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 days 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 type, 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 thej^ 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 were 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.