February, '17] hewitt: insect behaviour 89 



a modified negative phototropism and migrate in a northwesterly 

 direction. These facts are of practical value in controlling outbreaks 

 of this insect. 



The two previous tropisms, operating together, constitute perhaps 

 the most widely operative of all environmental stimuli as affecting 

 insect life. The daily activities of insects, their movements on the 

 soil, on vegetation or in the air,^are largely governed by them. And 

 in referring to the dual influence of these stimuli it may be remarked 

 that the various types of stimuli are very frequently cooperative. 

 Years ago, when collecting Diptera by sweeping, Wheeler was im- 

 pressed with the fact that there must be a regular diurnal up and 

 down migration of insects in the low vegetation, comparable to the 

 phenomenon exhibited by the pelagic fauna in the sea. The insects 

 descend to the ground at night and with the return of Hght and heat 

 rise until they reach the upper surface of the plants. There is little 

 doubt that this diurnal migration is of economic importance and 

 demands further careful study. Its dependence on the coopera- 

 tive effect of several stimuli such as light, heat, and probably air 

 currents, indicates the necessity, which should always be borne in 

 mind in studying insect behaviour, of a careful analysis of tropic 

 reactions. 



The relation of the dispersion of insects to air currents is an aspect 

 of insect behaviour that has had wide recognition since, and perhaps 

 before, the locusts descended on the land of Pharaoh. Anemotropism 

 is well exhibited in the case of the Rocky Mountain locust which 

 moves with the wind and when the air current is feeble is headed away 

 from the source. The brown-tail moth owes its distribution in New 

 England and eastern Canada largely to wind-spread and the investiga- 

 tions of Collins and his associates have shown that the general spread 

 of the gipsy moth in New England is most probably due to the fact 

 that the first-stage larvae are carried in a north and northeasterly 

 direction by the warm prevailing winds rather than to dispersal by 

 artificial means, as formerly believed. The practical value of knowl- 

 edge of this type of behaviour is shown by the experiments of Le Prince 

 and Zetek on the flight of Anopheles with the use of Quinby inter- 

 cepting planes in Panama. Le Prince has suggested that where anti- 

 malarial work is to be undertaken in badly infested regions observa- 

 tions on the flight direction will indicate which of several possible 

 production areas is the source of the particular species of mosquito it 

 is desirable to eradicate. 



If time permitted it would be profitable to discuss other types of 

 tropisms and to show how this line of study throws light on the com- 

 plicated instinctive behaviour of insects, particularly those exhibited 



