ever, the parasite quickly destroys it by consuming its vital tissues. It then completes its own growth, pupates and eventually emerges as a very active, highly organized and beautifully colored fly, provided with a splendid nervous system, exquisite sense-organs and powerful locomotor organs in the shape of legs and wings. It is either a male or a female and, if of the latter sex, soon proceeds to place its offspring in immediate contact with the host. Although the larval Ichneumon exhibits modifications of structure almost as extreme as those of the adult Sacculina, these produce no effect on the organization of the adult insect. The association of the larva with its host is the work of the mother insect, a creature gifted with complex instincts that enable her to ferret out the host even in the most intricate concealment. The large size and small number of her eggs and her highly specialized method of oviposition indicate very clearly that chance, which plays such a role in the life-cycle of the tape-worm and Sacculina, has given way to an almost inevitable association of the parasite with its host.
Of course, the Ichneumon represents only one of many forms of parasitism among insects. I have chosen it because it is the most characteristic and most highly specialized. There are insects like the Strepsiptera and the Rhipiphorid and Meloid beetles which seem to combine the Sacculina with the Ichneumon type in that they produce many small eggs that hatch as very active triungulin larvæ and only later develop into legless, bag-like larvæ of the Ichneumon type. It is interesting to note that in the Strepsiptera the adult female prolongs the parasitic habit of the larva, while the adult Meloidæ or oil-beetles are rather sluggish and seem to show other after-effects of their larval life. There are also many insects, like the true lice and bird lice which are, to all intents and purposes, permanent parasites comparable with the ectoparasitic flukes, though they never exhibit such extreme modifications. And, finally, there are other animals besides insects that have parasitic larval and free adult stages, e. g., the fresh-water mussels.
Zoologists have naturally been deeply impressed by such wonderful parasites as the tape-worms, flukes and Sacculina and have regarded these as fine examples of degeneration or degradation. Many, indeed, have dwelt on these words in a manner which leaves no doubt that they are used in a purely anthropomorphic sense as implying deterioration or "an impairment of natural or proper qualities" in the parasites, notwithstanding Ray Lankester's assertion that "degeneration may be defined as a gradual change of the structure in which the organism becomes adapted to less varied and less complex conditions of life." It is easy to trace the source of this anthropomorphism to the atrophy of the parasite's neuro-muscular system, a system by which we as intel-
- Cf. Lefevre and Curtis, "Reproduction and Parasitism in the Unionidæ," Journ. Exper. Zool., IX., No. 1, 1910, pp. 79-115, 5 pls.
- "Degeneration," p. 32.