40 
limited to a knowledge of the methods of destroying them or pre¬ 
venting their injuries. It is often scarcely less useful to be able to 
predict the amount of their injuries and the length of time over 
which these will probably extend ; and such prediction must almost 
always take into account the variety, number, rate of multiplication 
and activity of their natural enemies. The gardener, for example, 
whose produce seems threatened by hordes of plant-lice, may rest 
easy when he sees that the number of their parasites or carnivor¬ 
ous enemies is rapidly increasing, since the time must be short until 
these entirely check the multiplication of their prey. Again, al¬ 
though no successful attempt has yet been made to increase the 
number of our insect friends by special or artificial measures at any 
given time or place, the possibility of the final success of such 
efforts is always to be borne in mind. Cases are not infrequent, 
however, in which it is possible to avoid involving the enemies of a 
pest in measures taken for the destruction of the pest itself; so that 
the beneficial species may easily be made to preponderate relatively 
to the number of the injurious species remaining; but for this a 
thorough knowledge of. the economy of both is of course essential. 
Finally, since the conditions of insect life vary greatly from year to 
year and even from generation to generation, a species of hitherto 
trivial significance may hereafter rise to first-class importance as a 
check upon the ravages of an insect enemy. 
For these and other reasons, it has been customary'for all writers 
on economic entomology to give descriptions and life histories of all 
known enemies of the injurious insects treated. 
The earliest reference to insect enemies of the chinch-bug, which 
has come to my attention, is in a paper by Mr. B. D. Walsh, upon 
insects injurious to vegetation in Illinois, published in the fourth 
volume of the Transactions of the Illinois State Agricultural Society, 
for 1859-30. In examining a field of sweet corn in September, he 
noticed numerous chinch-bugs in the husks (some imagos and some 
pupae), and found also quite a number of specimens of four very 
common species of lady-bugs (Coccinellidae), all the known American 
species of which are more or less carnivorous. With the exception 
of the chinch-bugs, and a few individuals of an allied species of 
Hemiptera, there were no other insects under the corn husks. “The 
idea at once occurred to me,” he says, “that these lady-bugs were 
depredating upon the chinch-bugs, and I was confirmed in the 
opinion upon finding a pupa, wdiich was evidently that of some coc- 
cinellid, probably Coccbieila munda, Say, in the same situation. 
Now, since the pupa was there, the larva must also have lived there, 
for it is not the habit of these larvae to get into holes and corners 
to complete their transformations; and if the larvae lived there, 
there was nothing else for them to live on but the above mentioned 
two species of bug, the smaller of which never occurs in any great 
numbers like the larger and more mischievous chinch-bug. That 
the lady-bugs were then and there preying upon chinch-bugs, I have 
but littie doubt; but it does not necessarily follow that they habit¬ 
ually prey upon chinch-bugs. They might have been driven to prey 
upon them for lack of more agreeable food; as a cat will sometimes 
eat bread, when she cannot obtain meat. Nothing but actual experi¬ 
ment and observation can determine the truth in this matter.” In 
