THE REACTIONS OF DROSOPHILA 75 



4. Effect of centr if ligation. — The centrifuge consisted of a 

 table mounted on a base capable of revolving, on a fixed axis 

 in a horizontal plane. A small water motor attached to an 

 ordinary faucet furnished the motive power. On the table could 

 be fastened glass tubes of various lengths and bores. In these 

 tubes, the ends of which were tightly corked, one or more flies 

 were placed in the desired position; the tube was then revolved 

 about its middle point as a center at a known speed, the time 

 of revolution usually being one minute. 



In the preliminary trials it was found that at a certain speed 

 the flies in the ends of the tube crept toward the center and 

 remained there. If the speed was greatly increased, they were 

 thrown out to the ends. It became necessary therefore to de- 

 termine the maximum and minimum limits within which a 

 definite response could be noted. The calculation was made 



according to the formula, F = where F represents the 



r 



centrifugal force, m the mass, v the velocity of revolution and r 



the radius. Experiments showed that when F was equivalent to 



gravity the flies began creeping toward the center. When it was 



considerably larger than gravity the flies were thrown out 



to the ends. Furthermore when F was just equivalent to gravity 



the flies crept toward the center until they reached a point 



where the force was less than gravity, the speed remaining the 



same. To induce further creeping toward the center the speed 



had to be increased, since the shorter the radius of revolution 



the greater the speed necessary to generate the same force. 



The tube ordinarily used was 50 cm. long with a diameter 



of 2 cm. Applying the formula, n = , which can be de- 



47r2r 



rived from the previous one, the speed necessary to generate a 

 force equivalent to gravity is easily calculated. When the flies 

 are in the ends of the tube, therefore, it must revolve approx- 

 imately once every second; as they move toward the center 

 the speed must be gradually increased. But since the flies can 

 creep against a force much greater than gravity without losing 

 their equilibrium, a constant speed can be found at which they 

 will creep all the way to the center. This is about 85 revolu- 

 tions per minute. Experiments carried out in darkness, in dif- 



