Page:Phylogeny of cynipid genera and biological characteristics.pdf/19

 cases are of a sort that might be explained as due entirely to the differences in the parts of the plant affected. If it is the young, growing shoot which is affected by one generation in mid-June, and the young, leaves and undeveloped stems in the buds which are attacked by the next generation earlv in the following May, the galls (e. g., Neuroterus batatus and N. noxiosus) may be rather different in appearance but will be of exactly the same type: an irregular swelling of the part affected, the gall quite inseparable and not even distinct from the parent plant, with the same irregular distribution of larval cells, which cells are of the same degree of complexity in the two generations. Such a case amounts to no more than a seasonal dimorphism among the adults, with the differences in the galls due to seasonal factors and, except for the more complex alternation found in others of the Cynipidæ, one would not ordinarily think of such instances as alternations of generations.

Indeed, it is hard to understand how differences in the forms of the galls of successive generations could be often avoided where the two generations of a species are produced in a single year. The part of the plant affected is usually different for each generation, for it must always be a rapidly growing part of the plant which receives the egg. This last fact explains sufficiently why different parts of the plant are attacked by the insects in. the different generations. It may be too much to suggest that in all the instances of extreme differences in the successive generations the differences are due primarily to the extreme differences of the parts of the plants affected, for we do not know enough about the factors influencing gall production to compare adequately the effects; but, in the simpler cases mentioned, we feel warranted in ascribing the most of the differences which appear to this difference in the part of the plant affected. It may be possible to secure more definite information on this subject by experimentally breeding and raising the galls.

That the second generation became fixed in the life cycle of each species only after a period of struggle to find a suitable method of obtaining an existence is apparent from observations already made on two species. Adler (1881) described the way in, which Cynips pallida (also named Biorhiza terminalis, and the alternate was called Biorhiza aptera) arrives at the successful location of the eggs of the second generation only after unsuccessful attempts to find the suitable place. The bisexual generation is developed in galls on the terminal buds of young shoots, and oviposits usually in the bark of the roots, but also in the buds and leaf petioles. Galls begin development on the buds and petioles but, as far as is known, do not reach maturity. The galls on the roots are