Page:Popular Science Monthly Volume 79.djvu/443

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INSECT PARASITISM

lectual beings necessarily set great store, and the hypertrophy of the alimentary and reproductive organs, which, notwithstanding their immense biological significance, have nevertheless been assigned a very inferior place in our scheme of ethical values. But parasites may properly be regarded as more advanced organisms than the predators, for they have not only had a more eventful phylogenetic career, but, during their long history, have learned to use other organisms in a very economical manner as instruments of nutrition. From a consistent biological point of view, therefore, and from one embracing insect as well as vermian and crustacean parasites, it is evident that the peculiar convergent complexions of these organisms should be attributed to specialization. "Degeneration" is properly a pathological term, and parasites, however pathogenic they may be, are, of course, no more pathological or diseased than predatory animals. There is some evidence to show that the myzostomes have persisted in their modern form since Silurian times, with a conservatism equalled only by that of their Crinoid hosts. If all the generations of these peculiar annelids have been pathological for millions of years, they should long since have disappeared from the waters of the globe, but we find that though many or all of the original species have doubtless become extinct, this was probably due simply to the extinction of their hosts, for nearly every extant species of Crinoid supports at least one species of Myzostoma. Moreover, if we regard parasitic modification as an expression of degeneration, we must suppose that such forms as the adult Ichneumon are produced by a post-larval regeneration. Apart from adding an unusual meaning to the word "regeneration," this fails to express the actual conditions correctly. The whole ontogeny of such insects is in reality very highly specialized, the adult representing in many particulars as great a departure from the primitive insect type as the larva, albeit in a very different direction. In discussions of this subject I would therefore substitute the words "parasitic specialization" for such terms as "degeneration" and "degradation." Together with these, another term, "retrogression," should be avoided, for the reason that the parasitic modifications of structure to which it is often applied can be more properly attributed to "arrest of development."

It will be seen from the foregoing discussion that the leading peculiarity of insect parasitism, at least in such groups as the Hymenoptera and Diptera, which are almost the only ones of value in controlling noxious insects, is the restriction of the parasitic habit to the sluggish larva and the specialization of the free adult for the purpose of disseminating the species and of placing the coming generation in intimate contact with the host. No one who observes one of our large, graceful Ichneumonids, such as Thalessa lunator, alighting on a tree-trunk and then conveying its greatly attenuated eggs by means of its long hair-like ovipositor through some three inches of hard wood into