Page:Phylogeny of cynipid genera and biological characteristics.pdf/16

 majority of the oak-gall-producing Cynipidæ, only the alternate generations are siInilar, every agamic generation being succeeded by a bisexual generation, which in turn produces an agamic generation. Where this heterogeny occurs the origin and development of an agamic generation has been so involved with the origin and development of the alternation of dissimilar generations that the problem is more properly discussed in the section of this paper on the alternation of generations. The simpler case, where only agamic generations exist, will be discussed here.

That agamy is a more specialized method of reproduction than a bisexual method need hardly be argued. The evolution of agamy among the Cynipidæ is easily understood, for among the species still existing today we find all gradations from the primitive bisexual to the specialized agamic condition. Among the species of Aulacidea, the sexes are produced in about equal numbers and normal sexual reproduction occurs. Here and in the very closely related genera are the only instances of normal sexual reproduction found in the whole family, except such as occur in the sexual generation of heterogenous species. In Diastrophus the male sex is much less abundant than the female, constituting only about thirty per cent of the total number of individuals, and it is therefore very likely that parthenogenetic reproduction occurs frequently in the group. But that the eggs are still often fertilized is quite certain; I have many times seen copulation in the case of Diastrophus nebulosus. In Rhodites, however, it is definitely known that for several species the parthenogenetically produced eggs are the regular means of reproduction, and Adler (1881, p. 153) went so far as to call the males of these species superfluous. Though it is still likely that the males do occasionally fertilize the females in these species, it is quite certain that, as Adler put it, “we can predict that they [the males] will probably become extinct in the course of time.” In Disholcaspis we have instances where apparently only females are ever produced, but it is still possible that there is a bisexual, an alternate generation yet to be discovered among those species. At any rate complete agamy has been attained by at least those five species of Andricus studied by Adler.

The accompanying table giving some data as to the ratio of the males to females is based on my own observations, supplemented in a few cases by records froin other observers. The total amount of the data is very meager, but is enough to indicate what are undoubtedly the true conditions among several of the groups. The order of the species is that of the diminishing abundance of the males. This is largely a generic arrangement of the species, which indicates that the sequence