Page:Popular Science Monthly Volume 79.djvu/451

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INSECT PARASITISM

planation, there seems, at first sight, to be something mysterious in the insect parasite, for when we see an adult organism, such as an Ichneumon coming from a distance—out of the blue, so to speak—and seeking out a concealed larva in which to deposit its egg, we are tempted to turn to some teleological explanation, such as is implied in the common conception of "instinct," or perhaps to something in the nature of a "divinatory sympathy" between parasite and host. Although such conceptions are necessarily anthropomorphic, I would not deny them a certain, albeit provisional, value. As biologists, however, we are fortunately in a position to suggest a simpler explanation. The intimate practical knowledge (sit venia verbo) which the mother Ichneumon possesses of the host, loses much of its mystery when we stop to consider that she has, during her own larval life, devoured just such an insect, for the same reason that we may be said to have an excellent practical knowledge of an orange after we have eaten it. The Ichneumon is therefore familiar with the location, feeling, odor and taste of the creature in which she will lay her eggs, if we make the not improbable assumption that the results of her own larval experience persist as mnemic factors, notwithstanding the profound morphological and physiological changes which she has undergone during metamorphosis. There would then be nothing surprising in her tropism-like reactions to the mechanical and chemical stimuli represented by the host larva and its immediate environment.

As the time at my disposal is nearly exhausted, I must bring my discussion to a close. Having made the pilgrimage to the American Mecca of experimental zoologists, I could hardly hope for salvation if I departed without at least saluting the Kaaba. This I can do most effectively, perhaps, by calling attention to the great need of experimental work in animal and especially in insect parasitology. Biologists, during the romantic period of Darwinism, made much of the parasites. These organisms, in fact, supplied them with no end of ammunition in defence of natural selection, the influence of the environment and the biogenetic law. Then came the period of morphological minutiæ with its tacit assumption that particles of a dead organism are vastly more interesting and illuminating than the whole of a living one. During this period the parasites were, of course, sectioned and studied in the same manner as other organisms, but, since it is impossible to explain a living whole by pulling it to pieces and sticking the inert fragments together again, parasitism, which is a process and not a thing, retained its ethological interest mainly for biologists who were engaged in the practical applications of their science.[1]

Now that we have reached the third period, or that of emphasis on experiment with the living organism as the best means of elucidating

  1. In support of this statement the reader may be referred to the following general articles on insect parasitism, written by well-known economic entomolo-