Vol. 1.] ^YooclwortJt. — Wing Veins of Insects. 51 



vein-like structures exist in the wing which have nothing to 

 do with veins, prove, upon analysis, to be untenable. The 

 typical vein structure is connected by a complete series of 

 intermediate forms with all the other vein-like structures in 

 the wing. The lumen is merely the space left betw^een the 

 semi-cylindrical halves, and disappears if these two elements 

 remain flat. Tracheoid structures are simply modifications 

 of one or both of the layers and are exceedingly variable. 

 Trachea' likewise are found to l)e variable enough to discredit 

 their importance as means of defining veins. Neither size nor 

 pigmentation affords any better method of distinguishing 

 foreign elements in a venation. On the other hand, the less 

 tangil)le influences, such as are expressed in folds, hairs, and 

 color pattern, do conform to and belong to the venation scheme. 



VEIN DEVELOPMENT. 



While many investigators have studied the development of 

 the wings of insects, most of the principal groups having 

 received attention, practically nothing has been given, even 

 incidentally, upon vein development. Doubtless the reason 

 for this is the fact that most of these investigators have been 

 concerned with the question of the origin of the wing, the 

 method of its expansion after molting, the clothing scales, or 

 the tracheation; all matters much more conspicuous than vein 

 development. While there is nothing peculiar enough to have 

 attracted students to the study of the formation of veins, still, 

 for the purpose of the present study, the matter becomes of 

 more than usual interest. 



The development of the wing, in most of the lower insects, 

 begins by a slight modification of the " margin " of the meso- 

 and metathorax, though sometimes, as in Odonata, where there 

 is no "margin," by a minute tubercle; in the higher groups, by 

 a much more complicated process involving an invagination of 

 the tegmentary epithelium and a subsequent evagination of 

 the organ like that which occurs in other imaginal disk- 

 producing appendages. In the lower groups of insects the cells 

 of the wingpad are not distinguishable in any respect from 

 other hypodermal cells in similar situations, such as those in 

 the narrow fold at the posterior edge of most segments. In 

 the invaginated wings the cells are very different from those of 

 the body wall, but not distinguishable by any histological 



