CHAPTER XII 

 PARASITISM 



Introductory Remarks, p. 315. Host-selection, p. 318. Phases 

 OF Parasitism. Multiparasitism, p. 322 ; Superparasitism, p. 325 ; 

 Hyperparasitism, p. 326. 



Introductory Remarks 



In discussing parasitism among insects it is necessary, in the 

 first instance, to define the Hmits of the subject. A generally 

 accepted definition describes a parasite as a living organism 

 which takes up its abode temporarily or permanently on or within 

 some other living organism (its host), and at whose expense it 

 obtains its nourishment. This definition is generally held to 

 imply that the host has developed tolerance to the presence of 

 the parasite, and consequently suffers no fatal effects from its 

 activities. 



(a) Among insects there exists a considerable number of species 

 whose behaviour is covered by the above definition. They are 

 true parasites and include the kinds discussed in text-books of 

 medical entomology and parasitology. Such parasites are almost 

 always insignificant in size as compared with their hosts which, 

 it may be added, are almost always vertebrates. Anoplura, (Estrid 

 larvae, Aphaniptera, Streblidse, Nycteribiidge and Hippoboscidae 

 are familiar examples. 



(h) There are, however, a much larger number of insect parasites 

 which exhibit a different behaviour. They belong to various 

 families of Diptera and Hymenoptei^a, but are very rare outside 

 those orders. They are parasites only as larvae, while the adults, 

 into which they transform, are highly developed, free-living 

 creatures. Their hosts are nearly always members of their own 

 class, and are nearly always destroyed as the result of the 

 parasitism. Furthermore, such parasites are relatively large in 

 size in comparison with their hosts, and this fact appears mainly 



315 



