though the characters of the gall might have thrown considerable light on the question. In revising the genera of the Cynipidce, I shall give great consideration to lines that the gall characters may indicate, then, of course, looking for confirmation of these by the less evident characters of the insect morphology.
The galls of each genus are not only of the form typical for the group, but also of a degree of complexity which is usually quite uniform within the group, with considerable variation between groups. The galls of species of a wide range of genera studied by Melvin T. Cook (1902–4), Cosens (1912), and others all show a similar differentiation of the plant tissue into three zones, the epidermal, the parenchymal, and the nutritive, with often the addition of a fourth zone, the protective. But, although all cynipid galls are formed on fundamentally the same pattern, the developments of that pattern are surely much more complex in certain cases than in others. It seems proper to call those galls most simple in which the extent of proliferation of the plant tissues is least in amount (relative to the amount in that part of the plant normally), where the separation of the zones of tissues is least definite, and where the gall is still an integral part of the host plant; and to call those galls more complex where the amount of new tissues is relatively greatest and the separation of the zones is most complete, resulting in the complete independence of some of the zones in some cases (especially in the production of a separable larval cell) and where the separation of the gall from the host is most complete, the gall in many cases becoming almost or entirely a separable organism, connected with a food supply by only a small amount of tissue or in some cases actually developing in size and in formation of new tissue after the gall has left the parent plant. Involved with these characters of complexity will also be a reduced number of cells or a single larval cell within each gall; the smaller the number of larval cells in a single gall, the greater the amount, relatively, of new tissues produced by each individual iinsect, and the moredefinite the gall structure; consequently the monothalamous gall indicates greater specilization than either the polythalamous or the agglomerate gall.
In accordance with our understanding that the galls of each genus of gall-wasps show related characters, we find (as detailed in the accompanying table) that the degree of complexity of gall structure shows the generic relations of the insects producing the galls. It is to be noted that in many of the genera the galls are all of a single degree of complexity. Without an exception, the galls of Autlacidea are of about one degree of
