HOST-SELECTION 321 



The outstanding feature with respect to host-selection by 

 ectoparasites is that they very rarely affect hosts living openly 

 and exposed. This peculiarity is very constant and it may 

 possibly have originated as an adaptation whereby the exposed 

 parasites are enabled to secure a measure of protection from the 

 attacks of hyperparasites. Among parasites exhibiting this kind 

 of host-preference the Chalcid Melittohia acasta affords an 

 instructive example, since it selects a wide range of victims, which 

 are distributed among the orders Diptera and Hymenoptera. 

 As Thompson and Parker point out, in all cases the hosts are 

 enclosed in cells or nests of mud or " paper," in silken cocoons or 

 in chitinised puparia. Also, in their analysis of features common 

 to these hosts they find that the latter are at least twice the size 

 of the parasite and are surrounded by a protective envelope which 

 is separated from the body of the host by an intervening space. 

 Genieys (1925) has made an experimental study of the Braconid 

 Habrobracon brevicornis, which is an ectoparasite of naked and 

 more or less sedentary Lepidopterous larvae living in galleries in 

 plants or immersed in stored vegetable products. His efforts 

 to induce this species to parasitise hosts that live exposed or are 

 densely hairy usually failed ; in such cases oviposition either did 

 not occur or the young parasitic larvae generally failed to develop 

 after hatching. Certain pupal parasites of Cyclorrhaphous 

 Diptera are sensu stricto ectoparasites, since they live attached to 

 the outsides of the pupae just beneath the puparial shell. The 

 larval stages of several species of the Staphylinid genus Aleochara 

 and of the Chalcid Mormoniella vitripennis (Nasonia brevicornis) y 

 for example, are passed in this manner. 



In a few exceptional instances, exposed, free-living hosts are selected. 

 Certain species of the Chalcid genus Euplectrus are ectoparasites of 

 Noctuid larvae which feed openly on Jow plants. This genus is also 

 exceptional in that it has acquired the habit of cocoon-formation 

 which is wanting in other Chalcids {vide Thomsen, 1927). The cocoon 

 substance is a product of the Malpighian tubes which is discharged 

 through the anus, and it would seem that its utilisation in this manner 

 has been evolved as a protection for the vulnerable pupae. In another 

 Chalcid, Schizonotus sieboldii, Cushman has shown that its larvae are 

 ectoparasites of the exposed pupae of the beetle Melasoma interruptum. 

 The Ichneumon Paniscus parasitises Noctuid and other caterpillars 



K.A. ENTOMOLOGY. 11 



