INSECT BEHAVIOR 355 



Phototropism and thermotropism, either together or singly, 

 as Wheeler suggests, may explain the up and down migration 

 of insects in vegetation. " On cold, cloudy days few insects 

 are taken because they lurk quietly near the surface of the soil 

 and about the roots of the vegetation, but with an increase in 

 warmth and light they move upwards along the stems and 

 leaves of the plants, and, if the day be warm and sunny, escape 

 into the air." 



Thermotropism. Ants are strongly thermotropic; they 

 carry their eggs, larvae and pupae from a cooler to a warmer 

 place or vice versa, and thus secure optimum conditions of 

 temperature. Caterpillars and cockroaches migrate to regions 

 of optimum temperature. 



In thermotropism it appears that the direction of heat rays 

 has little or no effect as compared with differences of intensity. 



Tropisms in General. Other kinds of tropisms are known, 

 for example, tonotropism, or the control of the direction of 

 locomotion by density, and electrotropism; not to mention any 

 more. 



All these phenomena are responses of protoplasm to definite 

 stimuli and are almost as inevitable as the response of a needle 

 to a magnet. 



The tropisms of the lower organisms have been experi- 

 mented upon by many skilled investigators, whose results fur- 

 nish a broad basis for the study of the subject in the higher 

 animals a study which has scarcely begun. Even in the 

 simplest organisms, behavior is the resultant effect of several 

 or many stimuli acting at once, and the precise effect of each 

 stimulus can be ascertained only by the most guarded kind of 

 experimentation ; while in the higher animals, with their com- 

 plex organization, including specialized sense organs, the study 

 of behavior becomes intricate and cannot be carried on intelli- 

 gently without an extensive knowledge of the behavior of 

 unicellular organisms. The properties of protoplasm are the 

 key to the behavior of organisms, though comparatively little 

 is known as yet in regard to these properties. Furthermore, 



