. Elements of ecology. Ecology. 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. ). 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
. Elements of ecology. Ecology. 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. ). 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 Fig. Arrangement of experiment to measure the geotactic orientation at angle 0 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. ). As the caterpillar turns from its init
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