HELIOTROPISM 



133 



produce also changes in the degree and sense of hehotropism. I had 

 found that the caterpillars of Porthesia chrysorrhoia are outspokenly 

 hehotropic only as long as they are not fed. After having begun to 

 eat, their heliotropic sensitiveness diminishes or disappears completely; 

 and in later stages of their growth and development their heliotropism 

 becomes very weak, even if they are caused to starve again. In ants 

 the intensity and the sense of hehotropism seem to be connected with 

 the development of their sexual products. At the time of sexual ma- 

 turity the males and females are markedly hehotropic ; * while in 

 the workers not a trace of hehotropism is demonstrable. 



Many animals change the sense of hehotropism during their devel- 

 opment. The larvae of Limulus polyphemus are positively hehotropic 

 immediately after hatching, while they become negatively hehotropic 

 in later stages. The larvae of the common house fly are negatively 

 hehotropic at the end of their larval period, while this reaction neither 

 exists in the earher stages nor in the imago stage. It is not impossible 

 that in all these cases the real cause for the changes in the sense and 

 intensity of heliotropism is to be found in chemical changes which 

 accompany sexual maturity or larval development. Larvae of Poly- 

 gordius (a marine Annelid) are, when caught, negatively hehotropic; 

 in about two hours, however, they become positively hehotropic. I 

 found that they could be made positively hehotropic at any time by 

 cooling the sea water to about 7° C, or below. It was also possible 

 to make positively heliotropic larvae negatively heliotropic by raising 

 the temperature of the water. Larvae which were positively hehotropic 

 at 24° were rendered negatively hehotropic by raising the temperature 

 to 29° C. Larvae which were positively hehotropic at room tempera- 

 ture became much more positively hehotropic when the temperature 

 was lowered; while those which were already negative at room tem- 

 perature remained so when the temperature was raised.f It was pos- 

 sible to make the same larvae in succession negative or positive at desire ; 

 it was only necessary not to raise the temperature too suddenly above 

 25°, as this apparently injured the animals. The immediate effect 

 of temperature in this case was possibly a chemical one. 



Results similar to those obtained by changing the temperature 

 could be obtained by changes in the concentration of the sea water. 

 When Polygordius was suddenly put into sea water diluted with fresh 

 water, those that were positively hehotropic before became negatively 

 heliotropic, while those that were already negative continued so. It 



* Kellogg has observed that bees also become outspokenly positively heliotropic at the 

 time of their nuptial flight, Science, 1904. 



t Loeb, Pfliiger's Archiv, Vol. 53, p. 81, 1893. 



