1918 ENTOMOLOGICAL SOCIETY. 87 



all Nature is linked together into a system, one part dependent upon another 

 in an intricate web of life. Disturbances in one portion of the system are fol- 

 lowed by disturbances in another. We have already indicated in previous sections 

 some of the relations between insects and plants, between insects and birds, and 

 between insects and their parasites. Numerous other relations might be mentioned 

 but these are sufficient to show that a knowledge of these relations is an important 

 part of the equipment of the economic entomologist who would deal successfully 

 with the problems confronting him. 



In a region undisturbed by man the various parts of the system of Nature 

 have practically reached a state of balance through the ceaseless action for long 

 ages of the " struggle for existence." Plant struggles with plant, animal with 

 animal, and both with the environment. With the advent of man, however, the 

 balance has been disturbed- by the clearing of the forests, the cultivation and 

 drainage of the land, the growing of crops, and the introduction of foreign plants 

 and animals, since the new set of conditions will be favorable to the increase 

 in numbers of certain plants and animals, including insects, and unfavorable to 

 others. This disturbance is often widespread. Favored insects will multiply 

 rapidly on account of the abundant supply of food furnished by the cultivated 

 crops, faster at first than their parasitic enemies; and insectivorous animals such 

 as snakes, toads, birds and predaceous insects will be deprived of the necessary 

 shelter and hiding places by the clearing of the land, and become less abundant. 



On the other hand insects not favored by the destruction of their food 

 plants under the new conditions will diminish in numbers, as will also their para- 

 sites, both sometimes no doubt to the verge of extinction. If, however, as is 

 sometimes the case, conditions again favor the insect it will multiply very rapidly 

 because the development of the parasite lags behind its host. Moreover, there 

 is always a limit to the increase of the parasite, otherwise it would exterminate 

 its host, and eventually itself. 



Again, the development of insects is sometimes influenced by the soil condi- 

 tions. For example, sandy or gravelly soils seem to favor the multiplication 

 of such insects as the plum curculio, and the grape root worms. But another 

 factor, namely, the influence of the soil on the plant, must not be overlooked. 

 Plant growth on sandy and gravelly soils is retarded and is to a certain extent 

 abnormal, and the plant is less resistant to attacks of insects. On the other hand, 

 strong rich soils may induce vigorous growth, also to a certain extent abnormal, 

 when the plant is preyed upon by certain insects like plant lice and scale insects 

 which thrive best upon succulent growth. 



This relationship of soil insects to climate and soil conditions has been 

 recently discussed in the Agricultural Gazette by Dr. A. E. Cameron, of the 

 Entomological Branch, Ottawa, who is attempting to get some definite information 

 out of the chaos of many apparently conflicting observations, a condition due 

 mainly to the imperfect determination of the measure of the operation of many 

 factors. He finds that phytophagous insects of the soil frequent those soils where 

 their food plants thrive, but as these plants depend on the type of soil — its 

 structure, texture and composition, temperature and humidity, it is clear that 

 these insects depend on the type of soil. 



Again, predaceous soil forms are dependent on the presence of phytophagous 

 soil forms, and a change in any one of the factors constituting a habitat will 

 have an influence on the fauna. 



As a physical index of the varied conditions controlling soil insects it is 

 believed that the evaporating power of air is the most important one, inasmuch 



