Twenty flies are placed in each of four glass tubes. The tubes are sealed. Tubes I and II are partially covered with black paper; Tubes III and IV are not covered. The tubes are placed as shown. Then they are exposed to blue light for five minutes. The number of flies in the uncovered part of each tube is shown in the drawing.These data show that these flies respond to (respond means move to or away from):

Correct answer: A. Blue light but not gravity

A. Blue light but not gravity
B. Gravity but not the blue light
C. Both blue light and gravity
D. Neither blue light nor gravity

Explanation

Comparing tubes III and IV, we can see that when there was a height difference in tube III, there were more flies in the upper half (19) than in the lower half (1). Meanwhile when tube IV was examined, the left and right halves - which were at the same height - had an equal number of flies (10). This shows that the flies did indeed respond to gravity.When we compare tubes II and IV, both of these are horizontally placed but in tube II, one half is covered with black paper while the other half is exposed to blue light. As a result, there are 18 flies in the exposed region and 2 in the darkened region. When this is compared to tube IV which had an equal number of flies in either half, we can conclude that exposure to blue light is detected by and responded to by the flies.

Last updated 3 September 2026

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