42 THE TROPISMS 



extended studies prove that both phototaxis and photokine- 

 sis are factors which, in varying degrees, determine the 

 behavior of this species. 



Positive phototaxis is often combined with photokinesis. 

 In the large amphipod Talorchestia there is a marked photo- 

 taxis. Specimens placed in a glass dish before the window 

 or an artificial light keep hopping toward the light and strug- 

 gling to get as near it as possible for hours at a time. Yet if 

 individuals chance to get into a shaded region where they are 

 less strongly stimulated they often remain there. Among 

 positively phototactic insects similar behavior is not uncom- 

 mon. Many insects are quite spasmodic in their photo- 

 taxis. They may run about in apparent unawareness of 

 light and then suddenly become seized with an impulse to 

 go toward it. These insects frequently keep in shaded 

 localities most of the time and would ordinarily be thought 

 to be negatively phototactic while in reality they only 

 manifest a tendency to rest in shaded places into which 

 they happen to wander. The proclivity to crawl under 

 objects is commonly also, in part at least, a manifestation of 

 positive thigmotaxis. 



Among animals which regularly go toward or away 

 from the light there are considerable variations in the method 

 employed. While in many forms there is a direct orientation 

 to the rays, in others the orientation is only brought about 

 indirectly. Larvse of blow flies commonly crawl away 

 from the light, but by a method which I have elsewhere re- 

 ferred to as the selection of random movements. To quote 

 from my previous account, "When strong light is thrown 

 on a fly larva from in front, the anterior end of the creature 

 is drawn back, turned toward one side, and extended again. 

 Often the head is moved back and forth several times before 

 it is set down. Then it may set the head down when it is 



