Page:Phylogeny of cynipid genera and biological characteristics.pdf/12

 study of even these instances will show that there are constant enough differences between the galls of the species. On the contrary, a few instances might be cited where the same insect is found inhabiting galls so different in form that they have been considered as distinct species. In these cases it seems that the influences of the host plant organization have come to the front, for where two galls are thus dissimilar they are found always upon host plants of different species. Thus the smooth form of the gall of Rhodites dichlocerus is found on species of rose whose stems are comparatively unarmed, and the very spiny form of the gall, produced by the same species, is found upon species of rose whose stems are normally more densely covered with prickles. But such cases are the exceptions. It is only rarely that the importance of the plant in determining the character of the gall becomes more than secondary; the specific qualities of the insect inhabitant are the primary factors in determining the form of the gall.

The factors inciting gall formation have more than this specific nature. They show qualities which are generic for the insect producing the gall, but not for the host plant on which the gall occurs. This fact has been vaguely recognized by students of cynipid galls for inany years. It has been natural to refer to a “typical Disholcaspis” or a “typical Amphibolips” gall, but such statements have never been carried to the point of defining the characters of the galls of any of the genera of the Cynipidæ. After an elaborate study of the histology of galls of many species, Melvin T. Cook (1902) arrived at the conclusion that “The morphological character of the gall depends upon the genus of the insect producing it rather than upon the plant on which it is produced, i. e. galls produced by insects of a particular genus show great similarity of structure even though on plants widely separated; while galls on a particular genus of plants and produced by insects of different genera show great differences.” This bore out the experience of all who had carefully examined galls. It was valuable to have the confirmation of an histological study, but an examinationi of the gross miiorphology of the structures offers as satisfactory information, and there seems to be no reason why it is necessary to study the microscopic structure of the gall before one can perceive its generic characters.

With such brilliant guides available to indicate generic relations of the insects, it is surprising that no student has employed gall characters on any large scale in classifying the Cynipidæ. Strange inconsistencies are found in our present-day classifications which have arisen because the generic characters of the insect were not sufficiently apparent,