similar pattern. The galls of four of the European species studied are leaf-galls, all these being monothalamous and separable from the plant, with differences between alternate generations amounting to differences of form and not of plan of structure. The gall of the European Neuroterus aprilinus is a bud-gall, the alternate of which is more distinctly different than with other species of this genus. The American species of Neuroterus of which the life histories are known show still less distinction between the galls of the two generations, having no greater differences than what would be necessitated by the differences in the state of the plant at the times of the year in which the galls are produced. The adults of the two generations in this genus are likewise very similar, differing primarily in being bisexual in one generation and agamic in the other. Concerning the European species Adler said (1881, p. 26): “If we compare the flies of the two generations belonging to any of the species above described, we shall find the differences at first sight very slight. The difference of colouring is unimportant, and is chiefly observable in a slight variation in the colour of the legs; nor is the size of the body very different, while the form and surface markings agree in many points.” And he then pointed out that the only important differences are those of form of abdomen and of ovipositor, due to the different modes of reproduction of the two forms. Among our American species the differences between the adults of the two generations are even less marked. There is no doubt that the heterogeny found among the species of Neuroterus amounts to no more than seasonal dimorphism with agamic reproduction in alternate generations. This is in marked contrast to the great differentiation between alternate generations of the species of other cynipids where heterogeny occurs. No other instances of differentiation of alternate generations as slight as in Neuroterus are known among the other cynipids. Again we find proof that the genus Neuroterus is more primitive than the other oak gall-wasps.
In conclusion, we believe that Neuroterus is one of the most primitive groups of the Cynipini, derived rather directly from Aulacidea, and that a considerable evolution has occurred within the genus. Consequently, in this group we may expect to find the primitive stages of several of the biological characteristics of the oak gall-makers, and considerable attention given to the study of the life histories of species of Neuroterus is likely to be well repaid by discoveries which will cast light on the evolution of the highly specialized cynipids.
