316 PARASITISM 



accountable for their fatal activities. At the outset they behave 

 as true parasites, deriving their sustenance mainly from the blood 

 of their hosts, which suffer little ill-effects and continue to feed 

 and grow. Sooner or later the fat-body and less vital organs are 

 attacked, and the parasites become, in the biological sense, 

 internal predators. Growth of the hosts at this stage virtually 

 ceases, but they continue to live, and often feed voraciously. 

 As the parasites near maturity their carnivorous propensity 

 intensifies, and all food-yielding parts of their hosts are ultimately 

 consuined until only the exoskeleton remains. As their final 

 economy they, as often as not, conduct their pupal transformation 

 within the husks of their hosts, and thus sheltered in these grim 

 cells they are able to dispense with cocoons. " Fatal " parasites 

 of this kind have been termed by O. M. Reuter, and subsequently 

 by W. M. Wheeler, " parasitoids." Among other entomophagous 

 parasites the Strepsiptera are to be regarded as non-fatal 

 parasitoids, since their hosts are not usually destroyed as the 

 direct result of the parasitisation ; in this respect they are 

 annectant between a and b. 



(c) Parasitoids may live either externally or internally in 

 relation to their hosts. It seems tolerably clear that the 

 ectoparasitic life is the simpler condition of the two, and that the 

 endoparasitic method is a development from it. There is every 

 probability that the parasitoidal mode of life originated from 

 simple predatism. A predaceous insect is one which seeks out 

 and devours a succession of living organisms or prey until its 

 needs are satisfied. It follows, therefore, that a true predator 

 must have its locomotory and sensory organs sufficiently developed 

 to equip it for this mode of existence. In the eventuality of a 

 single individual prey being large enough to provide a carnivorous 

 insect with all the food it needs, there would seem to be little 

 difference between a predator and an " ectoparasitoid." Since, 

 however, the larval life of the latter is spent in close and continuous 

 association with a single host, and it exhibits the degeneration 

 correlated with such an existence, it shares the characteristics of 

 a parasite rather than of a predator, and this distinction is the 

 one commonly adopted. 



