INSECTS IN RELATION TO INSECTS. 553 



attacks, but not insects from the spiders, for almost all of the latter 

 feed upon insects only. Indeed there are no parasitic spiders, for they 

 are all absolutely animals of prey, which capture insects either by 

 cunning or by nets, they then kill them, and suck out all their 

 nutrirnental juices. A great portion of the Acari, however, are parasitic 

 upon insects, and not, indeed, as we find the parasitic Acarl upon 

 birds and Mammalia, beneath the epidermis, but upon it, and chiefly 

 where a delicate connecting membrane binds two plates together^ 

 We thus find Gamasus coleoptratorum especially upon the dung and 

 carrion beetles, namely, Necropkorus, Scarabcetis, and the Histers ; 

 several of the Trombidia upon different winged insects, namely, 

 LibeUula;, gnats., aphis, &c., and Ocypela rubra, as a small cochenille 

 spot, upon several also often very small Diptera, and, lastly, the genus 

 Aclysia, even upon the water-beetles. They dwell also upon their 

 connecting membrane, and are besides enveloped in a peculiar case, 

 whence through a particular aperture the creature projects its mandibles, 

 and bores into the skin of the beetle *. 



Much more numerous are the relations, considered from our present 

 point of view, in which insects of different families stand to each other. 

 We might assert that the predaceous beetles and those which live upon 

 animal substances, carry on as it were a war against those which feed 

 upon vegetables, and upon which they feed with the same voracity as 

 the latter do upon plants. Some, as the Carabodea, Dytici, and 

 Staphylini, do this, particularly in their perfect state, hunting down 

 the larvae of the herbivorous ones, and devouring all that they can 

 catch ; whereas others, and these especially are the most dangerous 

 enemies, seek to lay their eggs in the bodies of the larvae, thus presenting 

 their young with food in the body of a living creature. We find this 

 habit among the Ichneumons and the Tachince. Both select for this 

 purpose almost exclusively the caterpillars of Lepidoptera, and as a 

 caterpillar thus pierced must sooner or later die, they therefore consi- 

 derably restrict the influence such caterpillars have, by the destruction 

 of plants, upon the advance of the vegetable kingdom. The larvee of 

 the minute Pteromali live in a similar manner in the bodies of the 

 Cocci and Aphides, the influence of which is not less upon the de- 

 crease of the vegetable kingdom than that of the caterpillars of the 



* Viet. Audouin in Mm. de la Soc. d'Hist. Nat. dc Paris, torn. i. p. 98, PI. V. f. 2. 

 Diet, dcs So. Nat. Art. Aclysia, 



