220 WILLIAM BRODBECK HERMS 



larvae, quite the reverse from a sudden increase of illumination. 

 For C. vomitoria, where the after-effects were so pronounced in 

 experimenting with low intensities, no results were obtained to 

 illustrate this principle in the same manner, but fig. 25 is substi- 

 tuted and represents the matter quite as clearly. The courses 

 seen in this series of figures (fig. 16 to 25) show without further ex- 

 planation that the random or trial movements are characteristic 

 for low intensities, and become fewer with the increase in inten- 

 sity until finally the orientation is to all intents and purposes 

 direct, i.e., the animal turns directly away from light of high inten- 

 sity. Thus there is almost a perfect gradation between the in- 

 direct method of random movements and the direct tropism scheme 

 of behavior with regard to orientation. Therefore it seems prefer- 

 able in reference to the organisms in question to designate 

 this as a combination method of orientation, uniting the two general 

 schemes in one. The larger number of random movements pro- 

 duced by C. vomitoria is again indicative of its lower degree of 

 sensitiveness as compared with L. caesar. 



With the orientation of the animal to the source of light there 

 is still to be considered the second step in the process of photo- 

 taxis, i.e., the movement away from the stimulus. This step is 

 accomplished, as is evident from an examination of any of the 

 figures, by the action of the light rays operating bilaterally on the 

 organism, it must keep its general direction. Any deviation from 

 this predetermined path is, however, corrected by trial movements 

 in a more restricted sense. The circus movements produced by 

 larvae when one side of the head is pigmented is further evi- 

 dence that these organisms when once oriented follow the direct 

 or tropism scheme of behavior. 



C. Why is there this behavior to light? It has been shown that 

 the sarcophagid fly-larvae respond to light, orient negatively and 

 creep away from the source of light, in short are negatively photo- 

 tactic. What is the relation between this behavior of the larvae 

 and the conditions under which they exist normally? 



It will be recalled that the eggs of the adult females are deposite 1 

 on dead animals found in light situations. The larvae while feed- 

 ing are largely protected from light and birds by the carcass. 



