February, '17] hewitt: insect behaviour 83 



extensive changes in environmental conditions, but such development, 

 particularly in agriculture, has necessitated, among other things, the 

 importation of large quantities of the natural products, including 

 vegetation, from older countries with the inevitable introduction of the 

 insects affecting those products, thereby not only modifying the en- 

 vironmental conditions for the native insects in the new country, but 

 also introducing into a new environment insects from another country 

 and from a native environment more conducive to stability in behaviour. 

 Thus the conditions are altered for both the insects native to the 

 new country and the insects fortuitously introduced. 



Formerly, the investigations of the entomologists did not extend 

 very far beyond a studj^ of the Kfe-histories of insects, and control 

 measures were largely based on such knowledge supplemented by a 

 limited study of the insects' habits; the idea being, as we were told, 

 "to find the weak spot in the insect's life-history." The limitations 

 of such methods of solving entomological problems were demonstrated 

 by an inability either to account for the outbreaks of certain insects or 

 to discover effective means of control. Not until the behaviour of 

 insects, that is, their reactions to their environment, to each other and 

 to the different biological constituents of that environment, was 

 studied with true appreciation was it possible to make satisfactory 

 progress in the control of certain serious pests. The corn root-aphis 

 (Aphis maidi-radicis) furnishes a good example of this fact. It was 

 not until Forbes and his assistants worked out the relation of this insect 

 to the ant Lasius niger americanus, on which it depends for its well- 

 being, that any success in controlling this serious corn pest could be 

 attained; and such control measures as the breaking up of the ant 

 colonies in the spring and the destruction of weeds on which the ants 

 plant their wingless aphid captives before the growth of the corn, are 

 based solely on a knowledge of behaviour. 



The reaction of an organism to environmental influences is known 

 as a tropic reaction or a tropism. The external stimulus may induce 

 a physiological state that exhibits response in movement, or the physi- 

 ological condition of the organism may be changed in a more funda- 

 mental manner with the result that not only is the organism itseK 

 affected permanently, but the progeny experience the effects of the 

 stimulus and react by a change in behaviour or even in structure. For 

 the sake of convenience we may term such tropic reactions as indi- 

 vidual and racial, respectively. 



It will not be possible in the time at my disposal to deal with more 

 than the main types of tropic reactions to physical factors and a treat- 

 ment of these must of necessity be brief in character. Let us, there- 

 fore, consider the chief tropisms: Chemotropism, thermotropism, 



