Page:Phylogeny of cynipid genera and biological characteristics.pdf/18



The alternate generations of certain species of gall-wasps show more remarkable differences than do the alternate generations of any other group of animals in which this heterogeny is known to occur. Because of the careful experiments of Adler (1881) and of others, most students of biology are thoroughly familiar with the extreme differences presented by successive generations of these Cynipids. I havre given detailed accounts of the life histories of some of our American species in an earlier paper; and a review of this and of Adler's work will show to what the extremes of differences amount. The galls of two successive generations may be produced on different parts of the plant and may present entirely different forms, e. g., the gall of one generation of Andricus operator is mainly a large mass of wool on the young stems, leaves, or aments of the oak; the second generation gall is a small, seed-like, naked capsule between the cup and the acorn. The insects of the two generations of some species may be so entirely different as to warrant believing them to belong to different genera, if the relationships of the two forms has not been discovered. In fact, Adler indicated that in practically every instance the two generations had been placed in different genera.

We have been so impressed with these extreme instances of alternation that we have failed to recognize the less remarkable cases; and the origin of the perfected phenomenon of heterogeny has remained a riddle just because we have not paid sufficient attention to these less extreme instances.

An adequate conception of the characters of the adults and of the galls of the species concerned in this discussion can be gained only by an examination of specimens or by a careful study of the papers already referred to, Adler's paper including most of the European and my paper including most of the American species of which the life histories are known, but the accompanying table will summarize the essential characters for each species.

It is remarkable that for several of the species, all of which belong to the genus Neuroterus (of genetic significance), the successive generations are very similar. Indeed, the differences between the adults is so slight as to be practically indiscernible. The main difference is physiological, for one generation reproduces agamically and the other by fertilized eggs. The galls of these two forms are likewise exceedingly similar and, although they may appear quite different in most cases, they are still of fundamentally the same type. The differences in those