Vol. 1.] Wood'worth. — Wing Veiits of Insects. 61 



ing the homologies exhibited by the venation is to make out 

 the tracheation of the wingpad. Comstock and Needham 

 ('98-99) are the only ones who have attempted to put into 

 practice in any extensive manner this plan of studying vena- 

 tion. They found that in several groups the tracheation could 

 not in any manner be depended on for this purpose. In other 

 cases, however, including most of the lower orders, these au- 

 thors were convinced that the tracheation afforded the sound- 

 est criterion for the establishment of homologies. 



The only other theory to account for venation that has been 

 elaborated and seriously defended is that of Adolph, as fol- 

 lows: This author does not attempt to explain the origin of 

 venation, but bases his explanation of the present condition on 

 the alternate arrangement of the veins in two series, one occu- 

 pying the crests, the other the depressions of a series of regu- 

 lar plications, best seen in the mayflies. He supposes that the 

 more dorsal veins, which he denominates the convex veins, are 

 different in origin, structure, and significance from the ventral 

 (his concave) veins, and that the orderly alternate arrange- 

 ment of these will aid in the recognition of obscure homologies. 

 According to this theory the wing represents the fusion of two 

 semi-venations alternating with each other. Adolph's theory 

 does not appear to have gained many adherents, though it has 

 attracted a good deal of attention and not a little criticism — 

 first, by Brauer and Redtenbacher ('88), and later by many 

 others. The idea has strongly influenced later students, even' 

 when they have rejected it as unsound. This is because it 

 directed their attention to facts that are very evident and, as 

 I believe, of unquestionable importance in the development of 

 the venation of many groups, though exactly what they signify 

 has never been clearly pointed out by any of my predecessors. 

 I shall presently endeavor to explain their meaning. 



Still another theory has been suggested, but not elaborated, 

 in a recent work Ijy Packard ('98). This latest idea is that 

 the original venation was determined l)y the mechanical 

 necessity of flight. Most authors have supposed that this 

 factor has been an important element in the specialization of 

 particular types of structure, but none had previously ascribed 

 any of the features held in common by all types of wings to 

 this cause. Packard does not explain any of the details of 

 the genesis of venation in accordance with this principle, and 



