THE PALEONTOLOGICAL DATA 87 



Before entering upon a discussion of the data bearing on the primitive 

 type of wing-venation it may be well to examine briefly the data bearing 

 upon the origin of wings. 



(6) ON THE ORIGIN OF WINGS 



The question as to the origin of the wings of insects has been discussed 

 for nearly or quite one hundred years ; and although a large number of the 

 more eminent entomologists of this period have taken part in this disais- 

 sion, the writers of to-day hold widely different opinions on this subject. 



A detailed discussion of this question does not fall within the scope of 

 this essay. The reasons that have led to the adoption of the uniform 

 terminology of the wing-veins of insects are entirely independent of the 

 question as to the method in which wings were originally developed. The 

 important fact in this connection is that a study of the more generalized 

 members of the several orders of winged insects shows that the type of 

 venation is the same for them all, which indicates that wings have originated 

 but once in the class Hexapoda; or, to state the same thing in other words, 

 all of the orders of winged insects have descended from a common stock. 



In discussing the origin of wings, we must assume that they were 

 developed to a comparatively large size before they began to function as 

 active organs of flight ; for minute rudimentary wings would be useless as 

 active organs of flight. 



As to the function of the organs that were later modified into wings 

 there are two views, each of which have been widely advocated. One 

 school of writers believe that the wings are modified gills; that they were 

 derived from either the dorsal gills of the annelidan ancestors of insects or 

 from tracheal gills resembling those of the mnnphs of May-flies. Another 

 school of writers believe that the wings were evolved from lateral expan- 

 sions of body-segments which at first merely functioned as parachutes. 



The theory that the wings were evolved from lateral expansions of the 

 body-segments is the one that I believe is most strongly supported by the 

 available evidence; the nature of this evidence will be shown a little later. 

 A review of these two theories has been published recently by Crampton 

 (' 1 6) ; and the reader is referred to this paper for references to the literature 

 of the subject. 



The view that the wings of insects originated as dorsal backward 

 prolongations of the tergum is combatted by Tower ('03) who believes 

 "that the evidence points strongly to Verson's ('90) view that the wings of 

 Coleoptera and Lepidoptera are derived from the rudiments of the meso- 

 thoracic and metathoracic spiracles." Verson studied the embryo of 

 Bomhyx mori and Tower that of Leptinotarsa decemilineata. 



Powell ('05), however, as a result of a study of the development of the 

 wings of two species of the Scolytidce, shows that the wings do not arise 



