24 INVERTEBRATE PHYSIOLOGY 



If a horizontal beam of plane polarized light vibrating in the vertical 

 plane is presented the animal so that the light would enter only the lat- 

 erally directed lenses, the daphnid makes considerable effort to swim on 

 its side where the light will be excluded from the laterally directed cone 

 lenses. Further proof may be adduced by experiments described else- 

 where (Baylor and Smith, 1953). 



In the final analysis the response to polarized light in the natural en- 

 vironment is a relatively simple sort of positive phototaxis brought about 

 by an increased intensity of the light entering the cone lenses which are 

 directed anteriorly, posteriorly, dorsally, and ventrally compared to the 

 intensity of light entering the laterally directed cone lenses. If a daphnid 

 is presented with 360 degrees of light, i.e., inside an opal glass globe uni- 

 formly illuminated from the outside, movement appears completely random 

 horizontally and vertically. 



Color Dances 



Color dances are statistical behavior leading to food (Smith and Baylor, 

 1953) and maintaining the animal within a useful range of its food once 

 such food is found. This behavior is best seen with a light source from 

 above giving a uniform intensity over the entire aquarium. Under red light 

 (6,000 A and over) the population appears calm, the individuals dancing 

 upright in the water, with a small horizontal vector in their locomotion. 

 The vertical vector is larger and varies somewhat throughout the popula- 

 tion. The velocity of such locomotion is quite low, the animals occasionally 

 appearing to be suspended in the water. Under blue light (5,000 A or 

 shorter) the population is distinctly agitated, the individuals leaning well 

 forward in their dance and roaming about with a large horizontal vector 

 to their locomotion. Velocities are estimated at three to five times the 

 average velocity in red light. 



A change of the direction of the light beam from vertical to horizontal 

 shows that the color dances are oriented, not to gravity, but to the direction 

 of propagation of the light. The largest vector and hence the direction of 

 locomotion in the blue dance is always oriented at right angles to the line 

 of propagation of the light, while the largest vector in the red dance is 

 always directed parallel to the line of propagation of red light. When blue 

 light is introduced from the side, a vigorous wandering occurs in all direc- 

 tions in the vertical plane perpendicular to the light beam ; when red light 

 is introduced from the side, the red dance remains a quiescent dance with 

 a predominant vector, if any, in a line parallel to the direction of the light, 

 the animals swimming slowly away from the red light and returning more 

 rapidly at irregular intervals. 



Under white light from above the dances are discrete ; at any one mo- 



