14 
IIALF HOURS WITH INSECTS. [Packard. 
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 when 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 diffusion of the 
species left unrecorded. All animals and many trees and 
plants 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 injury, we have an immense number 
of insects high in the scale of organization, which subsist 
upon other insects only to kill and destroy them utterly. 
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 office 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 
