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Diapause has been most intensively studied in insects, where examples occur in all developmental stages. The common field grasshopper Chorthippus brunneus is a fairly typical example. This annual species passes through an obligatory diapause in its egg stage, where, in a state of arrested development, it is resistant to the cold winter conditions that would quickly kill the nymphs and adults. In fact, the eggs require a long cold period before development can start again (around five weeks at 0°C, or rather longer at a slightly higher temperature). This ensures that the eggs are not affected by a short, freak period of warm winter weather that might then be followed by normal, dangerous, cold conditions. It also means that there is an enhanced synchronisation of subsequent development in the population as a whole. The grasshoppers ‘migrate in time’ from late summer to the following spring.

the importance of photoperiod

Diapause is also common in species with more than one generation per year. For instance, the fruit‐fly Drosophila obscura passes through four generations per year in England, but enters diapause during only one of them. This facultative diapause shares important features with obligatory diapause: it enhances survivorship during a predictably adverse winter period, and it is experienced by resistant diapause adults with arrested gonadal development and large reserves of stored abdominal fat. In this case, synchronisation is achieved not only during diapause but also prior to it. Emerging adults react to the short daylengths of autumn by laying down fat and entering the diapause state; they recommence development in response to the longer days of spring. Thus, by relying, like many species, on the utterly predictable photoperiod as a cue for seasonal development, D. obscura enters a state of predictive diapause that is confined to those generations that inevitably pass through the adverse conditions.

Consequential dormancy may be expected to evolve in environments that are relatively unpredictable. In such circumstances, there will be a disadvantage in responding to adverse conditions only after they have appeared, but this may be outweighed by the advantages of: (i) responding to favourable conditions immediately after they reappear; and (ii) entering a dormant state only if adverse conditions do appear. Thus, when many mammals enter hibernation, they do so (after an obligatory preparatory phase) in direct response to the adverse conditions. Having achieved ‘resistance’ by virtue of the energy they conserve at a lowered body temperature, and having periodically emerged and monitored their environment, they eventually cease hibernation whenever the adversity disappears.