12 University of California Publications in Zoology [Vol. 19 



in light intensity is explained on the ground that specific chemical 

 processes are set up under the influence of light or darkness, and that 

 these processes "in iherer Art eben die Periode einer Erregung des 

 Stoffwechsels resp. dessen Ruhe verursachen." 



I believe that Menke's work has brought to light a factor that must 

 be considered in the matter of the depth migrations. I have pointed 

 out (Esterly, 1917b) that two species of Acartia exhibit a periodicity 

 of behavior that is not connected at the time with recurring external 

 stimuli; the same is true, though to a less extent, of Calanus. It 

 remains to be determined by experiment how general this factor is. 



The foregoing account of the literature has covered the papers 

 that deal with depth migration more extensively from the experimental 

 side. There are, in addition, some other references to this habit that 

 may be mentioned. Harper (1907) showed that the insect larva, 

 Corethra, is positively geotropic in strong light from above as well 

 as in that from below, and that it is negatively geotropic in dim light. 

 Harper suggests (p. 454) that the animals move down in the daytime 

 because of positive geotropism in strong light, and that they ascend 

 when the intensity decreases. 



My own studies of the reactions of Cyclops (Esterly, 1907) were 

 directed toward an understanding of diurnal movement. The females 

 of Cyclops albidus were shown to be negatively geotropic in the dark 

 and positive in the light, and it was suggested that phototropic re- 

 sponses are not even the main factors in causing the diurnal migra- 

 tions. The effect of light on the geotropism, however, offers a possible 

 explanation. 



There are, of course, very many papers dealing with responses that 

 might be applicable to the diurnal movement if the facts about natural 

 distribution were known for those particular forms. But it does not 

 seem desirable to include such accounts of behavior here. Similarly, 

 the extended literature dealing with results derived from field investi- 

 gations may be omitted since the purpose of this report is to give an 

 account of a set of experiments on certain forms and of their possible 

 bearing on the diurnal migrations of the same species. 



In discussing the literature it would not be well to omit mention of 

 the theories of Ostwald with regard to the diurnal movement. As is 

 well known, the chief point in his theory is that the movement is 

 largely due to alterations in the viscosity of the water, the viscosity in 

 turn being dependent on the temperature. The migration of the 

 animals is not so much due to their own activities as it is to the 

 physical condition of the medium in which they live. Ostwald states, 



