320 PARASITISM 



innate determining factors are physiological. Certain experiments 

 recorded by Thompson and Parker (1927) are significant in this 

 connection, who mention that although larvae of the Tachinid 

 Sturmia scutellata develop freely in caterpillars of Porthetria 

 dispar they die immediately after penetrating into those of 

 Hemcrocampa leucostigma and Notolophus antiquus, which belong 

 to the same family. On the other hand, Sturmia larvae develop 

 freely in caterpillars of Malacosoma, which belong to a different 

 family — the Lasiocampidae. Webber and Schaffner mention that 

 one of the numerous hosts of Compsilura concinnata is also 

 Porthetria dispar, but yet this Tachinid rejects caterpillars of 

 Malacosoma, which are acceptable to the Sturmia. The opinion 

 has been advanced that the activities of the phagocytes of the 

 hosts constitute a defensive mechanism against endoparasites. It 

 is claimed that species of the latter are destroyed by this agency, 

 and only those kinds specially adapted by physiological or structural 

 features are able to survive and become regular parasites of specific 

 hosts. The evidence in support of this contention is scanty, and 

 the theory is discussed in a recent paper by Thompson (1934, p. 433). 

 The host-preferences of ectoparasites present several peculiar 

 features. The ectoparasitic mode of life is unknown, for example, 

 in the whole family of the Tachinidae, whereas it is frequent 

 among diverse groups of Hymenopterous parasites. The factors 

 that determine whether a species lives within or external to its 

 host are evidently fundamental, since the endoparasitic mode of 

 life involves a complete change in methods of respiration. In this 

 connection it is remarkable that certain parasite genera, notably 

 the Chalcid Aphelinus, include among their species some which are 

 endoparasites and others that are ectoparasites. It is open to 

 speculation whether the internal physiological reactions of the host 

 are a determining factor which force, as it were, certain parasites to 

 live externally, while better adapted, but closely related, species 

 are able to pursue an endozoic existence. Whether such reactions 

 are a potent factor in this case or not is scarcely amenable to proof, 

 and, at any rate, the capacity of a species to undergo the correlated 

 respiratory changes involved in an endoparasitic mode of life is 

 at least as important. 



