Page:Phylogeny of cynipid genera and biological characteristics.pdf/39

 The galls produced by the insects of this genus vary considerably in degree of complexity. The galls of R. vernus and of R. fusiformans, for instance, are comparatively simple swellings of stems, entirely inseparable from the plant, and agglomerate or polythalamous-primitive characters. On the contrary, the galls of R. bicolor, R. nebulosa, R. rosæ, et al., are very complex developments, entirely separable from the plant, highly modified in form, and with a high development of the larval cell. All degrees of complexity between these two extremes are to be found. The larval cell is distinct in most of the galls and has a well-developed wall, but in no case that I know of is the cell separable from the rest of the gall. That is, the galls indicate a wide range in the degree of development, evolutionarily, of various species of the genus, but in no case are the galls as highly complex, i. e., as highly developed, as in some of the genera of the oak gall-makers.

The mode of reproduction within the genus undoubtedly varies considerably among the species. It would be especially interesting to observe the reproduction of a species like R. vernus, for instance, which in wing-venation (arcuate first abscissa of the radius) and simplicity of the gall appears to be a more primitive form. It is possible that the reproduction of that species is primitive, normal, sexual reproduction. But it appears that, on the whole, the species of this genus reproduce agamically, with the males still existent but not usually abundant enough to fertilize many, if any, of the females. As Adler stated concerning this group, “The few males that are still produced are thus superfluous, and we can predict that they will probably become extinct in the course of time” (Adler-Straton, 1894, p. 153). Males are bred much less often than are the females and, from what limited data we can gather, they appear (cf. Table IV) to constitute only about two or three per cent of the total number of individuals. But this does not always apply to the progeny of any one individual; my first breeding of Rhodites rosæ gave six females and seven males, although subsequent breedings of the same species have only rarely given me any males at all. Gradually the male sex is disappearing from the genus and in consequence agamic reproduction is, likewise gradually, becoming the sole means of reproduction for these insects. Adler (1880) secured normal galls and insects from the unfertilized eggs of Rhodites rosæ and R. eglanterice, and there is thus no doubt that these unfertilized eggs are entirely capable of maturing. This agamy, almost completely achieved, is undoubtedly more specialized, more recently evolved, than the normal, sexual means of reproduction.