254 BRADLEY M. PATTEN 



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one of a series of 'random movements' induced by the stimulus 

 and served to 'try' the conditions on the different sides of the 

 animal. He says ('05, p. 105) : "If a strong hght is thrown upon 

 a larva from one side it may swing the head either toward or 

 away from the light." Mast, repeating the experiment, recorded 

 the direction in which the anterior end first turned, in a large 

 number of trials. His table ('11, p. 189) shows a total of 177 

 turns away from the light and 165 towards it. At first sight 

 this would lead one to conclude that the direction of turning is 

 little influenced by the direction from which the light strikes 

 the animal. But other experiments at different intensities did 

 not confirm these observations, which like those of Holmes were 

 made under intense lateral illumination. Mast found that when 

 a larva was subjected to a moderate intensity of illumination, 

 the turns away from the light were much in excess. This, he 

 interprets in the following way ('11, p. 191): 



When the larvae are first exposed to sudden lateral illumination in 

 direct sunlight, they respond immediately by throwing the anterior end 

 toward one side violently, no matter in what position this end chances 

 to be. If it happens to be directed from the source of light when the 

 sunlight is flashed upon the organism, it turns toward the source of 

 light, and if the sunlight is immediately intercepted after the larva 

 turns, it will continue in the direction toward which the anterior end 

 points; if it is not intercepted, the anterior end is thrown in the oppo- 

 site direction, and then the larva may follow this turn and become ori- 

 ented immediately, or it may swing the end back and forth a few times 

 before becoming oriented. If the anterior end faces the light when it 

 is exposed to the sun, it is first thrown in the opposite direction and 

 orientation takes place just as described above. The anterior end is thus 

 turned in the direction opposite to that in which it is when the expo- 

 sure is made. It is therefore evident that under these conditions the 

 larvae will turn toward a strong unilateral illumination as often as 

 from it. . 



Apparently this is not the case when the light is less intense 

 (Mast '11, pp. 191, 192): 



If the larvae are carefully observed when they are suddenly exposed 

 to lateral illumination by diffuse light, it is found that they respond 

 immediately only if the anterior end is turned toward the source of 

 light when the exposure is made. If this end is in any other position, 

 there is no reaction whatever until the organism, in its normal process of 



