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 give a clue to the evolution within the genus. Kieffer has established (1902, Bull. Soc. Metz, (2) X, p. 96) a distinct genus, Lytorhodites, for these species, but there seem hardly enough other distinctive characters to warrant making of the group more than a subgenus.

In R. vernus Osten Sacken (=R. nodulosus Beutenmüller) the first abscissa of the radius is arcuate, a primitive condition of the vein. The only other groups in the family where this vein is arcuate are some of the genera of the Aulacini, and Rhodites is most likely descended from one of the groups of that tribe. All the other species of the genus have developed an angulate vein, in this respect showing specialization, but even in most of these species the vein is only slightly angulate and has not developed the angle to any great degree.

The second abdominal segment throughout this genus is well developed, occupying a half to two-thirds of the whole abdomen. This is a more specialized condition of the segment than occurs in Aulacidea and a less specialized condition than in most of the species of oak gall-makers.

The hypopygium of the insects of this genus is a remarkably developed structure. It is broad and very acutely pointed, “plow-shaped.” This is a very specialized form for that segment to assume; throughout the rest of the family it is quite inconspicuous and without a peculiar form.

The hosts of this group of gall-wasps are plants of the genus Rosa. Every known species of the genus occurs on roses. One species, R. rosæ, is believed to produce a gall, rarely, on plants of the related genus Rubus but, although this gall does resemble the mossy gall occurring on rose, I do not know that any one has bred the adult wasps from the blackberry gall and definitely proved the identity of the maker. Other records of species of Rhodites occurring on Rubus are undoubtedly errors in the determination of the host. There is no question that the insects are remarkably specialized in their choice of hosts. Compared with the distribution of the Aulacini upon plants of thirty-five different genera, this restriction of Rhodites (which genus contains about half as many species as the whole tribe of the Aulacini) undoubtedly indicates a higher degree of evolutionary development, a degree matched only by the concentration of the Cynipini upon the genus Quercus and only less remarkable than the specialization of the oak gall-makers because the number of the species of Rhodites is many times less than that of the Cynipini. The degree of specialization is complete in either case.