270 BRADLEY M. PATTEN 



Before the appearance of this paper, I had made a somewhat 

 similar experiment on the blowfly larva, with the same results that 

 Ewald obtained for Daphnia. Using the apparatus described on 

 page 220, one of the beams of light was cut down by a diaphragm 

 and the other by an episcotister, so that the light coming from one 

 side was a steady beam of low intensity, and that from the oppo- 

 site side an intermittent beam in which bright flashes alternated 

 with darkness. The apertures in the sector wheel were adjusted 

 so that the amount of light from each source was equal for a unit 

 time. It has already been established that when the larvae are 

 subjected to equal steady beams of light from opposite directions, 

 the aggregate response is almost precisely at right angles to the 

 line connecting the sources of light. The average angular de- 

 flection of 200 trails at equality (p. 240) was only 0.09°, when the 

 degrees represented a distance of but 1.5 mm. If the Bunsen- 

 Roscoe law holds for the phototactic response of the larvae, 

 they should orient perpendicularly to the rays of light when 

 subjected to the action of steady and intermittent lights of equal 

 energy per second. The experimental results based on 136 

 trails made under these conditions show an average angular 

 deflection of but 0.07° from the perpendicular. ^^ These results 

 seem to show that in the blowfly larva the phototactic reaction 

 follows the Bunsen-Roscoe energy law. 



Mast ('11, p. 234) says: 



There is no conclusive evidence, except perhaps in animals with 

 image forming eyes, showing that light acts continuously as a directive 

 stimulus, that symmetrically located sides are continuously stimulated. 

 . . . . (p.. 235) . Light no doubt acts on o-ganisms ^vithout a change 

 of intensity much as constant temperature does, making them more or 

 less active and inducing changes in the sense of orientation; but there is 

 no conclusive evidence showing that light acting thus ever functions in 

 the process of orientation. 



Abrupt changes of intensity no doubt cause stimulation and 

 are very effective in orientation under certain conditions'^ b^t 

 they are not, as Mast maintains, the only ways in which light 



^^ In these experiments the number of light impulses was 115 per second. 

 •* See orientation orblowfly larva to single light, pp. 252 to 272. 



