270 'Journal of Comparative Neurology and Psychology. 



Case 5. Larva oriented with back above and longitudinal body- 

 axis at right angles to direction of light ra^'j'— When the larva 

 was oriented as above and the rays were introduced at right angles 

 to the longitudinal axis (Fig. 13, a\ c-) the behavior was similar 

 to some phases of Case i, Cor. i. The larva swung directly 

 away from the source of light until its longitudinal axis was par- 

 allel to the light rays, with the head directed away from the source 

 of light. Obviously the swing might cover from 1° to 90° and 

 either positive or negative progressive orientation might follow. 



If the larva was lying with the back below, but otherwise 

 oriented as in the previous instance to the directive influence of 

 the rays, the reaction was the same; namely, a swing to one side. 

 This resulted in placing the longitudinal axis parallel to the rays 

 of light. Frequently, in such case, the larva would undergo a 

 rotation on its own axis, so that it assumed a position with the back 

 uppermost and the head directed away from the source of light. 

 Whether or not this "righting reaction" occurred, appeared to 

 depend largely upon the degree of freshness. Individuals which 

 had undergone fatigue more frequently refused to rise from the 

 bottom. It was at no time possible, however, to fatigue the larv<Te 

 to such an extent that they would not give the "swinging-reaction" 

 into line with the light rays. By alternately changing through 

 an arc of 30° the direction of the light which struck the larvae 

 from behind (Fig. 15, a, b, c), they could be made to swing, pivoted 

 on the middle or end of their longitudinal axis, in an arc of equal 

 degree. This pendulum-like activity in answer to the change in 

 direction of the light-stimulus was extremely constant and in no 

 case was it observable that the reaction was diminished by fatigue 

 in spite of long periods of such alternate directive stimulation. It 

 may be added here that prolonged direct stimulation from behind 

 never produced a change in the body-orientation of the larva. 

 The progressive orientation, however, might take place in either 

 the positive or the negative sense. 



Case 6. Larva oriented with back above; light enters from above 

 — Under the conditions mentioned above, the larva was forced to 

 give one or two reactions, depending upon the degree of intensity 

 and the suddenness of introduction of the light: 



(i) In some instances (especially when the light had the direc- 

 tion, b, Fig. 17), the larva first rotated through an arc of greater 

 or less curvature and finally assumed a new swimming position 



