14 HALF HOUIIS WITH INSECTS. [Pacicari>. 



which swarms over our wheat-field might easily be mistaken 

 by the farmer for what seems to him to be merely small or 

 "young" mosquitoes, so similar are the mature stages of the 

 two : though v.hen a larva the one preys upon our crops, and 

 the other does service to mankind as a scavenger of our 

 ponds and pools. So also the bed bug, which we can with 

 safety pronounce to be essentially a blood-sucker, has many 

 allies which are innocuous sap-suckers. Indeed our wonder 

 is continually excited at finding insects of very similar form 

 with habits of life most strikingly opposed. 



Any account of the natural history of an insect would be 

 very imperfect, were the habits and description of the pecul- 

 iar parasites that check the increase and diff"usion of the 

 species left unrecorded. All animals and many trees and 

 l^lants are exposed to annoyance from the continued attacks 

 of other species, without having their actual existence en- 

 dangered ; but among insects the term parasitism has an- 

 other and extraordinary meaning ; since besides those minute 

 forms of lowly organized life which only harass without 

 inflicting more serious injur}-, we have an immense number 

 of insects high in the scale of organization, which subsist 

 upon other insects only to kill and destroy them utterl}'. 

 Thus of the ordinary parasitic plants and animals which 

 always live on insects Dr. Leidy has given us in his "Flora 

 and Fauna within Living Animals" published in the "Smith- 

 sonian Contributions to Knowledge," some representations, 

 of remarkable delicacy and beauty, of miniature forests of 

 microscopic plants which line the alimentary canal of sev- 

 eral sorts of ground-inhabiting insects. There are not 

 wanting here even divers sorts of low and exceedingly mi- 

 nute worms, part of whose ofllce it may be is to restrain and 

 keep within bounds the vegetation which luxuriates in those 

 strange passage ways. 



In the other form of parasitism the insect devours all the 

 soft parts of the body of its victim, leaving but the empty 



14 



