HELIOTROPISM OF ANIMALS 55 



in their existence. In the experiments of Lubbock the 

 workers contained in the nest not only collected under red 

 glass, but also carried their larvae there. The animals are 

 therefore negatively heliotropic. 1 



All these facts, however, do not yet exhaust the connec- 

 tion between sexuality and heliotropic irritability. The 

 heliotropism of the male and female ants is also different, 

 inasmuch as it requires more intense light to cause helio- 

 tropic movements in females than in males. In isolating 

 the males and females of the same swarm I noticed that the 

 females had ceased to execute heliotropic movements before 

 it seemed as if twilight had really begun. The males how- 

 ever still collected on the window side of the tube long after 

 sunset. Experiments with colored glasses succeeded in males 

 when the light was so faint that I had difficulty in dis- 

 tinguishing the color of the glasses. On dark, cloudy days 

 females showed no heliotropic reactions toward the window, 

 while the males did. It harmonizes with this observation 

 that on cloudy afternoons I saw occasionally winged males 

 leave the nest, but no females. 



As soon as the intensity of the light had become so small 

 that heliotropic phenomena were no longer produced, another 

 form of irritability appeared in the winged ants, especially 

 in the females, namely, stereotropism. The animals then 

 crowded into all crevices. I placed the animals in a dark 

 box, and laid a small, folded piece of velvet into one corner 

 of it. After a few moments they had crept into the folds of 

 the velvet. With the males it took a much longer time than 

 with the females. This irritability, however, did not appear 

 as long as the light was sufficiently intense to call forth 

 heliotropic phenomena. When exposed to light, the animals 

 crept neither under the piece of velvet nor into crevices. It 

 is very probable that a similar difference in heliotropic irri- 



i The observations recorded in Lubbock's paper admit another possibility. [1903] 



