322 PARASITISM 



which usually form their cocoons shortly afterwards ; Polysphincta is a 

 parasite of spiders and the Vespoid Rhopalosoma pceyi is similarly an 

 ectoparasite of the tree cricket Orocharis. 



Phases of Parasitism 



Multiparasitism. Multiparasitism is the simultaneous infesta- 

 tion of the same individual host insect by two or more species of 

 primary parasites. The phenomenon is more usually known as 

 superparasitism, but it is proposed to limit the use of the latter 

 expression to instances in which an individual host is attacked by 

 tw^o or more parasites pertaining to the same species. 



The prevalence of multiparasitism is frequently dependent 

 upon whether or not the female parasites, at the time of oviposition, 

 are able to distinguish between unattacked healthy hosts and 

 those already parasitised. In other words, its occurrence would 

 appear to be dependent upon errors of instinct. It also obtains 

 in other cases as the result of pressure of competition induced by 

 a high ratio of parasites to host-population, when oviposition 

 in at least some proportion of already parasitised individuals 

 becomes unavoidable. Irrespective of the inducing cause, multi- 

 ])arasitism plays an important part in the economy of parasites, 

 but it has so far been subject to very little exact experimental 

 investigation. The manifestations of the phenomenon are various, 

 and the following classification covers the phases which prevail. 



1. Multiparasitism Involving the Survival of a Single Parasite 

 Species. This occurrence appears to be very prevalent and the 

 survivor may bring about the death of the other parasite or 

 parasites in one of two ways, viz., either by direct attack or by 

 the indirect effects of its presence. The studies of Pemberton 

 and Willard (lois) on the inter-relations of fruit-fly parasites in 

 Hawaii have shown that when the Braconids Diachasma tryoni 

 Cam. and Opius humilis Silv. occur together in the same individual 

 fruit-fly larva? the last-mentioned species was usually killed. It 

 succumbs as the result of wounds and lacerations inflicted upon 

 it by the long, curved, sickle-like mandibles of the newly hatched 

 Diachasma larvae. They mention that from a total of 627 fruit- 

 fly puparia examined, 143 contained larvae of the two species of 

 parasites, and that in 133 of these the Opius larvae were found to 



