Page:Phylogeny of cynipid genera and biological characteristics.pdf/6

 until one of them is supported by other sorts of evidence. The important fact is that the vein considered does clearly show a path which was taken in the evolution of the insects. That the path itself does not indicate in which direction the journey was made is no reason for ignoring the existence of the path. Other signs beside the road will supply proof enough of the direction of the movement along that road.

To anticipate the conclusions which will be drawn later, it may be said that all other considerations, without an exception, point to those insects which have the arcuate vein as being most primitive; those having the angulate vein are by all means the most specialized of the gall-wasps; and an arrangement of the groups of intermediate grades of specialization strictly parallels an arrangement of those groups on the basis of venation only. Such strict parallelism of the indications of evolution of the groups demands parallel interpretations of the direction in which the evolution has progressed. It seems that we must believe that the angulate first abscissa of the radial vein in the wings of the cynipoids has developed by gradual transitions from an arcuate vein.

An enlarged abdominal plate may be the result of the fusion of two or more plates or the actual increase in size of that plate in the course of evolution. The primitive condition, somewhere in the more or less remote ancestry of any group of insects, showed the segments more or less equal in development, and any condition which shows a single segment especially developed is undoubtedly the result of later evolution. Whether the degeneration of a segment that has become thus specialized ever occurs is a question to be debated for each group studied.

The dorsal plate of the second segment of the abdomen of Cynipidæ is always more developed than any of the other dorsal plates and in many. instances the plate has become so enlarged as to cover almost the entire abdomen. That this enlargement in the true gall-wasps is not the result of fusion is evidenced by the continued existence of the other plates underneath the enlarged plate. A study of the character of the segment among all of the gall-wasps shows it to have, very apparently, generic significance, and this has been recognized in the foundation of many of the genera. In the Andricus-Cynips group of wasps the stricter use of this character will help solve the true relationships.

A study of the figures on Plate XXXII will illustrate the following. In the cynipoid genus Ibalia the second dorsal plate does not show any special modification, the segmentation appearing of a more primitive