484 Journal of Comparative Neurology and Psychology. 



that the immersed end of the box was directed toward a window 

 the positively phototropic flies, introduced into the elevated end, 

 could be made to creep toward the hot or cold region, since this 

 region lay in the direction of the source of light. 



Reaction to increased temperature. The water surrounding 

 the immersed end of the box was raised to 45° C, a temperature 

 that is soon fatal to Drosophila. Flies introduced into the oppo- 

 site end of the box, which was practically at room temperature, 

 crept more or less steadily toward the light. This movement 

 brought them gradually to a region of increasing temperature. 

 Upon arriving near the lower water-line {D) the creeping flies 

 turned about, describing curved paths, and headed back toward 

 the cooler end of the box, their positive phototropism apparently 

 being overcome by the repelling effect of the heat. In no instances 

 did creeping flies, whether on the floor or the roof of the box, pass 



Fig. I. Sectional plan of apparatus. A B, glass box, 38 cm. long, 23 cm. wide, and 8 mm. deep, 

 partially immersed in water; C, upper water-line; D, lower water-line. 



beyond this lower water-line. Occasionally one would creep along 

 the line in a zigzag manner, as though alternating between the 

 two antagonistic directive influences, but finally it would yield 

 to the negative stimulus, and creep back to the region of lower 

 temperature. The majority of the return excursions were made 

 as the result of a uniform and continuous deflection from the 

 heated area. Since there was no satisfactory evidence of random 

 movements involving a "trial and error" method of reaction, this 

 behavior may conveniently be spoken of as a "tropism." This 

 term is here used merely in a descriptive sense for an orderly turn- 

 ing away from a stimulating region. It does not carry with it an 

 implied theoretical explanation of the precise effect of the stimulus 

 on the organism. 



Reaction to decreased temperature. When the water in which one 

 end of the glass box was immersed was cooled with ice, the other 



