Page:Phylogeny of cynipid genera and biological characteristics.pdf/28



We have been handicapped in discussing the phylogeny of the Cynipidæ by our lack of knowledge of fossils of the group. The only fossil gall-wasps known are three species which I have recently described. All three of these belong to the genus Aulacidea, but whether this is significant may not be determined on the basis of such meager data. Nevertheless, it is remarkable that no cynipid galls are described from fossil oaks. The only fossil galls known are quite certainly not cynipid. I am inclined to believe that the family Cynipidæ is of recent origin, possibly not much older than the Oligocene fossil I have described.

Certainly the family has not existed for any long time with the characteristics shown in the present day by the more specialized groups, for it is inconceivable that the insects could have survived for long any struggle for existnce while impeded by the specialized habits possessed by most of the species, as the following brief data may show.

The necessity of the larva passing its life within a gall greatly reduces the parent's chance of finding the proper place in which to oviposit. The parent gall-wasps of most species must find a plant of the one genus and possibly of the one or two species of that genus on which alone it can survive. In most species, moreover, the parent must find a particular part of that special host and, failing to arrive at that spot within the very few hours of its adult life, the insect dies without effecting reproduction. Such a failure is not as likely to occur with forms which are less specialized in their choice of host and part of host affected. Insects of this latter sort are found in the genus Aulacidea.

Effective as a highly complex gall may prove as a protection to the larval wasp, it has evident disadvantages. It serves as a prison in which the larva is trapped. The larva has become a degenerate, legless, inactive, helpless organism; and, if once its gall defence is broken, the insect is quite at the mercy of the enemy. The defence offered by the gall itself is, I think, largely theoretical, and exists mostly in the minds of superficial observers. The hardest galls, or galls most complicated with hairs, spines, wool, loose larval cells, etc., or galls rich in tannin crystals, etc., are all abundantly parasitized and, indeed, some of the most heavily parasitized galls are those best equipped (theoretically) against enemies (e. g., Amphibolips confluens has 95% parasites, Callirhytis furnessæ has 50% parasites, Biorhiza forticornis has 96% parasites, Rhodites rosæ has 15% parasites, etc.). The smallest amounts of parasites usually occur with those species of Aulacidea which make the simplest galls.