THE PHOTIC REACTIONS OF SARCOPHAGID FLIES 195 



region was 70.69 CM. and it gradually diminished to nothing 

 at the other extreme. Of the two extreme lengths mentioned, 

 the longer resulted in a gradation of 7 CM. per centimetre and 

 the shorter in a gradation of 35 CM. per centimetre. 



Since the sarcophagid fly-larvae are negative to light, one would 

 expect them to turn to the darker portion on entering such a field 

 of graded light. This would also be expected when one considers 

 the results obtained by means of light on two sides, as illustrated 

 by figs. 3 and 4. The courses of the larvae always lay more or 

 less transverse to the rays, the greater deflection being toward the 

 light of lower intensity regardless of the size of the luminous 

 field. When the larvae were placed midway between two balanced 

 lights of like luminous area (likewise when of unlike area, but like 

 intensity), their course lay transversely to the rays, as illustrated 

 by fig. 5. These results are in accord with the statements made by 

 Loeb ('05): ''If there are two sources of light of different inten- 

 sities, the animal is oriented by the stronger of the two lights. 

 If their intensities be equal, the animal is oriented in such a way 

 as to have symmetrical points of its body struck by the rays at the 

 same angle." 



If, now, the negative fly larva is in a field of graded light, one 

 would expect it in creeping to take a course toward the darker side 

 of the field until stimulation (light from above)became equal 

 on both sides, and decreasing in intensity. Clearly the opposite 

 direction would be out of the question, since that involves a grad- 

 ual increase in intensity, though there would be a chance for equal 

 bilateral stimulation. Since locomotion is involved and conse- 

 quently directive stimulation in order to bring the larva in right 

 relation to the field of light, it was necessary to start the animal off 

 in the proper direction by means of a light from behind the larva, 

 which could be controlled. This was first accomplished b}^ means 

 of a 7 cp. incandescent light. Thus the larva was properly oriented 

 and would continue traveling in the same direction for some time 

 after the light was turned off. The more frequent result on reach- 

 ing the graded field was that the larva passed into this area of 

 light, exhibiting the usual random movements, but the after 

 effects of the directive light, though the light was turned off ten 



