240 THE SPICULIFERA. 



the insects of nearly every order are more or less ex- 

 posed to the attacks of particular species, and as a 

 general rule each species of these parasites appears 

 to have some particular insect which it is specially 

 appointed to keep in check, and in which alone it 

 deposits its eggs. And in this respect nothing per- 

 haps is more remarkable, or more conducive to show 

 us the intricacy of the mechanism by which the 

 balance of power is maintained in the oeconomy of 

 nature, than the circumstance that many of the 

 species of these parasitic insects, whose duty is evi- 

 dently that of keeping down the excessive increase of 

 their vegetable-feeding brethren, are themselves in 

 turn kept in check by other species, which, by some 

 infallible test, discover the concealed abode of their 

 larvae, and thus avenge the hapless victim upon whose 

 substance they are remorselessly preying. Swift tells 

 us that 



Naturalists observe, a Flea 

 Hath smaller fleas that on him prey, 

 And these have smaller still to bite 'em, 

 And so proceed ad infinitum ; 



and although this may not be literally true of the 

 active little tormentors of our own persons, it cer- 

 tainly applies pretty well to the Ichneumons. 



The larger species as a general rule only deposit a 

 single egg in the body of the insect which they select 

 as their victim, and they appear to possess some 

 mysterious sense, by which they are enabled to judge 

 whether the creature on whom they propose to confer 

 their favours has already received a similar ill-omened 

 guest. Of the smaller species, those which deposit 

 their eggs in insects much larger than themselves 

 usually lay a considerable number in each individual. 



