THE PHOTIC REACTIONS OF SARCOPHAGID FLIES 203 



One must conclude from the summary of the experiments in 

 table 4 that the rate of movement increases with the intensity. 

 Davenport and Cannon ('97, p. 32) say for Daphnia, ''Since there 

 is no close relation between diminished intensity and the longer 

 time required for migration, it seems more probable that this 

 longer time is not the result of lower intensity, but that it is 

 due to diminished precision of orientation showing itself in hesi- 

 tating movements." Although this statement was made for a 

 positive organism, it might nevertheless be inferred that the same 

 conclusions would hold for negative organisms as well. This 

 is in part true of the sarcophagid fly-larvae. Certainly there are 

 many more random movements in very low intensities, as illus- 

 trated by fig. 16 (p. 219); these movements disappear largely 

 if not entirely at high intensities, e.g. fig. 19 (p. 219). But it 

 cannot be concluded that the increase in rate of movement in 

 high intensities is due entirely to precision of orientation, since 

 the light not only has a directive but also a dynamic effect. In 

 table 5 it is shown that there is such a result even when the direc- 

 tive element has ceased. By closely observing the larva as it 

 travels in high intensities, e.g., 1000 CM., it can readily be seen 

 that the crawling movements are greatly accelerated. This was 

 particularly noticeable when the larva was under the conditions 

 shown in fig. 4. When the animal passed into the field of higher 

 intensity (1056 CM.) its longitudinal contractions and elonga- 

 tions were perceptibly increased in rapidity after re-orientation, 

 and the sidewise movements of the head were visibly less. 



Again, in a series of experunents discussed on page 199, in which 

 the larvae were moving away from the source of light in a field 

 gradually increasing in intensity, both phenomena were evident, 

 viz., the sidewise movements decreased in number, while the rate 

 increased. The sidewise or random movements were, however, 

 greatly exaggerated under the conditions already pointed out. 



Yerkes ('00) in a further study of Daphnia also concludes with 

 Davenport and Cannon that the increase in rate for these forms 

 depends chiefly upon precision, but found evidence of a ' ' quicken- 

 ing of the swimming movements." 



