66 STUDIES IN GENERAL PHYSIOLOGY 



long test-tube a (Fig. 4). The upper half was surrounded 

 by a wide hollow cylinder 6, the bottom of which was com- 

 posed of the cork stopper c, into which a fitted water-tight. 

 The lower half of a extended into the hollow cylinder d. b 

 and d were filled with water to a given height. When the 

 temperature of the water in the cylinder b was 

 34, while that of the water in d was 18, the 

 animals in the upper part of the tube became 

 very restless, but did not creep into the cooler 

 part of the tube. Nor were the animals oriented 

 when the temperature in the lower part of the 

 cylinder was higher than in the upper. Such 

 FIG. 4 an orienting effect of temperature was observed 

 only when an animal came to the boundary between the 

 warmer and the cooler zones at c. In such a case the animal 

 moved into the zone having the lower temperature, but not 

 into the other. 



By means of diffuse daylight, however, it was an easy 

 matter to drive the animals from a place of lower tempera- 

 ture to one of higher. This is possible because the light 

 orients the animals and dictates to them sharply the direc- 

 tion of their progressive movement, while the same is done 

 by a source of heat only to a slight extent. It was therefore 

 possible to drive the animals from diffuse light into direct 

 sunlight, notwithstanding the difference in temperature. 



At low temperatures, even -f 10, it is scarcely possible 

 to demonstrate the heliotropism of fly larvge. Heliotropic 

 experiments in these animals succeeded best at a tempera- 

 ture of 20-25. 



The orientation of Musca larvce toward chemical stimuli. 



-If on a summer day a piece of meat is set in the open, 



blow flies collect on it in great numbers and deposit their 



eggs. There can be no doubt that a chemical stimulus 



attracts the animals and causes them to lay their eggs. 



