180 NATURE STUDY REVIEW [9:6— Sept., 1913 



Open so that the observer can watch the animals throughout the 

 experiment. The experiment shows the light optimum for these 

 animals; that is, they could collect in either brighter or in less 

 bright light, but do collect at the margin of the bright so that this 

 must be their light optimimi. Also one will see, from time to 

 time, animals that become reversed in their light reaction. These, . 

 swim to the opposite side of the dish and collect in the darker 

 regions. 



Thigmotaxis. 



The tendency of animals to collect in comers is due to a thig- 

 motactic response. Warning has already been given that to test 

 for a light response it is often necessary to round out the corners 

 of the pans with wax. But even with this precaution the thig- 

 motactic reaction cannot be entirely eliminated for the smaller 

 animals generally collect along the lower edges even in darkness. 

 In testing for thigmotaxis, however, it is possible to entirely 

 eliminate the light stimulus and thus more clearcut results are 

 obtainable. 



I. Purpose. — To test for reaction to touch stimuli. 



Materials. — Two bread pans; black paper; thin glass plates 

 or layers of mica. 



Procedure. — Fill two bread pans with water to the depth of one 

 inch. Place several pieces of mica or glass plates in one pan as 

 follows : Lay one piece on the bottom and rest one end of another 

 piece upon it. Build the other pieces upon the first in the same 

 manner. Not more than half of the pan should be covered. Put 

 ten to fifteen animals in each pan. Cover tightly or place in dif- 

 fuse light such that no shadow is cast and let stand an hour or 

 more. Compare the position of the animals in the experiment 

 with those in the control. If the bodies of the animals are touch- 

 ing a large amount of surface, they are positively thigmotactis. 



Results. — Isopods give a strong positive response under the 

 conditions of this experiment. That is, they come to rest in the 

 angles formed by the overlapping of the glass or mica plates. 

 Amphipods may be used and will also give positive results. In 

 either case the animals in the control will tend to remain scattered 

 over the entire pan. 



Discussion. — When isopods or amphipods are used, this experi- 

 ment may be varied by exposing the end of the pan containing 

 the pile of glass (or mica) plates to light while the end that is free 



