202 The Animal Mind 



whereas if kept in light they would have been positive. A 

 sudden change in light intensity, either brightening or 

 darkening, has the effect of making positive Daphnias 

 temporarily negative (532). 



Temperature changes influence response to light. The ob- 

 vious suggestion here would be that since increased tempera- 

 ture often accompanies increased intensity of light, animals 

 that are positively phototropic only up to a certain degree of 

 illumination ought to become negative when the tempera- 

 ture is decidedly raised. This, however, is by no means 

 always the effect produced by increased temperature. 

 Strasburger's swarm spores became positive in higher tem- 

 peratures, negative in lowered ones (695). Orchestia agilis, 

 which we have just seen becomes temporarily negative on 

 being brought from strong into weak light, may be made 

 positive again if the water is slightly warmed. When the 

 same animal is dropped into water, it becomes strongly 

 negative, but it will show a positive response if the water is 

 heated almost to a fatal point (330). Essenberg (209) 

 finds that certain aquatic insects are more strongly positive 

 when the temperature is increased. On the other hand, 

 the copepods and annelid larvce studied by Loeb were made 

 negative by increased, positive by lowered, temperature. 

 Other crustaceans, e.g. Daphnia (808, 185), had their 

 responses to light unaffected by a fairly wide range of 

 temperature changes. 



Increasing or decreasing the density of the water will also 

 affect phototropism. In some copepods diluting the water 

 produced negative responses to light, while increasing its 

 density brought about those of the opposite sign (425). 

 Diluting the water produced negative phototaxis in the 

 larv^ of Palaemonetes (451). Parker failed to find any 

 such effect in the case of the copepods studied by him (534). 



