NOTE ON THE HELIOTROPISM OF PALyE- 

 MONETES LARV/E. 



F. P. LYON. 



The striking peculiarity of the light reaction of these larvae 

 is that they orient themselves with the anterior end away from 

 the light and then swim backward toward the light. If orienta- 

 tion alone is the criterion of a tropism, these organisms are nega- 

 tively heliotropic. But they gather on the side of the disk 

 toward the light and as ordinarily observed would surely be con- 

 sidered positive. 



Blue light produces these effects as strongly as daylight. 

 Red light is almost ineffective. 



The orientation of the larvae is usually extremely precise. 

 The instant that light is admitted to a dish containing the larvae, 

 they all turn so that their heads are directed away from the 

 source of light and the body axis is in the line of the rays. In 

 many the abdomen is strongly extended so that the body axis is 

 a straight line. These swim backward practically in a straight 

 line. Very frequently they swim on their backs in a perfectly 

 straight course to the light side of the dish. Others swim with 

 ventral side directed down. 



Besides the individuals which hold their bodies extended and 

 exhibit very accurate orientation there are many which swim 

 with the abdomen more or less flexed ventrally. These are 

 inclined to rotate, but their general course is quite exact toward 

 the light. 



A study of the water currents, when India ink was added, 

 showed that the backward movement was the result of strong 

 forward beats of the swimmerets. The details have not been 

 studied. The animals can move forward, but this form of loco- 

 motion is quite different from the backward type, being accom- 

 plished by a series of very rapid jumps or plunges. So quick 

 were these forward movements that I could not accurately ob- 

 serve how they were accomplished. I believe that they were 



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