342 ^"^ ANIMAL KINGDOM 



into the light in search of another shelter. Kinesis does not result in any 

 particular response or produce movement in any particular direction, 

 but is expressed only as an increase in the rate of general activity. 



The opposite of kinesis is immobilization. Night-flying moths, for 

 example, are "put to sleep" by light. Many organisms settle down to 

 rest if their body is touching several surfaces, but not if contact is pre- 

 vented. A sudden and complete immobilization is called "death feign- 

 ing." Many insects fall immobile to the ground if the leaves on which 

 they are sitting are jarred. In this immobile state they can sometimes be 

 picked up and squeezed without producing resistance or spontaneous 

 movement. This plastic immobility superficially resembles human cata- 

 lepsy. The walking stick (Fig. 17.16 A), a wingless orthopteran, will feign 

 death if rubbed in the presence of light. Its legs can then be moved in 

 various ways, and the walking stick will hold such postures for several 

 minutes. Death feigning has an obvious advantage as a defense against 

 those predators that recognize their prey by its motion. 



A stimulus may produce an orientation response, the turning or 

 locomotion of the animal toward or away from the direction of the 

 stimulus. Commonly effective stimuli include light, gravity, wind and 

 water currents, odors, sound and radiant heat. Maggots, for example, 

 have a negative reaction to light; if they are placed on a paper beside a 

 light they crawl rapidly away from it. Many other such orientations 

 are known. A fly on a flat surface will turn to face the wind, male moths 

 will fly toward the odor of the female, crayfish are attracted to dead 

 flesh, and daphnia turns its back to a side light. 



Orientation to a given stimulus is seldom the same under all con- 

 ditions. It may be altered or even reversed if certain other stimuli are 

 present. The maggots can be made to seek light, if they are grown in 

 the presence of ammonia or other harmful chemicals. Also, a maggot 

 about to pupate will spontaneously go toward the light. Such response 

 patterns are adjusted to suit the survival of the organism. Maggots live 

 in manure and rotting flesh and require moisture. Eggs are laid on the 

 surfaces of such food, and a strong negative reaction to light ensures 

 that the maggots will burrow in. If the material contains harmful 

 chemicals the maggots would do better to leave and take their chances on 

 finding another food source. Although the maggot requires moisture, 

 the pupa would mold in a moist environment. Hence, crawling toward 

 the light just before pupation places the pupa in a drier situation. 



Many orientations are easily reversed. Caterpillars tend to go toward 

 light if hungry, but crawl away from it if they are full. Some orienta- 

 tions are sensitive to a variety of other stimuli. One of the best known 

 of these is the light orientation of daphnia, studied for over 100 years. 

 Daphnias will swim toward or away from a light depending upon the 

 brightness of the light, the color of the light, the temperature of the 

 water, the amount of carbon dioxide present, and their state of hunger. 

 Bright light, short wavelengths, high temperatures, low carbon dioxide 

 and hunger all favor a negative response. Other modifying stimuli, too 

 complex to be presented here, also influence the orientation of daphnia. 



A somewhat more complicated orientation to the direction of a 



