No. 497] 



TROPISMS OF INSECTS 



299 



Such are the results of experimentation where the trans- 

 parent sheet of glass has been allowed to change the nor- 

 mal conditions of environment. Let us suppose, however, 

 that the fly be allowed to pass out of the window into 

 a more nearly natural environment. It immediately flies 

 into the open, but does not endeavor for any length of 

 time to continue its positively phototropic movements. 

 Once unrestrained it is soon again following the normal 

 pursuits of its particular species, which in the case of the 

 aforementioned Drosophila is principally to locate decay- 

 ing vegetable matter which will be suitable for food, 

 breeding, and oviposition. It is evident in this case that 

 the universal and quick response of the Drosophila to 

 light when confined is due to some added factor in the 

 experiment which. I hope to point out on a later page. 



Another paragraph in the same paper refers to results 

 obtained under still more unnatural conditions. 



"The exposure of Drosophila to light of high intensity is accom- 

 panied by an increase in the kinetic effect. Under the influence of 

 the highest intensity used, that of a 250 c.p. arc light at 40 cm., the 



When we realize that these insects are as quick as 

 ourselves to appreciate sudden changes in light and shade 

 at short distances, it is not astonishing that they are un- 

 able to orient themselves under such startlingly unnatural 

 conditions, placed only sixteen inches from an arc light, 

 and their behavior here can not be compared with their 

 reactions to normal daylight or sunlight. In fact the per- 

 version of instinct induced by electric arc lights is a 

 common experience with many nocturnal insects which 

 are attracted in countless numbers, while their normally 

 phototropic diurnal relatives are disturbed scarcely at 

 all by the presence of the lights. 



This too, is a matter of common experience, which has 

 found expression in the adage of the moth and the flame. 

 Most of the Lepidoptera heterocera are negatively photo- 

 tropic, venturing out at dark and concealing themselves 



