HELIOTROI-ISM or ANIMALS 



Positive heliotropism can be demonstrated in flics by the 

 same experiments as in plant lice; only it must be noticed 

 that flies are provided with several more kinds of irrita- 

 bility than plant lice, and that in consequence heliotropism 

 may be obscured when other stimuli besides light come into 

 play. In one experiment, for example, I observed 

 that light, gravity, and heat were without effect 

 on the flies, because the animals always remained 

 on the cork stopper in the test-tube. Some sub- 

 stance was probably on the stopper that attracted 

 the flies; for when I put the animals into a flask 

 with a clean glass stopper, they reacted to light. 



I am indebted to Professor Ernst Mach for a beautiful 

 observation on the influence of light on the orientation of 

 the house fly : 



Several years ago I accidentally made an observation which I 

 have never been able to follow further. While adjusting 1 my rotat- 

 ing polarization apparatus in a dark room, by the help of sunlight, 

 whereby a bright quadratic picture a some 16 cm. across (Fig. 5) 

 \\;is rotated three or four times per second in a circle of a radius of 

 30 cm., a fly F (Fig. 6) happened to enter 

 the bundle of rays LL, went through the 

 whole rotation as though stunned, and fell 

 upon the table. I was able to repeat this L 

 \periment twice. The fly, which was appa r- 

 ently sound, escaped while I was giving niy 

 attention to something else. FIG. c 



In this case, then, the same effect was produced by rotat- 

 ing the rays of light as by revolving the fly on a centrifugal 

 machine. 1 



1 R&dl has recently runic to tin- conclu-ion that tin? reactions of insects on a 

 centrifugal machine are indeed caused by the li^'lil. Jf this is correct, they are 

 identical with Maeh's ob^-rvat ion. [r.in:;] 



