INTRODUCTION 3 



for these beneficial forms, which prevent the normal increase, many 

 of our common injurious insects would become so numerous as to 

 practically prohibit the growth of crops affected. 



Value of study of insects. The strictly economic aspect of insect 

 life is not, however, the only phase worthy of our attention and 

 study. The apathy with which the study of entomology was for- 

 merly treated was unquestionably due to the general lack of interest 

 in biology until recent years. During the last generation it has been 

 more and more appreciated that man is but a child of nature, and 

 that he can learn much in the proper conduct of his affairs by a 

 study of the laws of life in general, whether of the uncivilized races 

 of mankind, of insects, or of microscopic bacteria or protozoa. Our 

 grandfathers hardly knew that bacteria existed ; to-day most of the 

 science of pathology, and much of the practice of medicine, is based 

 on an understanding of their life. It would seem, therefore, that 

 insect life should furnish a large field for the student of general 

 biology, and more and more biological problems of fundamental 

 importance are being worked out through studies of insects. 



That this should be the case is extremely obvious when we 

 remember that there are over 300,000 known species of insects, 

 including over four fifths of the described species of animals, and 

 that at the rate at which they are being described, it has been esti- 

 mated that over a million species exist. The immense number of 

 insects, both of species and of individuals, is undoubtedly due to 

 their varied structure, which enables them to live under all possible 

 conditions. Thus the larvae of many different species are adapted 

 so that they live entirely in water, others bore in trees and plants, 

 some are subterranean, while still others inhabit the tissues of do- 

 mestic animals or of other insects. By the aid of their wings the 

 adults spread rapidly and are thus able to migrate when necessity 

 arises. Thus the insects possess such diversity of structure and 

 habit that they are able to live under all external conditions, and 

 on account of their immense numbers they have been able to adapt 

 themselves to a changing environment which would have entirely 

 obliterated classes or species few in number. 



Not only are insects the most abundant form of animal life, but 

 they exhibit the highest degree of intelligence of any of the lower 

 or invertebrate animals. The wisdom of the ant and the industry 



