Page:Popular Science Monthly Volume 79.djvu/448

444, therefore, that the existence of parasites of the Ichneumon type, with free, active and highly developed adults is rendered possible by an inhibition of gonadic growth during larval life; whereas parasites which begin to reproduce while still living with their hosts are thereby prevented from either leaving them or undergoing further morphological differentiation.

3. A third primitive peculiarity of holometabolic insects, which seems greatly to have favored parasitism, is the astonishing rapidity of their larval metabolism and growth and the equally remarkable quiescence of their pupal stages. These have, of course, converted insects into the most wonderful opportunists, through enabling them to take advantage not only of the changing seasons and the very diverse physical conditions of our planet, but also of the most evanescent supplies of food, both living and in process of decomposition.

4. Parthenogenesis may also be cited as a widely prevalent phenomenon, which has been put to good use by parasitic insects. Like polyembryony, it has an economic significance, because it enables such noxious parasites as the plant-lice to multiply enormously under conditions that would preclude reproduction in non-parthenogenetic species, and for the same reason greatly assists many Hymenopterous parasites in checking the undue multiplication of these and other plant-destroying insects.

Although I may have had little difficulty in convincing you that parasitism is a very specialized kind of behavior, you will probably still be of the opinion that there is something inherently and radically wrong with animals that resort to it rather than to predatism, mutualism or some other means of maintaining their vital activities. It must, of course, be admitted that in becoming satellites of their hosts, parasites have renounced the primitive, wasteful and erratic freedom of the predator and are compelled to mould their activities on those of the host. This necessarily puts them in a condition of such abject dependence that their very existence as individuals and species is imperilled whenever they overstep that margin of vitality which the host,