INSECT PARASITISM 445 



like every other healthy organism, can afford to sacrifice to the acci- 

 dents of its environment. 17 



The parasite not only tends to restrict itself to the use of a particu- 

 lar host as a food-procuring instrument, but is also compelled to exer- 

 cise the most exquisite care in the use of this instrument. From the 

 very nature of the situation, therefore, parasitism is an extremely pre- 

 carious type of behavior. But this is true also of all highly specialized 

 behavior, that of biologists included, and points the way to, but does 

 not constitute, the real difficulty with parasitism. This, I take it, is the 

 suppression of the voluntary movements, which necessarily results from 

 the intimate host relations, especially when these are confined, as is 

 so often the case, to some one particular organ or tissue. It is not, 

 therefore, the parasite's habit of taking something for nothing from 

 another organism, that is so fatal, for all creatures, in matters relating 

 to nutrition, find it more blessed to receive than to give, but the ac- 

 ceptance of the most important supply of its energy under conditions 

 that preclude an exercise of the muscular and hence also of the sensory 

 and nervous activities and restrict its vital activities to a round of as- 

 similation, metabolism and reproduction. This unbalancing of func- 

 tions is probably hastened by a kind of intraorganismal parasitism or 

 " Kampf der Theile " in Eoux's sense, the alimentary and reproductive 

 tissues drawing their nutriment not only from the host but also from 

 the more inactive tissues of the parasite's own body. That this torpor, 

 or inactivity of the neuromuscular system is at the bottom of the 

 peculiar disability of parasites is shown by many non-parasitic organ- 

 isms, which have easy access to an abundant food supply consisting of 

 dead or inorganic substances. Most plants and many invertebrates, 

 such as the barnacles, and especially the scavengers among insects, ex- 

 hibit essentially the same modifications as parasites. In fact, the larval 

 stages of many insects that feed on carrion or decomposing animal 

 and vegetable matter, are quite indistinguishable from parasitic larvae. 

 This and the further fact that plant-eating species are not generally 

 regarded as parasites by entomologists have led to considerable con- 

 fusion in certain accounts of insect parasitism. 



While most parasites among the lower invertebrates have never suc- 

 ceeded in freeing themselves from the tyranny of the host relation and 

 the fatal torpor to which it inevitably leads, this is, as we have seen, by 

 no means true of the typical insect parasites. To the ontogeny of these 

 organisms the dictum " once a parasite, always a parasite " most cer- 



17 Within this ' ' margin of vitality ' ' must also be included the reproductivity 

 of the host species. Thus certain ants, like Formica fusca, throughout the north 

 temperate zone, are able to survive the inroads of a number of parasitic ants 

 (Polyergus rufescens, Formica sanguiyiea, F. rufa, F. exsecta and many allied 

 species), largely on account of its great reproductive powers, coupled with an 

 ability to live in the most diverse physiographic conditions. 



