66 STUDIES IN GENERAL PHYSIOLOGY 
long test-tube a (Fig. 4). The upper half was surrounded 
by a wide hollow cylinder b, the bottom of which was com- 
posed of the cork stopper ¢, into which a fitted water-tight. 
The lower half of a extended into the hollow cylinder d. 6 
and d were filled with water to a given height. When the 
temperature of the water in the cylinder b was 
84°, 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 
E 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 + 10°, it is scarcely possible 
to demonstrate the heliotropism of fly larve. Heliotropic 
experiments in these animals succeeded best at a tempera- 
ture of 20-25 °. 
The orientation of Musca larve 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. 
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