MOTOR RESPOXSES IN INVERTEBRATES 599 



Bewegungsreflexe and the process of orientation are independent. But 

 they do not support him in the contention that their independence is due 

 to different receptors involved. They maintain that the one process is 

 correlated with rate of change in light intensity and the other with con- 

 tinuous action of light. 



Color Vision. — Beginning with Bert (13) numerous investigators have 

 attacked the problem of color vision in Daphnia. Some concluded that 

 Daphnia is color-blind, others that it is not. Koehler (115) reviewed the 

 literature in detail and presented the results obtained in an intensive 

 investigation of the problem. He maintains that if Daphnia is fully 

 adapted to light of moderate intensity, it is positive in red, orange, 

 yellow, or green, and negative in blue or violet, no matter whether these 

 colors are brighter or darker for Daphnia than the white light in which 

 it is adapted, but that if they are fully dark-adapted, they are negative 

 to all colors provided the increase in intensity is low and that if they are 

 very strongly light-adapted, they are positive to all colors. 



He concludes that if Daphnia is fully dark-adapted or very strongly 

 adapted to intense light, it is color-blind, but that if it is adapted to 

 light of moderate intensity, it can distinguish at least two colors. "Sie 

 unterscheiden der Wellenlange nach, und unabhangig von der Intensitat, 

 mindestens zwei Qualitaten voneinander und von farblosen Lichtern, 

 namlich langwelliges (Rot, Gelb, Griin) und kurzwelliges Licht (Blau, 

 Violett)" (page 135). Merker (166a) maintains that he has proved 

 that Daphnia "sees" ultra-violet, but he gives no information concerning 

 the rest of the spectrum. 



INSECTS 



More observations and more speculations have been devoted to motor 

 responses to light in insects than in any other group of invertebrates. 

 These observations and speculations concern the processes involved in 

 orientation, the effect of light on activity, the rate of adaptation to light 

 and to darkness, and vision. 



ORIENTATION 



Many insects orient very precisely in light, some are photopositive 

 or photonegative and some tend to hold any given direction of move- 

 ment that has once been assumed in relation to the direction of the most 

 intense illumination. In light from two sources, the former usually go 

 toward or from a point between the two sources, the latter usually 

 directly toward one of the two sources. 



If a photopositive insect is laterally illuminated in a beam of light 

 from a single source, it turns toward the light until the two eyes are 

 approximately equally illuminated, and then proceeds in a fairly straight 

 course. If it is exposed in a field of light consisting of two beams crossing 



