678 



SCIENCE 



[Vol. LV, No. 1434 



it should be placed in shallow water on a slop- 

 ing bank and partially embedded in the mud 

 or sand so that the bottom of the funnel is 

 even with the bottom of the pond. The rest 

 of the trap extends out toward the deeper 

 water. A dead fish wired securely to the bot- 

 tom of the trap makes an excellent bait. 

 Attracted by this bait, the crayfish crawl into 

 the trap and seem to be unable to find their 

 way back out. A single night-set with such a 

 trap will reward the trapper with at least a 

 water bucket full of crayfish for laboratory 

 use, or for the more immediate purpose of 

 supplying the camp with an exceedingly de- 

 lectable breakfast. 



E. C. O'ROKE 

 South Dakota State College, 

 Brookings, South Dakota 



SPECIAL ARTICLES 



NOTE ON THE RELATION BETWEEN THE 

 PHOTIC STIMULUS AND THE RATE OF 



LOCOMOTION IN DROSOPHILA 

 It is a fact demonstrated by many investi- 

 gators that Drosophila melanogaster (ampelo- 

 phila) is negatively geo tropic and positively 

 phototropic. In addition it is also known that 

 light acts as a kinetic stimulus as well as a 

 directive one. When the individual is illum- 

 inated, therefore, its movement is determined 

 by the three factors operating simultaneously. 

 If light acts in opposition to gravity the rate 

 of upward crawling of the fly is lowered; and 

 if light acts with gravity the rate is increased. 

 Since the stimulus of gravity is always con- 

 stant, and the photokinetic stimulus constant 

 within wide limits, the rate of upward crawling 

 is a measure of the effect of the phototropic 

 stimulus. 



Definite quantitative results have been ob- 

 tained by measuring with a stop-watch the time 

 necessary for wild flies to crawl to the top of a 

 glass cylinder under three different intensities 

 of light. Illuminated from above with a light 

 of 1,500 candle meters the time taken for 50 

 per cent, of the experimental flies to reach the 

 top (a distance of 172 mm) v\-as found to be 

 6.17 seconds. With an intensity of 750 cm., 

 7.6 seconds; and with an intensity of 75 cm., 



10.89 seconds. Each of these determinations 

 is the average of 50 trials with 87 animals 

 selected from five different cultures. The age 

 of the flies varied between six and nine days. 

 Under the illumination of a rubj' lamp giving 

 only enough light to enable observation, the 

 time consumed in reaching the top was 11.3 

 seconds. There is then a definite relationship 

 between the intensity of illumination and the 

 rate of movement, which may be expressed by 

 the Weber-Fechner law, as was done in the 

 case of the Japanese beetle.^ Figure 1 ex- 



IIUE IN 



Fig. 1. Two graphs indicating the relation be- 

 tween light intensity and tlie phototropic orien- 

 tation of Drosophila. The circles are points, at 

 100 



which Rate = , plotted 



Eeaetion time in seconds 

 against tlie log of the intensity. The solid dots 

 show the reaction time plotted against the 

 intensity. 



presses this relationship. The broken line is 

 obtained by plotting the logarithm of the inten- 

 sity against the rate of locomotion, where rate 

 equals 100 divided by the reaction time in sec- 

 onds. From- this graph it may be concluded 

 that the sensation is proportional to the log- 

 arithm of the intensity of the stimulus. The 

 continuous line is obtained by plotting the reac- 

 tion time in seconds against the intensity of 

 light and leads to the same conclusion. 



It was found by McEwen- that the mutants 



1 Moore, A. R., and Cole, W. H. : "The response 

 of PopilUa japonica to light and the Weber- 

 Peehner law," Jour. Gen. Physiol., 3: 331, Jan- 

 uary, 1921. 



- McEwen, E. S. : " The reactions to light and 

 to gravity in Drosophila and its mutants, ' ' Jour. 

 Exp. Zool., 25: 49, February, 1918. 



