1S92.] Recent Literature. ~t" ;, » 



of two unequal lights the more intense is followed by caterpillars. In 

 all the experiments the stimulus continues as long as the light remains 

 unchanged: the caterpillars remain at the end of the tube though the 

 other end may be open. 



Other stimuli that might vitiate the result ascribed to light are to be 

 mentioned under the titles of negative geotropisni, contact and tem- 

 perature. The first phenomena are those seen when the animals 

 creep up a vertical surface, but this is overcome by the action ol light 

 as shown by illuminating the tube from below through t narrow slit in 

 an opaque covering, then the animals overcome the negative geotro- 

 pic tendency and move down towards the light. 



The effects of contact stimuli are seen in the tendency of the cater- 

 pillars to collect on the convex edges and corners of solids— a tendency 

 that controls their position upon the buds of trees as well as on the 

 objects offered them in the laboratory. 



A source of warmth is unlike a source of light in that the cater- 

 pillars move away from it : this is made evident from experiments 

 upon the animals in opaque boxes when brought near a closed stove. 

 The movements are, however, not in the line of direction of the heat 



These experiments in Porthesia have been repeated upon nearly 100 

 species of insects but with the same results, so that they will serve as 

 type of the reaction of a positively heliotropic animal. 



Certain special cases of interest remain to be discussed : first the 

 old problem of the moth and the candle. 



When Sphinx euphorbias or other nocturnal moths are kept in a 

 glass box they fly towards the window side at dusk, or in the day- 

 time if disturbed. When an artificial light and a window are at 

 opposite sides of the room the moth goes to one or the other according 



Here also it is shown that it is the blue and not red light which 

 directs the flight. It seems that these nocturnal moths have periods 

 of sleep followed by periods of greater sensitiveness to light-being 

 sensitive in the night. This rhythm is moreover not easily disturbed 

 for when kept several days in a dark box the moths still continue to 

 be restless in the evening. During the period that they are heliotro- 

 pic thev agree with the Porthesia caterpillars in all respects. 



Plant lice in the winged state exhibit strong positive heliotropic 

 movements and are also negatively geotropic and forced to move 

 away from heat just as are the larva? of Porthesia. 



