Orientation 211 



extended and will make longer sweeps. As the animal swims in this 

 posture, it will tend to turn toward the source of light. Turning will 

 continue until the amount of light received in the two eyes is equal, 

 whereupon the legs will function symmetrically and the animal will 

 swim straight ahead (Fig. 6.12). In many such instances the turn- 

 ing of the animal appears to be produced quite mechanically, and 

 such reactions were referred to as "forced movements" by Jacques 

 Loeb (1918) who originated the theory of orientation just described. 

 In other instances orientation appears to come about by a trial-and- 

 error procedure in which the animal changes its direction when it 

 encounters unfavorable conditions and tries other directions until it 

 finds a course in which conditions no longer stimulate it adversely. 



Fig. 6.13. Arrangement of experiment to measure the geotactic orientation at 

 angle of caterpillar Malacosoma placed on plane inclined at angle a. Lateral 

 movement of head indicated by h and h'. ( Crozier, 1929, Copyright, Clark Univ. 



Press. ) 



Whether one adopts or rejects a mechanistic viewpoint in inter- 

 preting the reactions of organisms, the fact is that tropistic and 

 tactic responses of plants and lower animals play a major role in 

 their lives under natural conditions, and their orientation often ap- 

 pears to be rather rigidly controlled. An example of the mechanical 

 way in which an orienting force may act is provided by an experi- 

 ment involving the negative geotaxis of the tent caterpillar, Mala- 

 cosoma. If this caterpillar is placed horizontally on a steeply sloping 

 surface, an unequal stimulus of the proprioceptors located within 

 the two sides of the body will result, owing to the animal's weight. 

 A differential response in the tonus of the body muscles will then be 

 produced, with the result that, as the animal creeps, it will turn to 

 move up the slope (Fig. 6.13). As the caterpillar turns from its 

 initial horizontal position toward the vertical, the difiFerence in stimu- 

 lation of the two sides of the body becomes progressively less, and 

 eventually reaches a threshold beyond which no further turning is 

 elicited. If the plane upon which the caterpillar is creeping is only 



