group. 



The 2 pairs of wings are appendages of the meso- and meta-thorax 

 of the Insects. There are never more than 2 pairs. Their narrowed 

 basal portions are articulated with the dorso-lateral parts of the meso- 

 and meta-thorax. Strongly developed wing muscles serve to move 

 them (see section on musculature). 



The problem of the phylogenetic origin of the wings of insects is extremely 

 difficult, and as yet by no means solved. The rise of such organs is not explained 

 by saying that they are integurnental folds, which gradually increased in size, 

 stood out from and eventually articulated with the body. The wings must in all 

 stages of their phylogenetic development have performed definite functions. It is 

 impossible that they were originally organs of flight. "What function it was they 



