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. Larve 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 
