Many insects, perhaps most, do not develop continuously all year round, but avoid some seasonally adverse conditions by a resting period (section 6.5) or migration (section 6.7). Summer dormancy (aestivation) and winter dormancy (hibernation) provide two examples of avoidance of seasonal extremes. The most predictable environmental indicator of changing seasons is photoperiod — the length of the daily light phase or, more simply, day length. Near the equator, although sunrise to sunset of the longest day may be only a few minutes longer than on the shortest day, if the period of twilight is included then total day length shows more marked seasonal change. The photoperiod response is to duration rather than intensity and there is a critical threshold intensity of light below which the insect does not respond; this threshold is often as dim as twilight, but rarely as low as bright moonlight. Many insects appear to measure the duration of the light phase in the 24 h period, and some have been shown experiment- ally to measure the duration of dark. Others recognize long days by light falling within the “dark” half of the day.
Most insects can be described as “long-day” species, with growth and reproduction in summer and with dormancy commencing with decreasing day length. Others show the reverse pattern, with “short-day” (often fall and spring) activity and summer aestivation. In some species the life-history stage in which photoperiod is assessed is in advance of the stage that reacts, as is the case when the photoperiodic response of the maternal generation of silkworms affects the eggs of the next generation.
The ability of insects to recognize seasonal photo- period and other environmental cues requires some means of measuring time between the cue and the subsequent onset or cessation of diapause. This is achieved through a “biological clock” (Box 4.4), which may be driven by internal (endogenous) or external (exogenous) daily cycles, called circadian rhythms. Interactions between the short time periodicity of circadian rhythms and longer-term seasonal rhythms, such as photoperiod recognition, are complex and diverse, and have probably evolved many times within the insects.