28o Animal Instincts and Tropisms 



that the presence of some substance secreted by the 

 sex glands may cause the intensification of the heHo- 

 tropism which leads to the nuptial flight. 



In certain species of Daphnia, fresh-water copepods, 

 and of Volvox, a trace of CO 2 suffices to make negatively 

 heliotropic or indifferent specimens violently positively 

 heliotropic. ^ Certain forms of marine copepods and 

 the larv^ of Polygordius can be made positively helio- 

 tropic by lowering the temperature^ and the larvas of 

 the barnacle can be made negatively heliotropic by 

 strong light. ^ It is quite possible that a change in the 

 sense of heliotropism by temperature and light is to 

 some extent at least responsible for the periodic depth 

 migrations of heliotropic animals. Many if not all 

 positively heliotropic animals can be made negatively 

 heliotropic by exposure to ultra-violet light. "* 



A most interesting example of the role of heliotropism 

 in the preservation of a species is shown in the cater- 

 pillars of Porthesia chrysorrhcea. The butterfly lays 

 its eggs upon a shrub. The larvae hatch late in the fall 

 and hibernate in a nest on the shrub, as a rule not far 

 from the ground. As soon as the temperature reaches 

 a certain height, they leave the nest; under natural 



* Loeb, J., Arch. f. d. ges. Physiol.^ 1906, cxv., 564. 

 'Ibid., 1893, liv., 81. 



3 Groom, Theo. T., and Loeb, J., Biol. Ceyitralbl., 1890, x., 160; 

 Ewald, W. F., Jour. Exper. ZooL, 1912, xiii., 591. 



4 Loeb, J., Arch. f. d. ges. Physiol., 19C6, cxv., 564; Moore, A. R., 

 Jour. Exper. ZooL, 191 2, xiii., 573. 



