18 
INSECTS IN GENERAL. 
species which are injurious to mankind, chietly by depredating upon 
valuable cultivated crops, are much more numerous, although consti- 
tuting but a very small proportion of the whole insect world. It is im- 
portant to bear in mind that in these destructive operations insects oc- 
cupy an exceptional or abnormal position, and that we ourselves have 
been the means of bringing about this state of things, by the excessive 
cultivation of certain plants, whereby a corresponding increase of cer- 
tain species of the insects which feed upon them has been induced. It 
is very rarely that any such loss of balance between the insect and the 
vegetable worlds takes place iu the state of nature ; and yet, such oc- 
currences are not wholly unknown. This has happened most remark- 
ably in the case of wood -eating insects, there being instances on record 
in which extensive tracts of forest trees have been destroyed by the 
larv® of some of the more minute wood-boring beetles. 
But, as just stated, it is in their depredations upon some one or other 
of the more valuable cultivated crops that insects have come into 
the most direct and serious contlict with human interest. These depre- 
dations, as is well known, have often been of a most extensive and ruin- 
ous character, causing the annual loss of crops to the value of many 
millions of dollars, and in some seasons and localities, necessitating the 
total abandonment of some of the most valuable and staple produc- 
tions, such as wheat, barley and potatoes, and also some of our choicest 
fruits, such as the plum and the peach ; and sometimes threatening the 
destruction even of the most valuable fruit of all — the hardy and widely 
distributed apple. Theso destructive operations of insects have neces- 
sarily attracted to them the most earnest attention of both practical 
and scientific men, and many valuable treatises and reports have been 
written which have been devoted chiefly to the practical treatment of 
the subject. It is our present intention to treat of insects from a more 
general and comprehensive point of view. 
GENERAL UTILITY OF INSECTS. 
From what has just been said, it is evident that it is in the nature of 
their food and their food-taking habits, that insects hold the closest 
relationship to human interests ; and this is true not only in the direct 
manner above described, but also indirectly, by means of the important 
parts which they fulfill in the economy of nature. Indeed, the opera- 
tions of insects in this last respect are of such vast importance, that it 
would be safe to say that if theso should cease, the earth would soon be- 
come uninhabitable by mankind. These operations consist chiefly, first, 
in the destruction of other insects by the predaceous and parasitic kinds, 
whereby the excessive increase of the former is held in check ; secondly, 
