88 JOURNAL OF ECONOMIC ENTOMOLOGY [Vol. 10 



by diffused moisture in the air. Similarly, moisture in the soil affects 

 the behaviour of insects considerably, as Wheeler has shown in the 

 case of many species of ants, and as Parker has demonstrated in his 

 study of the sugar-beet root-louse (Pemphigus hetce Doane) in which 

 it was found that soil moisture is a very important factor in controlling 

 the rate of increase in colonies of this insect. The attraction of the 

 so-called " watershoots " of trees such as apple for aphides should be 

 regarded as being in effect a hydrotropism. 



The importance of moisture as a factor in insect behaviour is strik- 

 ingly illustrated in the case of some of our most important grain insects. 

 Forbes has discussed the effect of drought and rainfall upon the abund- 

 ance and suppression of the chinch bug in Illinois. In Canada we 

 find that the prevalence of the western wheat-stem sawfiy is governed 

 by humidity. A lack of precipitation causes a dearth of flowering 

 stems among the grasses in which this insect normally breeds, resulting 

 in a decrease, the abundance of the insect depending primarily upon 

 the prevalence of suitable grass stems. Similarly, a lack of moisture is 

 an important natural check on the Hessian Fly, a dry season being 

 generally recognized as prejudicial to the fly. In Manitoba, Griddle 

 finds that the partial second brood is frequently destroyed completely 

 by the premature ripening of the grain due to the hot weather condi- 

 tions in late July. Further instances might be given of the effect of 

 moisture on other classes of insects but sufficient has been said to 

 indicate the diversity of the hydrotropic type of behaviour. 



Reaction to light pdays a prominent part in insect behaviour and 

 numerous are the examples that might be given, were* it necessary, of 

 phototropism in insects. But while entomologists are familiar with 

 the manner in which adult insects such as Lepidoptera are attracted 

 to light and with the negative phototropism of many larval fornis, and 

 of adult insects such as Anopheles, we are still far from anything ap- 

 proaching a working knowledge of this reaction. Such knowledge will 

 undoubtedly place a valuable weapon in the hand of the applied ento- 

 mologist. In some cases we are able already to take advantage of this 

 type of behaviour. Swaine finds that the destruction of piled logs by 

 the wood-boring larvse of the sun-loving Monohammus can be prevented 

 by forming a dense shade over the logs by means of brush. In his 

 study of the army cutworm (Euxoa auxiliaris) in Alberta, Strickland 

 found that the larvse are negatively phototropic and hide beneath the 

 soil till about four or five o'clock in the> afternoon when they come to 

 the surface and feed. With the weaker light they become positively 

 phototropic and a general migration in a westerly direction takes place. 

 When food is scarce hunger may overcome their aversion to sunshine 

 with the result that the larvse come above ground, but they still display 



