THE PHOTIC REACTIONS OF SARCOPHAGID FLIES 201 



whereas under usual conditions with like intensity euch diagonal 

 courses were reduced to about 15 per cent in the same number of 

 trials. Out of 50 trials, 4 per cent (two trials by the same larva) 

 resulted in a return to the point A, the larva refusing to go into 

 regions of higher intensities. This larva in its remaining trials 

 (three in number) took a sharply diagonal course. 



Under the heading of "Accuracy of Orientation," Walter ('07, 

 pp. 79-80) notes that the negative planarians subjected to direc- 

 tive light showed a strong tendency to take a path in a diagonal 

 direction, and calls attention to the similar case found by Smith 

 ('02, p. 469) for the earthworm. Walter believes this to be due in 

 the case of planarians, to imperfect orientation resulting from the 

 crescentic pigment shields of the eye, which would permit a diag- 

 onal path to the right or left to a certain degree without allowing 

 light to stimulate the retina. In the eyeless earthworm, though 

 Walter does not suggest this, the diagonal path may have been 

 due also to imperfect orientation. Furthermore, the arrange- 

 ment of the sense organs on the segments, as shown b}' Harper 

 ('05), would certainly permit a more or less diagonal course, i.e., 

 a turning from side to side to a certain degree as the worm crawls 

 would be possible without subjecting the sense organs to stimu- 

 lation from light. 



A very much more perfect orientation is possible on the part of 

 the fly-larva because of the localized condition of the photore- 

 ceptive function as discussed on page 205, etc. In these organisms 

 the receptive surface is restricted to the extreme anterior pole, 

 and as the animal travels away from the light, this part of its 

 body lies in its own shadow, so that the creature is continuously 

 oriented within a narrow range of shadow. 



6. Intensity and rate of 7novement 



While experimenting with various intensities of light, it became 

 apparent that the larvae crawled more rapidly as the directive 

 light became more intense. The question naturally arose, — What 

 is the relation between the intensity of the light and the rate of 

 movement. This matter was tested in the following manner. 



