Evolution and Taxonomy 53 



shows on the prothorax, transparent projections of triangular 

 form and a netted venation similar to that of the fore wings*. 



Whatever the origin of wings may have been, I think that 

 this much is clear : they were developed to comparatively 

 large size and were furnished with numerous veins before they 

 began to function as active organs of flight. This large size 

 and rich venation may have been the result of a natural selec- 

 tion of those forms best fitted to act as a kite or a parachute. 

 But this type of wing is not well adapted for active flight. 

 As soon as there arose a tendency for the wing to function in 

 this way, there began, doubtless, the extensive series of mod- 

 ifications of which we have records both in the rocks and 

 among living insects. 



We know almost as little regarding the origin of the veins 

 of the wings as we do of the wings themselves. Still we may 

 be allowed to speculate regarding the matter. L,et us suppose 

 that the wings originated from 

 broadly expanded organs ; such j, 1 



organs would be furnished with jajg^^sa^ ^*"^<al'iV-N 

 tracheae, even if they were used ((^^^^^^^^^ I^y 

 only as a parachute ; for they ^^^^^^^^^S^=0^^^\ 

 would need to be supplied with ^^^^^^^^Wi ^^^^^ 

 air as are other parts of the body. ^^^^^^^P^ l^^^^W 

 On the modification of such or- ^^^mi^^ ^S^P 



gans into wings fitted for active 



flight, it would be important that ^'^^ ^--I^ithomantis. 



the main trunks of the tracheae 



should be protected in such a way that any bending of the 

 wing, which would compress them and thus choke off the 

 supply of air, would be avoided. Thus any tendency of the 

 membrane of the wing to become thickened along the lines of 

 the tracheae would be preserved by natural selection ; and cor- 

 related with the development of this firm frame-work, there 

 might be a thinning of the spaces between the main tracheae, 

 thus insuring lightness of the entire organ. 



*Josef Rettenbacher, Vergleichende Studien iiber das Fliigelgeader 

 der Insecten Ann. des k. k. naturhistorischen Hofmuseums, Wien, Bd. 

 I. 153-232. t. IX-XX. 



