THE PHOTIC REACTIONS OF SARCOPHAGID FLIES 215 



Loeb's ('90, pp. 70.71) conclusions are different, as the follow- 

 ing quotation shows: ''so konnte ich auch f iir die Muscidenlarven 

 nachweisen, dass sie unter dem Einflusse der Richtung der Strah- 

 len gezwungen waren, auch von Stellen geringer Lichtintensitat 

 in solche von hoher Lichtintensitat zu gehen." 



It is perfectly clear that sweeping statements with regard to 

 the action of intensity or direction of the rays cannot be made. 

 Whereas one species of organism is almost exclusively influenced 

 by the intensity, as in planarians, a second group of organisms is 

 chiefly influenced by the direction, as in the sarcophagid fly- 

 larvae, and in both cases the two factors are more or less involved 

 in each other. Furthermore, as has been pointed out by other 

 workers {e.g. Walter, '07, pp. 144-145), factors of a more or less 

 disturbing or obscuring nature enter into behavior in general. 

 Thus, there is an element entering into the behavior of the flies 

 under consideration which has received little or no attention by 

 former investigators either in fly-larvae, or in other lower organ- 

 isms, namely the after-effects of light stimulation. For at least 15 

 to 20 seconds after the light has been turned off, the larva of a 

 flesh-fly may continue creeping in a straight line without an in- 

 crease in the number of random movements. This phenomenon 

 is w^ell illustrated by the following series of figures (figs. 16-23 

 inclusive). In all cases the path of the larva was in total 

 darkness for a period of from 15 to 20 seconds preceding the 

 change in direction due to the sudden illumination from in front. 

 The after-effects are still more clearly demonstrated by figures 

 6-10 inclusive. The trails of the larvae in figs. 6 and 7 are sharply 

 deflected and the succeeding paths are in a new direction and as 

 straight as though the larvae were perfectly oriented to a contin- 

 uous bilateral stimulus. In figs. 8-10 the movements of the ani- 

 mals are again influenced by the after-effects. One would expect 

 the path of the individual to make a sharp angle at the edge of the 

 more intense field and to take up at once a course parallel to the 

 rays of the directive light. Instead, the path is in its beginning 

 diagonal, plainly influenced by the after-effects of the more in- 

 tense light. 



