362 WILLIAM L. DOLLEY, JR. 



while still others make circus movements for a period and then 

 orient fairly accurately. 



This marked lack of harmony between the results obtained 

 may in some measure, at least, be due to the fact that the num- 

 ber of sources of light was not the same in all of the experiments : 

 Parker does not state the conditions under which the specimen 

 of Vanessa antiopa used by him made circus movements. Radl 

 presumably performed his experiments before a window, i.e., 

 under conditions in which the animals received some light from 

 many different directions. The same probably held also for the 

 work of Holmes on amphipods and several insects. In some 

 experiments, however, as in those performed with Ranatra and 

 Notonecta, he worked in a ^darkened room,' and used for a source 

 of light a sixteen candle-power incandescent lamp. Brundin and 

 Carpenter also used a similar source of light. It is significant 

 indeed that in every case where a single source of light on the 

 same horizontal plane with the organism was used, at least some 

 trials are described in which no circus movements were made, 

 the anunals moving in a fairly straight course toward the light. 

 This was true of Ranatra, Notonecta, Drosophila, Bufo ameri- 

 canus, Orchestia traskiana, and Orchestia pugettensis. On the 

 contrary, in none of the experiments but one, where the light 

 conditions were not sharply defined, have the investigators re- 

 corded any other behavior than movements in circles. This 

 single exception is that described by Radl, in which Calliphora 

 vomitoria and Musca domestica with one eye blackened ran for 

 some distance directly toward a window. 



The experiments described in the present paper show that in 

 the case of Vanessa antiopa, at least, a knowledge of the number 

 of sources of stimulation is of great importance in a discussion 

 of circus movements; for the same animals, which, in a hori- 

 zontal beam, moved toward the source of light in a fairly straight 

 course, performed circus movements continuously when placed 

 before a window, or when the single source of light was placed 

 above the animal so that the light was non-directive.^ The 



' The term 'non-directive light,' as used in this paper, denotes diffuse illu- 

 mination. 



