214 The Animal Mind 



when the whole visual field moves, but when it is at rest, 

 and one object in it moves. Can it be, he asks, that 

 the moving object " fixes the attention" of the fish and 

 produces an apparent motion of the background in the 

 opposite direction, which motion the fish follows? (254.) 



Rheotropism in water arthropods may be similarly ac- 

 counted for, and in the opinion of Radl, this same tendency 

 explains the habit swarms of insects have of hovering over 

 the same place, a phenomenon which Wheeler thought 

 might be due to odors emanating from the soil (780). In- 

 sects will often be found to follow an object over or under 

 which they are grouped in the air, if it be moved (622). 

 Swarms of insects may be noted in the air over a country 

 road, following its windings and apparently oriented by the 

 contrast between the road and the dark banks on either 

 side. When, however, resting insects turn so as to keep 

 their heads to the wind, the reaction is evidently really 

 due to the wind and not to their visual surroundings (646). 

 Probably the disturbance to their wings produced by any 

 other position causes them to rest only in the " head-on" 

 orientation. 



The responses of animals to different intensities of heat 

 seem not to involve a definite orientation of the body. A 

 temperature above the optimum produces wandering move- 

 ments, which cease when the animal happens to reach the 

 proper temperature (480, 483, 808). 



