Page:Phylogeny of cynipid genera and biological characteristics.pdf/7

 sort except for the great development of the sixth segment, a line of evolution not pursued elsewhere among the Cynipoidea. The genus Aulacidea has the second segment less developed than in any of the other genera, a fossil species showing the plate still smaller. Timaspis and Aylax show conditions intermediate to Diastrophus, where the segment covers about one-half of the abdomen. In Rhodites the segment is larger that anywhere outside the group of the oak gall-wasps, and in this genus the peculiar production of the hypopygium also indicates an advanced degree of evolution. Of the oak gall-wasps, Neuroterus shows the most primitive second segment. In some of the species called “Andricus” the specialization reaches its extreme.

These and other instances are of a sort which invites interpreting the less developed segment as the most primitive. And such an interpretation agrees with the information concerning evolution furnished by other lines of evidence. But, in Amphibolips, Disholcaspis, and possibly some other groups which are clearly highly specialized in many respects, it is surprising to find the segment not as greatly developed as among some less highly specialized oak gall-wasps. It is possible that a degeneration of the plate has occurred in these instances, but it is also likely that there were several lines of evolution among the oak gall-wasps, along one of which the second segment did not develop as far as along another, even though other characteristics evolved farther in the first instance.

About 86 per cent of the known species of gall-wasps produce galls on species of Quercus and are confined strictly to that single genus of plants. Another 7 per cent are confined to species of the genus Rosa. The remaining 7 per cent are found on plants of thirty-five different genera, rauging all the way from the monocotyledons to the highest composites. That is, the hosts of a very small percentage of the species are distributed very widely throughout the flowering plants, while 93 per cent are restricted to plants of only two genera.

It is hard to understand how this extreme specialization has come about. That the oak has advantages as a shelter for an insect is readily seen, but it belongs to a comparatively small family of plants, the genus has only a limited distribution over the world today, and, whatever the qualifications of the oaks, it is not apparent why other plants should be so completely abandoned. That these other plants are capable enough