Page:The Gall Wasp Genus Cynips.pdf/49

 developed, with a more pronounced development of the collenchyma layer. In Besbicus all five layers are again present (with the parenchyma most so in mirabilis). In Philonix the development is chiefly that of a rather solid spongy parenchyma. In Atrusca it is a parenchyma with few but tremendously extended fibers. In Acraspis all five layers are developed among the species centering about Cynips mellea, but in the other specific stocks in Acraspis the protective and parenchyma zones seem absent and the collenchyma and epidermal layers are unusually well developed.

The gall-producing stimulus, whatever its origin, is evidently selective in its effects upon particular plant tissues; and, since the gall characters are usually of subgeneric significance, it is apparent that these peculiarities of the gall-producing stimuli are of as ancient standing as any of the morphologic structures of the insects. This means that these physiologic qualities have been constant in heredity for possibly ten or twenty million years during which the specific stocks of Cynips have been differentiated.

The second body of data on the physiologic inheritance of species is concerned with life histories in Cynips. Like most of the other higher gall wasps, these insects have an alternation of a bisexual and an agamic generation. Thruout the groups the life history data are so uniform that it may be readily summarized.

The agamic insect develops in a gall which appears on the leaves early in the summer. The gall is mature by the end of the summer, and the insect matures and transforms into an adult early in the fall. Then—the most unique feature of the genus—the adult continues in the gall, chewing an exit passage thru everything except the epidermis of the structure, but not emerging until very late in the fall or some time in the winter. Most of the emergence is in mid-winter. The agamic females oviposit within the scales of unopened buds on the trees. The bisexual galls do not begin development until the leaves begin to unfold on the trees in the spring. These galls are simple, seed-like or bladdery, thin-shelled developments usually not larger than the buds within which they occur. The bisexual insects mature within three or four weeks, emerging in the middle of the spring. They copulate and oviposit in the main veins, usually on the under surfaces