

INSECT BEHAVIOR 311 



strongly negatively phototropic and seek dark hibernation quarters, in 

 spite of warm temperatures that may prevail. If hibernating in a 

 warm place the mosquito becomes positively thigmotropic, and loses 

 its phototropism, prolonged exposure to strong lights producing no 

 response, though the insect responds actively to mechanical stimuli. It 

 is also negatively geo tropic, as it always assumes a position with the 

 long axis of the body perpendicular to the earth and the head pointing 

 upward. (H. B. Weiss.) 



Though adaptive in their results, these phototropic reactions can 

 scarcely be said to be performed on account of their usefulness. They 

 are performed anyway, and may result harmfully, as when they lead 

 a moth into a flame or, to take a more natural example, when they 

 expose an insect to its enemies. 



Phototropism and thermotropism, either together or singly, as 

 Wheeler suggests, may explain the up and down migration of insects in 

 vegetation. "On cold, cloudy days few insects are taken because they 

 lurk quietly near the surface of the soil and about the roots of the vegeta- 

 tion, but with an increase in warmth and light they move upwards 

 along the stems and leaves of the plants, and, if the day be warm and 

 sunny, escape into the air." 



F. Payne bred sixty-nine successive generations of the pomace fly, 

 Drosophila ampelophila in the dark without any resultant effect upon 

 either the eyes or the phototropism of the flies. 



Drosophila is usually positively phototropic, but R. S. McEwen 

 obtained a mutant which is not phototropic; this character being 

 "linked" with a characteristic "tan" color. 



Muscle Tension Theory. Experiments by Professor S. J. Holmes 

 with water scorpions (Ranatrd) showed that when the insect is illuminated 

 from the right side the legs on the right side are flexed and those of the left 

 side are extended, with a resultant locomotion toward the light. These 

 and other experiments "leave no doubt that the primary effects of light 

 consist in changes in the tension of muscles." (Loeb.) The muscle tone 

 is dependent upon the intensity of the light. If a positively phototropic 

 insect is illuminated from one side only it turns toward the light until the 

 muscle tension is equal on the two sides of the body ; then locomotion is 

 inevitably toward the source of light. The stimulus is received through 

 the eyes. . 



Artificial Heliotropic Machine.- As illustrating the purely 

 mechanical nature of the response to light, the artificial heliotropic 

 machine, as described by Loeb, may be referred to. Briefly, this 



