Page:Phylogeny of cynipid genera and biological characteristics.pdf/36

 of this character within the group, but the genus is closely related to, most likely descended directly from, Aulacidea or Aylax, the only genera where this vein is always arcuate.

The second segment of the abdomen of species of Diastrophus covers, dorsally, more than half the whole abdomen, in this respect being more developed than in Aulacidea, but not as developed as in Rhodites or most of the oak gall-producing cynipids.

The face of the species of this genus is peculiarly marked with fan-shaped striæ. This is a sort of specialization not found elsewhere among the true gall-wasps.

The bidentate claws may be some indication of specialization. The claws of other groups of the Aulacini are simple.

The hosts of this genus are mostly plants of the family Rosaceæ. One species is on Smilax, a liliaceous plant. One species is found on Fragaria, four species are on Potentilla, and seven species occur on Rubus. Thus, the choice of host plant is rather wide, indicating a relationship to the primitive, polyphagous groups of the Aulacini. But over half of the genus is confined to Rubus, and another third to the related Potentilla, and this is a specialization of a degree much greater than that found in any of the other Aulacini, but it is not comparable with the complete specialization achieved by the Rhoditini and Cynipini.

The galls of this genus are remarkably uniform in degree of complexity of structure, all but one of the species producing galls which are agglomerate or polythalamous swellings of stems, quite inseparable from the plant, and to this extent quite primitive. But the forms of the galls are fairly definite, and the formation of larval cells which are slightly separable from the rest of the galls indicates some degree of specialization. The gall of Diastrophus cuscutæformis appears to be an exception to this general type, but this species may best be considered to belong to the genus Gonaspis. That is, the galls of Diastrophus indicate rather primitive, but somewhat developed, relationships.

Reproduction within the group may be normal sexual reproduction at times. I have often seen the males of Diastrophus nebulosus copulating with the females, and undoubtedly fertilization often occurs. But the males constitute only about thirty per cent of the total number of individuals, and it is very likely that this scarcity of males results in unfertilized eggs being laid very often, and the parthenogenetic development of these eggs is not unlikely. Again, we find an instance of the rather primitive (bisexual) nature of the species of Diastrophus, showing at the same time some degree of specialization, in the gradual disappearance of the males.