4 THE NORTH AMERICAN SPECIES OF DROSOPHILA. 



II. BEHAVIOR. 



REACTIONS TO LIGHT. 



The adults of Drosophila react positively to light, i. e., they go toward 

 a source of light. This reaction has been studied by Carpenter (1905, 

 1908), Payne (1911), Lutz (1914), and McEwen (1918). Under con- 

 ditions of ordinary illumination the movement toward a source of 

 light is noticeable only when the flies are disturbed. It occurs, how- 

 ever, whether the animals walk or fly. 



McEwen, whose work is the most extensive and the most recent, 

 concludes that, in D. melanogaster, the young females react more 

 strongly than do the males. This sexual difference decreases as the 

 flies grow older, and eventually almost disappears. Both sexes react 

 most vigorously when about 4 daj's old. Removal of the wings causes 

 the flies to become less responsive to light, and the degree to which the 

 responsiveness is lost is roughly proportional to the amount of the 

 wing that is cut off. As was to be expected from this fact, the mutant 

 races, with parts of the wings gone or deformed show the same rela- 

 tions — the smaller the amount of normal wing present, the less marked 

 is the reaction to light. 



McEwen has also studied the reactions of several mutant eye-colors 

 to lights of different colors. His general conclusion is that for flies 

 with eye-colors lighter than that of the wild tj^pe, the order of decreas- 

 ing effectiveness for colored lights is violet, green, red. For the wild 

 type and for sepia, a darker eye-color, the order of effectiveness is 

 violet, red, green. 



REACTIONS TO GRAVITY. 



Adults of Drosophila react negatively to gravity, i. e., they usually 

 crawl up rather than down, especially when disturbed. This reaction 

 has been studied in D. melanogaster by Carpenter (1905), Cole (1917), 

 and McEwen (1918) . Cole found that the negative reaction was shown 

 by the animals when they were crawling, but not noticeably when they 

 were flying. He also found that they reacted negatively to a centri- 

 fugal force equal to or greater than gravity. When crawling they 

 reacted negatively to air-currents also; but when flying they usually 

 reacted positively to the same currents, even though they w^ere able 

 to fly against them. 



OLFACTORY REACTIONS. 



Barrows (1907) has studied the reactions of Drosophila melanogaster 

 to odorous substances. He finds that they react positively (are 

 attracted) to various organic compounds found in fermenting fruits — 

 amyl alcohol, ethyl alcohol, acetic acid, lactic acid, and acetic ether. 



