INSECT PARASITISM 433 



Boston. Here for several years past great numbers of parasites have 

 been received from Europe and northern Asia, carefully reared and 

 studied, and, when found to be sufficiently promising, liberated in the 

 hope that they will multiply and eventually control the gypsy and 

 brown-tail moths. 4 



The fact that such economic uses have been suggested for insect 

 and not for any other parasites seems to imply that the former must 

 be peculiar in certain important particulars. This I believe to be true 

 and has led to the following considerations. That I have chosen to 

 read them to you, who are primarily interested in the problems of 

 zoology in its broadest sense, is due to a conviction on my part that 

 many of the accounts of parasitism, even in the best of our zoological 

 hand-books, are more or less one-sided and anthropomorphic, probably 

 as a result of the stepmotherly treatment necessarily bestowed upon the 

 insects in such treatises. Before I say more about the insects, how- 

 ever, I wish to make a few remarks on animal parasitism in general. 



Parasitism is, of course, a form of "behavior," and may be de- 

 scribed as one of several complex types of the reactions of organisms to 

 the most important source of their energy, their food supply. Other 

 reactions to this element of the environment are predatism, commen- 

 salism, scavengerism and mutualism. There is in the main sufficient 

 consensus of opinion concerning the distinctions between these differ- 

 ent phenomena. Predatory animals kill other animals and devour them 

 wholly or in part. Parasites put other organisms in the position of 

 " hosts " by living directly on their tissues in such a manner as not to 

 cause their immediate death. The parasite thus draws indirectly on 

 the food supply of another organism by permitting or compelling it to 

 do the hard work of procuring the food and of converting it into much 

 more accessible and much more easily assimilable compounds. The 

 parasite may be said, therefore, to use its host as an instrument not 

 only for procuring, but for predigesting, its food. The commensal also 

 uses another animal as an instrument, but merely in gaining access to 

 a food-supply which the latter has procured but has not yet assimi- 

 lated. The scavenger, like the saprophyte among plants, may be de- 

 scribed as a parasite of the dead, deriving its sustenance from decom- 



4 Excellent general accounts of the subject here touched upon are contained 

 in the following papers : Marchal, ' ' Utilization des Insects Auxiliaires Ento- 

 mophages dans la Lutte contre les Insects Nuisibles a 1 'Agriculture, " Ann. de 

 I' Inst. Nat. Agronom. (2), VI., 2, 1907, 74 pp., 26 figs.; translation in part in 

 Pop. Sci. Monthly, LXXIL, 1908, pp. 352-370, 406-419; Silvestri, "Sguardo 

 alio Stato Attuale dell' Entomologia Agraria negli Stati-Uniti del Nord 

 America, etc.," Boll. Soc. Agric. Ital., XIV., No. 8, 1909, 65 pp.; Howard and 

 Fiske, ' ' The Importation into the United States of the Parasites of the Gypsy 

 Moth and the Brown-tail Moth," Bull. No. 91, Bur. of Ent., Dep. Agric, 1911, 

 312 pp., 73 figs., 25 pis. 



VOL. LXXIX.— 30. 



