INSECT PARASITISM 447 



planation, there seems, at first sight, to be something mysterious in the 

 insect parasite, for when we see an adult organism, such as an Ichneu- 

 mon 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 inti- 

 mate practical knowledge (sit venia verbo) which the mother Ichneu- 

 mon 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 Ichneu- 

 mon 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 improb- 

 able 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 experi- 

 mental work in animal and especially in insect parasitology. Biolo- 

 gists, 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 en- 

 vironment and the biogenetic law. Then came the period of morpho- 

 logical minutias with its tacit assumption that particles of a dead or- 

 ganism are vastly more interesting and illuminating than the whole of 

 a living one. During this period the parasites were, of course, sec- 

 tioned 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. 18 



Now that we have reached the third period, or that of emphasis on 



experiment with the living organism as the best means of elucidating 



18 In support of this statement the reader may be referred to the following 

 general articles on insect parasitism, written by well-known economic entomolo- 



