296 LIGHT AND THE BEHAVIOR OF ORGANISMS 



internal as well as external factors, and that if the inter- 

 nal processes, physiological changes, do account for the 

 variability in the reactions these processes cannot be run- 

 ning their course with any degree of regularity. 



Among the crustaceans and the higher forms varia- 

 bility and modifiability in reactions to light are com- 

 mon. Holmes (1905) found that Ranatra with the left 

 eye blackened tends to turn to the right in going toward 

 a source of light, but after several trials it goes nearly 

 directly toward it. " In the first trial the insect veered 

 over constantly to the left, passed by the lamp and went 

 off from the table before it turned around. In the fol- 

 lowing trials a marked tendency to turn to the left is also 

 shown; frequently the insect makes one or more complete 

 circus movements to the left before reaching the light. 

 At the eleventh trial its course is corrected for the first 

 time by a turn to the right side, but, instead of going 

 straight up to the light, it performed a complete circus 

 movement to the left before reaching it. The next time 

 the course was corrected by a sharp turn to the right and 

 the circus movement was dispensed with. At the next 

 trial the course was corrected in the same way, and at the 

 fourteenth attempt the insect deviated only slightly to 

 the left side and then turned to the right to reach the 

 lamp. In the following ten trials it reached the light by 

 a nearly straight path. Whenever it began to turn away 

 from the light to the left it corrected its course by a direct 

 turn in the opposite direction instead of going around in 

 a complete circle as at first." 



Spaulding (1904) observed that hermit crabs are ordi- 

 narily positive. They usually collect in the more highly 

 illuminated regions of an aquarium. But he found that 

 after shading the part of the aquarium in which the crabs 

 were fed every time that food was introduced, they soon 

 came to the part shaded even before food was put in, 

 quite contrary to their ordinary reaction to light. 



In even casually studying the behavior of bees, wasps, 



