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Lyall Watson – Supernature

Sensitivity to an annual cycle has obvious advantages.  It helps an organism predict seasonal changes in its environment and to make the necessary allowances for them.  A bird that spends its winters in the constant conditions of the tropics could use this sense to tell it when the time had come to return north for nesting.........

The tiny pale green plant lice, or aphids, that spend their summers busily plunging their mouth parts into plants and sucking out the juices, reproduce during the long days by a process of virgin birth in which no males are involved.  But when there is less than 14 hours of daylight, as autumn approaches, they start reproducing sexually and lay eggs that last the winter.  Many other animals change their appearance, rather than their habits, and adopt winter plumage.  Dull brown summer weasels turn in to resplendent white winter ones that can find concealment in the snow.  …....

In constant darkness fruit flies emerge from their pupae shortly after the temperature cycle reaches its lowest point, which in nature would be just before dawn.  Temperature can act as a time signal.  Hans Kalmus of London University found that grasshopper eggs hatched at dawn every day if kept at 22 degrees C, but that they hatched only at sunrise on every third day if kept at 11 degrees C