.156 NATURAL HISTORY OF WASPS. 
The stereotyped answer to the question of what 
good insects do is, that they destroy other insects. 
They impregnate flowers, and they do a great many 
other things which do not concern us just now; but, 
above all, they consume rubbish and noxious sub- 
stances. Under the term rubbish and noxious sub- 
stances many miscellaneous articles are included, 
wasps, for instance, things laid away in drawers, 
collections of insects perhaps and the most poisonous 
drugs. What is one creature’s food is another's 
poison, and the extent to which some of the most 
virulent poisons are consumed by insects is very 
singular. Some things which insects eat or drink 
more particularly greedily are poisonous to them, 
but, as a rule, it is by substances which reach them 
through their respiratory organs, rather than by those 
which they assimilate, that they are destroyed with 
the greatest facility. The low degree in which the 
nervous system of insects is developed explains, in a 
great degree, their immunity from the action of cer- 
tain of the more deadly poisons. But there is much 
in this whole subject which we cannot understand, 
because we cannot fairly examine the subject. We _ 
can analyze the ultimate results of some of the pro- 
cesses of digestion, but we cannot follow these pro- 
cesses in detail. 
The general principles of digestion in insects, 
however, may be expressed very briefly, and are 
plain enough. Thus, speaking with more particular 
reference to Wasps, the larva is supplied abundantly 
with food, while its respiration is very imperfect. 
The same result happens in a wasp’s cell as in a pig’s 
stye or a bullock’s stall; the food is imsufficiently 
