2~O J. E. WODSEDAL1 K. 



organism a- a \\lmle, rather ihan to the specific effect of acid or 

 other solution- on tin- chemical state of some postulated Mib- 

 stance. 



My experiments on the effects of chemicals on photoiaxis in 

 //. inter punctata nymphs were performed in a dark room and the 

 source of light was an incandescent bulb of seventy-five candle 

 power. In each experiment luenty fresh specimen- decidedly 

 negative in ilieir reaction to light were put in a long and narn>\\ 

 glass basin, three and a half bv t \u-nty-six inches, of about 2,000 

 c.c. capacity half filled \yith distilled water. The concentration 

 of the solution was then slowly and gradually increased by adding 

 constant small amounts of the given chemical at about tin or 

 twelve. minute intervals. A careful record was kept of tin- num- 

 ber of insects positive and the number negative at the different 

 concentrations throughout the experiment, h was noticed that 

 as the concentration of the solution was increased there was a 

 gradual increase in the number that became positive. For ex- 

 ample, in the experiment with hydrochloric acid at .01 per cent. 

 solution, of the twenty specimens four became positive; at .02 

 per cent, eight were positive; at .03 per cent, fifteen were positive: 

 and at .04 nineteen were positive. It was the same with the 

 other chemicals employed. As a rule, specimens that once be- 

 came positive remained positive throughout the experiment, and 

 usually the insects that seemed to be the least annoyed by t In- 

 light in fresh water were the first to manifest their positive photo- 

 taxis in the solution. In general the specimens that evim ed 

 their dislike for the light most obviously in fresh water were the 

 ones that made the most frantic efforts to get at the light when 

 they once became positive. Ho\\e\er. the specimens as a rule 

 were very persistent in their normal reaction, remaining negative 

 until death occurred. 



Of the different classes of chemicals emplo\cd the acids were 

 the most effective in reversing the photoiaxis of //. inter punctata 

 nymphs. A large majority, and in >onn- cases all ol tin- twenty 

 specimens became positive in tin- acid solutions. \Yhen ( '()o \\.i-- 

 bubbled through the water, again, a large percentage of tin- speci- 

 mens became positive. The concentrations sufficient for this 

 effect with the different acids u-ed are, -hydrochloric .04 per 



