INDIRECT BENEFITS DERIVED FROM INSECTS. 185 



the trunks of large trees, without leaving a particle behind ; and in places 

 where, two or three years before, there has been a populous town, if the 

 inhabitants, as is frequently the case, have chosen to abandon it, there 

 shall be a very thick wood, and not the vestige of a post to be seen. 



I observed in a former letter, that the devastations of insects are not the 

 same in every season, their power of mischief being evident only at 

 certain times, when Providence, by permitting an unusual increase of 

 their numbers, gives them a commission to lay waste any particular coun- 

 try or district. The great agents in preventing this increase, and keeping 

 the noxious species within proper limits, are other insects ; and to these I 

 shall now call your attention. 



Numerous are the tribes upon which this important task devolves, and 

 incalculable are the benefits which they are the means of bestowing upon 

 us ; for to them we are indebted, or rather to Providence who created them 

 for this purpose, that our crops and grain, our cattle, our fruit and forest- 

 trees, our pulse and flowers, and even the verdant covering of the earth, 

 are not totally destroyed. Of these insects, so friendly to man, some 

 exercise their destructive agency solely while in the larva state ; others in 

 the perfect state only ; others in both these states ; and, lastly, others 

 again in all the three stttes of larva, pupa, and imago. For order's sake, 

 and to give you a more distinct view of the subject, I shall say something 

 on each separately. 



The first, those which are insectivorous only in their larva state, may 

 be further subdivided into parasites and imparasites, meaning by the 

 former term those that feed upon a living insect, and only destroy it when 

 they have attained their full growth ; and by the latter, those that prey 

 upon insects already dead, or that kill them in the act of devouring them. 



The imparasitic insect devourers chiefly belong to the Hymenoptera 

 order ; and though it is in the larva state that their prowess is exhibited, 

 the task of providing the prey is usually left to the female, of which each 

 species for the most part selects a particular kind of insect. Thus many 

 species of Cerceris and the splendid Chrysida or golden wasps feed upon 

 insects of their own order. One of the latter (Parnopes incarnata) com- 

 mits her eggs to the progeny of Bembex rostrata: another (^Chrysis hiden- 

 tata) attacks the young of Epiyone spinipes. 



Bembex and MelUnus confine themselves to Diptera, the former preying 

 upon Eristalis tenax, Bombylii, and the like^ ; the latter, amongst others, 

 ridding us of the troublesome Stomoxys calcitrans. One of these last I 

 have observed stationed on dung watching for flies, which, when seized, 

 she carried to her burrow. The numerous species of Crabro Fab. also 

 store up chiefly dipterous insects in their cells, some confining themselves 

 to one and the same species, others apparently taking any that offer. 



Epipone spinipes, belonging to the family of Wasps, feeds upon certain 

 green apod larvae, of which the female deposits ten or twelve with each 

 egg. The common sand-wasp {Ammoyhila vulgaris) destroys caterpillars 

 of a larger size, and most of the other Vespoid and Sphecoid Hymenop- 

 tera, viz. Trypoxylon, Philanthus, Larra, &;c. assist in this great work. 



Pompilus, to which genus probably several species mentioned by Reau- 



' Latreille, Observations nouvelks sur les Hyminopteres. Annal. de Mus. 11. 



16* 



