1917] A Phylogenetic Study 341 



happened in the evolution of the head region of certain Diptera, 

 I would not imply that recent Diptera are descended from recent 

 Mecoptera, or that living Mecoptera are descended from living 

 Neuroptera. On the other hand, it is quite true that living 

 Neuroptera, Mecoptera, and Diptera have travelled together 

 along the same developmental "road, " so to speak, in following 

 out certain evolutionary tendencies. At some point along the 

 road, the Neuroptera branched off to follow their own path 

 of speciaHzation, but some of them wandered but a short 

 distance from the main line, and have remained as little changed 

 as certain of the fossil forms which fell by the wayside at an 

 early date. These "conservative" individuals have preserved 

 many features characteristic of the ancestors of the Mecoptera 

 and Diptera who continued together for a greater distance along 

 the road of evolution, before the Mecoptera in turn branched 

 off to follow their own path of speciaHzation. So too, among 

 the Mecoptera certain individuals wandered but a short distance 

 from the main line, and have preserved many features char- 

 acteristic of the ancestors of the Diptera, and the same process 

 was repeated when the Dipteron-like ancestors of the fleas 

 gave rise to the Siphonaptera. The study of these "conserva- 

 tive" forms among Hving insects is quite as instructive as the 

 study of fossil forms, and has the additional advantage of 

 enabHng one to examine the minute details not preserved in the 

 fragmentary fossil remains, and to take into account the bio- 

 logical habits, etc., which are of considerable importance in an 

 attempt to determine the relationships of the different groups 

 of insects. 



In the head region of nearly all adult Mecoptera, there is a 

 well marked tendency toward the formation of a "genal process" 

 or protuberance of the lower portion of the gense ("p" of Figs. 

 2, 5 and 9), and it is rather strange that such a widespread 

 tendency in the Mecoptera should not reappear in the Diptera — 

 although the process of the genal region labeled "p" in the 

 Dipteron shown in Fig. 6 may be homologous with the genal 

 process of the Mecoptera. In some of the Mecoptera (Fig. 9) 

 there is a tendency for the eyes to extend upward toward the 

 top of the head, and downward toward the mesal Hne below the 

 antennae, and the same tendency is evident in the Diptera shown 

 in Figs 8 and 6. 



