INSECT BEHAVIOR 



349 



parallel with the long axis of the trunk of the tree and the 

 head pointing upwards. When disturbed they fly off, but 

 very soon alight nearer the earth and again walk upward." 

 (Wheeler.) Coccinellidse and cockroaches are also negatively 

 geotropic. The latter insects, as Loeb has observed, tend to 

 leave a horizontal surface but come to rest on a surface that 

 is vertical or as nearly so as possible. 



Wheeler says, " Geotropic as well as anemotropic orienta- 

 tion is not altered for the sake of response to light. Even if 

 the insect be strongly heliotropic, as is the case in most Dip- 

 tera, it orients itself to the wind or to gravity no matter 

 whence the light may fall." 



Phototropism. It is a matter of common observation that 

 house flies, butterflies, bees and many other diurnal insects fly 

 toward the light ; and that cockroaches and bedbugs avoid the 

 light. These are familiar examples of phototropism, or the 

 " control of the direction of locomotion by light." The pho- 



FIG. 289. 



A, tracks made on paper by a larva of Lucilia ccesar moving out of a spot of ink 

 under the influence of light; A and B show respectively the first and second 

 directions of the light. B, tracks made in the dark. After POUCHET. 



