XXIII PARASITES 52 1 



Insects derive their sustenance primarily from the vegetable 

 kingdom. So great and rapid are the powers of assimilation of 

 the Insect, so prodigious its capacity for multiplication, that the 

 Mammal would not be able to compete with it were it not that 

 the great horde of six-legged creatures has divided itself into two 

 armies, one of which destroys the other. The parasitic Hymen- 

 optera are chiefly occupied in destroying the tribes of vegetarian 

 Insects ; the parasites do this by the simple and efficient device 

 of dwelling in the bodies of their hosts and appropriating the 

 nutriment the latter take in. The parasites do not, as a rule, 

 eat the structures of their host, many of them, indeed, have 

 no organs that would enable them to do this, but they 

 absorb the vegetable juices that, in a more or less altered state, 

 form the lymph or so-called blood of the host. The host could 

 perhaps starve out his enemies by a judicious system of absten- 

 tion from food ; instead, however, of doing this, he adopts the 

 suicidal policy of persistent eating, and as the result of his 

 exertions, furnishes sufficient food to his parasites, and then 

 dies himself, indirectly starved. Eatzeburg considers that the 

 traditional view that the larval of parasitic Hymenoptera live, 

 by eating the fat-body of their host is erroneous. They imbibe, 

 he considers, the liquid that fills the body of the parasitised 

 Insect.^ 



The wide prevalence of Insect parasitism is appreciated only 

 by entomologists. The destructive winter moth Cheimatohia 

 hrumata is known to be subject to the attacks of sixty-three 

 species of Hymenopterous parasites. So abundant are these 

 latter that late in the autumn it is not infrequently the case 

 that the majority of caterpillars contain these destroyers. Al- 

 though Lepidoptera are very favourite objects with parasitic 

 Hymenoptera, yet other Insects are also pertinaciously attacked ; 

 there is quite a host of Insect creatures that obtain their susten- 

 ance by living inside the tiny Aphididae, or " green-flies," that 

 so much annoy the gardener. A still larger number of parasites 

 attack eggs of Insects, one or more individuals finding sufficient 

 sustenance for growth and development inside another Insect's 

 e^g. As Insects have attacked Insects, so have parasites attacked 

 parasites, and the phenomena called hyperparasitism have been 

 developed. These cases of secondary parasitism, in which another 



^ Ichneumonen der Forstinsecten, i. 1844, p. 86. . 



