PHOTOTAXIS 51 



Amphipoda are usually positively phototactic while the 

 aquatic species are negative it occurred to me to try the effect 

 of throwing terrestrial forms into water. Specimens of 

 Orchestia agilis which were very strongly positive were 

 employed. As soon as they were hi the water they all 

 immediately became strongly negative and remained so 

 for days in various intensities of light. That the trans- 

 formation is not due to change of temperature was shown 

 by the fact that the same result was obtained whether the 

 water was warmer or colder than the air, or at the same 

 temperature. It is not improbable that it is the stimulus 

 of contact afforded by the water that caused the reversal 

 of the response. 



In the water scorpion Ranatra contact stimuli were found 

 to have a very marked effect on the insect's reaction to light. 

 Handling these creatures throws them into a death feint 

 which entirely inhibits, for a time, all phototactic response. 

 It frequently happens that on recovery from the feint there 

 is a negative response which later changes to positive. 

 If specimens which are swimming at the end of a dish nearest 

 the light are simply picked up by the breathing tube and 

 dropped back into the water they immediately begin 

 swimming with equal vigor in the other direction. They 

 may be caused to reverse their phototaxis hi this way 

 repeatedly. The same effect can be produced if they are 

 seized or stroked while under the water. Specimens which 

 are making frantic efforts to go toward the light when in 

 the air may be caused to become negative by simply dropping 

 them into water. When taken out they usually show a 

 marked negative response for some minutes, but later be- 

 come positive. 



The method by which animals orient themselves to light 

 naturally varies with their nervous and muscular organiza- 



