Vol. XI] CURRAN— REVISION OF THE PIPIZA GROUP 387 



to note the first appearance of the more specialized 

 forms. 



My first record of the ventral spur bearing species 

 is ontarioensis, at Guelph, Ontario, June 13, 1913. The 

 next record is trochanter ata, Illinois, May 10, 1914. 

 The former species is undoubtedly the lowest member 

 of the group, with the second following. Both of these 

 are larger than the average Cnemodon. C. myerma and 

 venteris were both taken in 1919. In appearance ven- 

 teris approaches more closely the typical Cnemodon. 



It must not be thought that the other genera are 

 devoid of evolutional interest. Pipizella and Pipiza 

 both present forms which are very important. In the 

 latter genus species occur with slender hind femora and 

 others with remarkably thickened femora with many 

 forms intermediate between these extremes. Heringia 

 shows a great deal of variation in the hypopygium and 

 in the specialization of the fourth segment, as well as in 

 other structures. In these genera many gaps occur and 

 it is not possible to trace the species definitely from one 

 to the other, although I have examined most European 

 and all North American species but one. Pipizella is 

 even more difficult to trace through although it is 

 quite evident that P. fraudulenta is either the highest 

 or the lowest form. 



A suggested tree of relationship is given on the next 

 page. It will be noticed that all the genera are traced 

 from a common ancestor, but all developed along dif- 

 ferent lines and for the genus Pipiza no direct connection 

 can be found. Its broad face, with practically nothing 

 in common with the other genera except the dark color 

 and the pilose face, eyes and abdomen, clearly indicates 

 considerable separation. 



