Tropisms — Relation to Modes of Behavior 63 



The positive reactions of many worms and crus- 

 taceans to contact stimuli keep them in protected 

 situations in various nooks and niches where they 

 escape many of their enemies. The positive chemo- 

 taxis of many animals leads them into situations 

 where they may find their food. 



But one of the chief considerations which makes 

 the study of tropisms of such importance is the fact 

 that the tropisms enter as components into more 

 complex activities of higher organisms. Tropisms 

 in their purity are met with only in the simpler ani- 

 mals. As we pass up the scale of life these primary 

 tendencies to action become broken up and com- 

 bined with other forms of behavior, so that they are 

 lost sight of in the more complex activities into which 

 they enter as component factors. 



A most interesting field of investigation in this con- 

 nection is presented in the relation of phototaxis 

 and vision. It is a field scarcely touched upon as 

 only one investigator, Radl, has entered upon it 

 with any seriousness. There seems to be a close 

 connection in many animals, and especially in insects, 

 between phototaxis and what are called compensa- 

 tory movements. Place a lady-beetle on a turntable 

 which is slowly rotated. The beetle begins to move 

 its head and then its body opposite the direction 

 of movement. Robber flies show the reaction espe- 

 cially well. The reaction depends upon the eyes 

 because it no longer occurs when the eyes are black- 

 ened over. It does not depend upon the rotation 



