NO. 1438. ]VIXG,S OF THE TENrHREDINOlDEA—MA cGILLI 1 'AM ) '. 575 



In the Hymenoptera, as we have shown, the courses of the branches of the forked 

 veins?, in those forms where they have been preserved, have been so modified that 

 these branches extend more or less transversely, making sharp angles with the main 

 stems. It is not strange, therefore, that the tracheae of the wings of the pupa lying 

 free within the wing-sac, have not followed these changes. 



It was found, however^ that this is not the explanation of the change. An exami- 

 nation of the wings of young pujia? of the honey-bee revealed the fact that in this 

 insect the laying out of the wing-venation precedes the tracheation of the wing. 

 After the wing-veins reach that stage of development in which they appear as pale 

 bands, the trachete grow out from the base of the wing into them. 



It is obvious that tracheae developed in this way will follow the paths offering the 

 least resistance to their progress; and that it is not to be expected that the tracheae 

 will preserve their primitive arrangement under these conditions. This brings us to 

 the conclusion, already announced, that in determining the homologies of the wing- 

 veins in the Hymenoptera we are forced to base our conclusions on a study of the 

 veins themselves, and that a method of study which is of the highest importance in 

 determining the homologies of the wing-veins in many other insects, is of little use 

 here for this special purpose. 



From the results just given it is evident that we nnist depend 

 entirely on a careful comparison of the wing-veins of the Tenthredi- 

 noidea, part by part, for a determination of their homology. Before 

 considering the special modifications of wing-veins, some discussion is 

 necessary of the manner in which the veins may be modified or reduced 

 in number and the resultant reduction or combination of cells. 



A reduction in the number of wing-veins may take place in two 

 ways — first, by the coalescence of two or more adjacent veins; and 

 second, b}^ the atrophy of a whole or a part of a vein. The first 

 method of reduction, coalescence, may proceed in three wa3^s — first, 

 by the coalescence of principal veins or branches from the base of the 

 wing toward the margin; second, by the coalescence of the tips of 

 veins or branches from the margin of the wing toward the base; and 

 third, by the coming together of two veins at some point more or less 

 remote from the margin of the wing and their coalescence for a greater 

 or less distance. This third type of coalescence is generally spoken of 

 as anastomosis. The modern hymenopterous type of wing has been 

 produced, as will be described later, by a combination of all three of 

 these methods. 



The second method of reduction, atrophy, or the fading out of the 

 whole or a part of a vein, is the means by which most of the changes 

 found in the modern hymenopterous wing are brought about. 



When two or more branches or any of the principal veins coalesce, 

 this fact is indicated by placing a plus sign ( + ) between the abbrevia- 

 tions of the veins that have combined. If, for example, R., and R^ 

 coalesce, the legend would be R2+3; while if any of the principal veins 

 combine, as R, M, and Cu, it would be written thus: R+M+Cu. This 

 implies that not only the branches of the same vein, but that the stems 

 of the difl'erent veins, as well as the bi-anches of diflerent veins, maj^ 

 coalesce. 



Proc. N, M. vol. xxix— 05 38 



