Circadian Rhythms

Circadian rhythms are clock-like cycles in physiological processes, and therefore in behaviour, that arise from an endogenous, temperature-compensated genetic oscillator that is naturally entrained by light and temperature cycles each day (Johnsson and Engelmann 2008; Saunders 2008; Saunders et al. 2002). They effectively allow an organism to predict cyclic environmental phenomena, giving them a means to prepare for and synchronise with predictable exogenous conditions, and often have periods that are not exactly 24 h long. They can be reset by exposure to light or heat (Johnsson and Engelmann 2008; Joplin and Moore 1999; Saunders et al. 2002), which leads to pauses or jumps in the tick of the developmental clock that needs to be considered when estimating PMI .

In calliphorids, circadian rhythms affect processes like oviposition (Tessmer et al. 1995), larval growth (Saunders 1972), onset of wandering (Kocarek 2001; Smith et al. 1981), eclosion (Joplin and Moore 1999; Smith 1987), adult activity (Smith 1983; Smith 1987) and even the termination of larval diapause (Tachibana and Numata 2004). Furthermore, eclosion can be entrained by light-dark cycles experienced prior to pupariation, even if the puparia are subsequently kept under constant lighting (Joplin and Moore 1999), for instance by burying. In short, the influence of circadian rhythms in natural settings is practically inevitable. In air-conditioned indoor situations where lights are left on, rhythms may become uncoordinated. When Phormia regina (Meigen) was reared under constant lighting, so that its circadian rhythm was allowed to run freely, variation (imprecision) in overall adult developmental time increased and there was a significant delay (bias) in development compared with cyclic light (Nabity et al. 2007), indicating that the cycle length was greater than 24 h.

The peaks in behaviour produced by circadian rhythms may be sharp or broad (Kocarek 2001; Smith et al. 1981), but offer no clue to the true duration of the preceding developmental stage. Specimens may wait the whole daylight period before wandering, leading to a maximum imprecision of about 12 h if peaks are broad, and more if they are sharp (Smith et al. 1981). This distortion of the developmental clock probably sets the limit to precision for most estimates of PMImin and introduced a variable bias that is not easy to estimate.

Little can be done about adjusting a PMI if there is evidence that circadian min rhythms have been in play. They are a prime reason for defining an asymmetrical window of prediction (Fig. 7.1) for an estimate. The window can be adjusted by other considerations such as weather conditions on the estimated day(s) of oviposition.

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