40 THE TROPISMS 
are not necessary organs for these movements (Lyon). 
In insects compensatory motions are easily observed. 
If. flies, beetles and many other forms are rotated on a turn- 
table they generally walk around opposite the direction of 
rotation. This reaction apparently depends upon the eyes 
since it is no longer performed if the eyes are blackened over. 
If robber flies or dragon flies are held in the hand and rotated 
the head will show reverse movements much as in the frog. 
In the crayfish and other higher crustaceans compensatory 
motions are shown by the eye stalks. If a specimen is 
rotated slowly about its long axis the eye stalks will move 
opposite the direction of rotation. Similar movements 
follow upon rotation about a vertical axis. Blacking over 
the eyes of the crayfish causes a diminution of compensatory 
motions upon rotation in a vertical plane, but produces 
little or no effect on movements of the eye stalks upon rota- 
tion about a dorso-ventral axis (Lyon). 
In many cases the effort to keep a constant visual field is 
an important element in determining compensatory motions, 
but it is not the sole cause of such motions in all forms. 
Sight is probably of more importance than any other single 
factor, but static organs in some cases seem to play a part 
also. In the dog-fish, however, Lyon has shown that com- 
pensatory movements of the eyes regularly occur, although 
somewhat weakened, in specimens in which both the optic. 
and the auditory nerves have been cut. 
PHOTOTAXIS 
Of the numerous papers on tropisms a greater number 
have been devoted to phototaxis than to any other subject. 
Only a very brief discussion of the results therefore can be 
attempted here. Investigation has shown that reactions 
to light do not fall under any one general scheme of explana- 
