INSECTS IN RELATION TO MAN 421 



>rincipal part of the science; for until these are clearly and completely 

 lade out for any given injurious species, we cannot possibly tell when, 

 rhere or how to strike it at its weakest point. 



"But besides this, we must also know the conditions favorable and 

 infavorable to it; the enemies which prey upon it, whether bird or insect 

 or plant parasite ; the diseases to which it is subject, and the effects 

 of the various changes of weather and season. We should make, in fact, 

 a thorough study of it in relation to the whole system of things by which 

 it is affected. Without this we shall often be exposed to needless alarm 

 and expense, perhaps, in fighting by artificial remedies, an insect already 

 in process of rapid extinction by natural causes; perhaps giving up in 

 despair just at the time when the natural checks upon its career are about 

 to lend their powerful aid to its suppression. We may even, for lack 

 of this knowledge, destroy our best friends under the supposition that 

 they are the authors of the mischief which they are really exerting 

 themselves to prevent. In addition to this knowledge of the relations 

 of our farm pests to what we may call the natural conditions of their life, 

 we must know how our own artificial farming operations affect them, 

 which of our methods of culture stimulate their increase, and which, if 

 any, may help to keep it down. And we must also learn where strictly 

 artificial measures can be used to advantage to destroy them. 



"For the life histories of insects, close, accurate and continuous 

 observation is of course necessary; and each species studied must be fol- 

 lowed not only through its periods of destructive abundance, when it 

 attracts general attention, but through its times of scarcity as well, and 

 season after season, and year after year. 



"The observations thus made must of course be collected, collated 

 and most cautiously generalized, with constant reference to the con- 

 ditions under which they were made. , No part of the work requires 

 more care than this. 



"This work becomes still more difficult and intricate when we pass 

 from the simple life histories of insects to a study of the natural checks 

 upon their increase. Here hundreds and even thousands of dissections 

 of insectivorous birds and predaceous insects are necessary, and a care- 

 ful microscopic study of their food, followed by summaries and tables 

 of the principal results, a tedious and laborious undertaking, a specialty 

 in itself, requiring its special methods and its special knowledge of the 

 structures of insects and plants, since these must be recognized in frag- 

 ments, while the ordinary student sees them only entire. 



"If we would understand the relations of season and weather to the 



