allee] simple EXPRIMENTS IN A NIMAL BEHA VIOR 185 



General Notes 



These experiments can easily be modified to suit local condi- 

 tions. They cannot fail to giv^e results because animals subjected 

 to a given set of conditions must respond in some way, providing 

 they are in good condition. If they do not move it means either 

 that they will not respond to that stimulus at all or that the 

 stimulus acting is not strong enough. This last possibility can 

 be tested by increasing the stimulus. Even if it is found that 

 animals give no response that is as important a fact, though not 

 quite as interesting, as if they responded. 



Always in experiments in animal behavior the relation between 

 the experimental response and the reactions in nature must be 

 borne in mind in order to make the experiment of any practical 

 value. Thus isopods live among decaying vegetation on the bot- 

 tom of ponds and streams. They come nearer the surface at 

 night and on cloudy days than at noon on bright days. In the 

 laboratory it is found that they are negative to strong, though 

 positive to weak light and this gives the explanation for the daily 

 migration. It is also found that they are positively geotactic and 

 this accounts in part for the fact that they are normally found 

 nearer the bottom than the surface. In another experiment 

 one finds that they are positive to touch stimiili and this together 

 with the fact that they are negative to light explains their presence 

 among decaying vegetation. Another factor that would bring 

 them among the decaying vegetation from time to time is that 

 here they find their food and find it perhaps by a more or less 

 decided chemotactic response. When the fact is added that 

 isopods in ponds where currents are absent are less positive to 

 water currents than are those in streams where the currents might 

 carry them down stream; also that their thermotactic reaction 

 tends to keep them in uniform temperature, there is assembled the 

 complete explanation of the reactions of isopods under experi- 

 mental conditions and the relations which these reactions bear to 

 their normal responses in nature. 



