28o LIGHT AND THE BEHAVIOR OF ORGANISMS 



specific. The chemicals appear to produce changes in the 

 general state of the organism as a whole or a unit. This 

 idea is strongly supported by the observations on Arenicola 

 larvae to be presented later, and by the work of Holmes 

 (1905, p. 317) on Ranatra. He found that any condition 

 which causes an increase in activity accentuates the posi- 

 tive reactions to light, while any condition which quiets 

 the organisms tends to make them negative. " The causes 

 that produce the negative reaction are, as a rule, those 

 which lead to diminished activity and excitement. Cold, 

 exposure to darkness, and the quieting effect of contact 

 stimuli lead to a condition of lessened excitability and, 

 perhaps as a result of this, to a negative reaction to light." 

 The same is probably true of many other insects. When 

 a moth becomes quiet it is likely to crawl into dark crevices, 

 but when it is disturbed it flies toward the light, and the 

 more it is stimulated the more energetically positive it 

 becomes. The pomace fly Drosophila is often found in 

 dark cavities in decaying fruit. If it is disturbed it im- 

 mediately flies out and escapes. Carpenter showed that 

 the stronger it is stimulated the more strongly positive it 

 becomes. Many similar instances could be cited. The 

 strong positive reactions to light in these forms may lead 

 them into fatal surroundings, but ordinarily they are of 

 the greatest importance, for they guide them to places of 

 safety. 



Original observations. — On May 29, 1908, a solution 

 containing numerous specimens of Daphnia, Cypris, Cy- 

 clops, a small w^ater spider about 0.5 mm. in diameter, and 

 various insect larvae, all taken from a shallow pond the 

 preceding day, were exposed in light of 200 ± ca. m. Some 

 of the individuals of the different species were negative 

 but most of them were neutral. In these there was no 

 apparent response to light. They remained equally scat- 

 tered throughout the aquarium and swam slowly about. 

 Pure CO2 was now allowed to bubble through the water 

 very slowly. Nearly all of the organisms except the water 



