72 



TROPISMS 



legs of the other side the tension of the extensors pre- 

 vails. As a consequence the fly has a tendency to move 

 in circles with the intact eye toward the center. 



Garrey has shown that when a fly with one eye black- 

 ened is put on a vertical stick, it still walks upward, but in 



spirals around the stick (Fig. 

 27), instead of in a straight 

 line. The asymmetry of loco- 

 motion changes only the geo- 

 metrical nature of the path in 

 which the animal moves, from 

 a straight line to a spiral, but 

 does not alter the forced move- 

 ment character of the reaction. 

 Bancroft has pointed out 

 that when in a positively helio- 

 tropic amphipod one eye is 

 blackened and the legs of the 

 same side are cut off, the ani- 

 mal 's path would be a combina- 

 tion of a circus motion induced 

 by the blackening of the eye 

 and of a rolling motion around 

 its longitudinal axis. Both 

 effects combined would result 

 in the animal swimming in a 

 spiral path, and if the animal is 

 positively heliotropic it would 

 swim in such a path toward the 

 light. This is the path which aquatic, asymmetrical posi- 

 tively heliotropic organisms, such as the flagellate 

 Euglena, describe in their motions to the light. 



Fig. 27. — Fly with one (right) eye 

 blackened can creep only in a spiral 

 on a vertical stick, while normally it 

 creeps in a straight line. (After 

 Garrey.) 



