Page:Phylogeny of cynipid genera and biological characteristics.pdf/8

 of producing galls is evident, for other gall insects of other families produce galls upon them in abundance. But, though we cannot understand the reasons for such a condition, the fact remains that there is this great specialization in the choice of host.

It need hardly be argued that this specialization has very likely been the more recent development in the evolution of the Cynipidæ. It is not unreasonable to believe that the primitive gall-wasps inhabited a wide range of plants and that these polyphagous forms have become relatively few as the numbers of the more specialized species increased. That the small percentage of the insects which are polyphagous today, which still possess the more primitive habit, are to be considered actual remnants of the ancient forms, can be readily shown.

In the accompanying table, which shows the number of species of Cynipidæ known to occur on each of the genera of host plants, the insects are listed by tribes. These tribes are well-defined groups, characterized primarily by distinct morphological structures, especially of wing-venation and of the abdomen, and the close relationship of the species included in each of the tribes is as certain as may be desired. Now, it is conspicuous that specialization in the choice of hosts is restricted to the species of particular tribes, and that practically every species in those tribes has adopted the special host. All but one of the species of the Cynipini are found regularly on oaks and all of the species of Rhoditini are found as regularly on roses, though rarely one or two of the species of each group will produce galls on a second plant as well as upon their own hosts. Finally, it is to be seen that all of the species which inhabit the plants of the many genera belong to the one tribe Aulacini. It is evident that the specialized host was not adopted by species of the Cynipidæ severally and at different times, but at two distinct periods, by two groups of forms, at a time about coincident with the differentiation of the tribes which those two groups now constitute. And it is as clear that the Aulacini comprise forms most like the primitive polyphagous species from which the Cynipini and Rhoditini have arisen at different times.

In confirmation of this, it is significant to find that not only are the Aulacini, as a group, polyphagous but that several of the species of the group are, individually, polyphagous. These are the only species of all the gall-wasps which are known to be regularly found on plants of different genera, though most of the cynipids will produce galls on several species of plants of the same genus. Aylax pisum lives in galls on Lygodesmia and on Stephanotis; Aulacidea turnida is found on Solidago,