98 University of California PuhUcations. [Entomology 



would seem to be the natural order of evolution, but this evi- 

 dence certainly does not point that way. 



PALEPHEMERID^. 



This family, founded on two Devonian insects, represents a 

 very much nearer approach to the Ephemerida3 of to-day than 

 the group last considered. The fragments are but fractions of 

 the wings, and while we can not know the whole venation, that 

 which is present indicates a shape quite similar to that of the 

 wing of the modern Ephemeridae. This group can be consid- 

 ered a near ancestral type intermediate between the previous 

 group and the modern Ephemeridae. 



EPHEMERID.E. 



The mayflies have, in recent years, occupied a very promi- 

 nent place in the discussions of the subject of venation. No 

 other group with wings that do not fold possesses so many 

 longitudinal veins. One of the easiest ways to trace homol- 

 ogies is to assume that the primitive organ contained as many 

 parts as are found anywhere within the group, and then to 

 imagine that ' specialization consists chiefly or wholly in the 

 cutting out from the supposed primitive form of such members 

 as is necessary to produce conditions resembling each of the 

 existing forms. On this principle the Ephemeridae have been 

 chosen by some writers as the nearest living representative 

 of the ancestral stem form. 



There is much reason for this assumption in the Ephemeridse, 

 on account of the rather simple undifferentiated venation 

 possessed by this group. On the other hand, if the geological 

 record is significant in this case, we must conclude that the 

 Ephemerid, instead of being a primitive insect, is really highly 

 specialized, and has the distinction of leaving fossil traces of 

 two distinct steps in the process of its evolution. On the 

 grounds of comparative anatomy there is the objection, that it 

 is more natural to look to an insect with two pairs of equal 

 wings (especially for the production of wings like those of the 

 grasshopper), than to a mayfly, as an ancestor. It is quite as 

 easy to derive all modern venations from one like a Sialid as 

 from an Ephemerid. 



The alternation in the level of the veins, resulting in the 

 so-called convex and concave veins of Adolph and his followers, 

 is to be considered as an acquired character rather than a 



