444 THE POPULAR SCIENCE MONTHLY 



elude, 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 pre- 

 vented from either leaving them or undergoing further morphological 

 differentiation. 16 



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 quies- 

 cence 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 phys- 

 ical 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 phe- 

 nomenon, 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 con- 

 ditions 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-destroy- 

 ing 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 de- 

 pendence that their very existence as individuals and species is im- 

 perilled whenever they overstep that margin of vitality which the host, 



16 This singular ability of the insect to inhibit the development and growth 

 of its gonads till adult life is not only significant in connection with the develop- 

 ment of parasitism within the group, but is also of fundamental importance in 

 the development of colonial life among all the social insects. In the worker 

 castes of these organisms the inhibition of the gonads, except under unusual 

 conditions, is simply prolonged into and throughout adult life. Perhaps in last 

 analysis this inhibition is merely a special manifestation of the extraordinary 

 independence of the insect soma and germ-plasm, as has been so beautifully 

 shown in the castration and transplantation experiments of Oudemans, Meisen- 

 heimer, Eegen and Kopec. For a discussion of this subject see my paper, "The 

 Effects of Parasitic and Other Kinds of Castration in Insects," Journ. Exp. 

 Zool, VIII., No. 4, 1910, pp. 377-438, 7 figs. 



