J. E. WODSEDAL1 K. 



were separated, and within another minute also left for the dark 

 area. Sometimes a specimen would return to the less obscure 

 end of the dish and not finding anything favorable to its thigmo- 

 tactic inclination, it went back whence it came from. Thi> ex- 

 periment was repeated several times, always with similar results. 



The light was next permitted to enter the trough obliquely, 

 the-^hicker end of the prism being next to the light. Again five 

 of the individuals made for the darker end of the trough almost 

 immediately, this time going against the rays of the light. About 

 half a minute later two other specimens followed and within 

 three minutes all had deserted the light area. Thi^ experiment 

 was repeated many times with ten different sets, and always in 

 less than one minute a large majority of the specimens were in 

 the dark portion of the trough. There were almost always a 

 few slow specimens which required a longer time, from one to 

 ten minutes and even much longer, to be moved. If left in the 

 trough, almost invariably at the end of a few hours the nymphs, 

 with some exceptions, would gather into a group in the obscure 

 region and remain there indefinitely. If the prism and light 

 were reversed, leaving the entangled colony undisturbed in the 

 more illuminated end, the colony would slowly break up and with- 

 in several hours again form in the more obscure territory. 



Another apparatus was so arranged that the length of the 

 prism could be increased or lessened by tipping the trough holding 

 the solution, at various angles, and thus augmenting or diminish- 

 ing the contrast between the two ends of the trough. It \\a- 

 observed that the proclivity of most nymphs to choose one end 

 of the dish in preference to the other varied directly in proportion 

 to the contrast in the illumination of the two extreme regions 

 of the trough. Here again, we find that some individuals react 

 far more readily than others. 



It was also observed that some nymphs under ordinary li^ht 

 manifest no signs of agitation when in a uniformly lighted area, 

 yet when an area in their neighborhood becomes slightly shad< d 

 they soon aggregate there. 



CONTROL OF PHOTOTACTIC RF..V riONS HY ( "in \IKALS. 

 It was found in the experimental \\ork on the reactions to 

 light that II. intcrpunctata nymph-- are practically all negatively 



