THE PHOTIC REACTIONS OF SARCOPHAGID FLIES 207 



with a mixture of lard and lampblack, and then illuminating that 

 portion, failed to produce any turning until the substance was 

 rubbed off in crawling. On blackening one side of this region only 

 and exposing the larva to light from overhead there were produced 

 the t^^pical circus movements with the pigmented side toward the 

 centre of the circle, as found by Holmes ('01) for negative terres- 

 trial amphipods. It should be said that each larva was first tested 

 for the normal reaction before the pigment was applied. 



It was also possible by careful manipulation to snip off the seg- 

 ment possessing the anterior hooks (the first segment), but the 

 results were most unsatisfactory, since the larva is almost wholly 

 dependent upon these hooks for locomotion, and consequently its 

 reactions were questionable. Moreover, larvae thus operated on 

 die in a few days. 



Another series of experiments was tried with the light pencil 

 apparatus, which gave further evidence toward the restriction 

 of photo-sensitiveness to a very limited region at the oral pole of 

 the larva. The individuals were started from a drop of tap water 

 on the slate stage (fig. 2, F) toward the light pencil {K) . As soon 

 as a fair start was made the light (L) was turned off. If the con- 

 clusion already reached is to hold good the fly larva should be 

 more or less sharply deflected to its left side on encountering the 

 pencil of light. The moisture on the animal from the drop of 

 water left a trail on the black stage so that its movements in the 

 dark could be traced after again turning on the light. 



Ten migrated larvae of C. vomitoria were tried, each five times, 

 and out of this total of fifty trials with the light striking the right 

 side of the individual 80 per cent of the courses were deflected to 

 the left. When the light pencil acted on the left side of the larva, 

 again 80 per cent of the courses were deflected, but this time to the 

 right. It is quite evident that this signifies a well balanced bilateral 

 condition of the sense organs. 



The remaining 20 per cent of the trials represent courses taken 

 straight through the light pencil without deflection; there were 

 no deflections toward the source of light. If any deflection was to 

 take place, it occurred each time when the head of the larva came 

 into the light; as soon as the head passed through the illuminated 



THE JOURNAL OP EXPERIMENTAL ZOOLOGY, VOL. 10, NO. 2 



