116 TROPISMS 



marine copepods could be made positive by lowering the 

 temperature, and positive larvae could be made negative 

 by slowly raising the temperature. Since in the latter 

 case the animals suffered from the high temperature the 

 results were not so striking as in the case of the positivat- 

 ing effect of lowering the temperature. The same effect 

 of the concentration of sea water and of temperatures 

 was observed by Ewald for the larvae of Balanus perfor- 

 ates. He found, moreover, the interesting fact that a 

 change of the ratio ~ in the sea water affected the sign 

 of heliotropism of barnacle larvae. An increase of Na 

 made them more positive, an increase in Mg more 

 negative. 144 



The larvae of Porthesia are strongly positively helio- 

 tropic before they have eaten, while they lose their helio- 

 tropism almost completely after they have eaten. 287 The 

 writer observed that male and female winged ants are 

 strongly positively heliotropic but as soon as they lose 

 their wings their heliotropism ceases. 287 McEwen 54£ * 

 has found that when Drosophila is deprived of its wings 

 its heliotropism ceases. 



Holmes found that terrestrial amphipods are posi- 

 tively, while the aquatic amphipods are negatively helio- 

 tropic. By putting a terrestrial amphipod into water it 

 became negatively heliotropic. 225 



That a reversal in the sense of heliotropism may be 

 due to a nervous effect is suggested by an observation 

 by Miss Towle 485 that a certain ostracod, Cypridopsis, 

 can be made positively heliotropic by mechanical shock, 

 and the writer noticed that indifferent fresh water Gam- 

 marus can be made negatively heliotropic by shaking 

 them. In both cases the heliotropism lasts only a short 

 time. 



