The Reversal of Tropisms 103 



power to reverse the response. 



Other tropisms are not so frequently reversed by 

 temperature changes. Parker found, however, that 

 in the copepod Labidocera the females were posi- 



C4,_ ?i 



tively yeotetactic in warm water and negative in 

 cold, the critical temperature being about 26 C. 

 The geotaxis of the males, like their phototaxis, is 

 weak. Dr. Dice observed that Daphnia pulex tends 

 to be positively <fp&etetactic at high temperatures 

 and negative at low ones. Several protozoans which 

 are negatively geotactic at ordinary temperatures 

 become positive a few degrees above o C., but it 

 may be that in some of these cases low temperature 

 produces a condition of inactivity that leads the or- 

 ganisms to settle at the lower end of their enclosure. 

 It is a curious fact that the sense of an animal's 

 response to light may be determined by the stimulus 

 of contact with some solid object. Contact stimuli 

 profoundly affect the irritability of many organisms 

 and consequently have a marked influence on their 

 behavior. The instinct of feigning death which is 

 usually elicited as a response to contact and which 

 we have elsewhere suggested is an outgrowth from 

 thigmotaxis, is usually accompanied by a tetanic con- 

 traction of the muscles and a reduced sensitiveness 

 to stimulation. It is probably on account of the 

 marked influence of contact upon the physiological 

 states of the lower animals that we sometimes find 

 contact stimuli causing a reversal of the response 

 to light. One of the first cases of this kind was 



