Rhythms and Seasonality

Terrestrial arthropods display rhythms in their activity, correlated with and adapting them generally to changes in their environment. Some of these rhythms are independent of signals from their surroundings (endogenous or circadian rhythms; Brady 1974). Examples of such functions are daily periods of sleep alternating with active locomotion or feeding and periods of singing or courting. Events in long-term life cycles are also cyclic and have internal controls interacting with changes in ambient stimuli, day length being a very strong one (Beck 1980). Although the physiological basis for such functions is still not understood, an underlying "biological clock" mechanism is postulated (Saunders 1982).

Long periods of quiescence commonly occur in insects and relatives, often to carry the animal through adverse seasons. This is called diapause and is characteristic of high latitude or high elevation species in the wintertime (hibernation) or of desert species during dry periods (aestivation). At these times, growth, development, and activity is attenuated. Finally, diapause is broken with the return of favorable conditions, and emergence occurs. Sometimes large numbers may return to action simultaneously, resulting in population explosions. Periods of dormancy are less profound in tropical than temperate insects because of more equable environmental conditions in the lower latitudes (Denlin-ger 1986).

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