The Wandering Phase

The onset of migration from the food resource follows the cessation of feeding and the consequent emptying of the crop, and fly larvae may spend several days crawling metres before they find a satisfactory place to pupariate (Gomes et al. 2006; Greenberg 1990). While larvae are still actively migrating, the larvae or puparia furthest from the food source should be amongst the oldest present, but paradoxically, smaller (and therefore apparently younger) larvae tend to be found furthest from the food (Gomes et al. 2006). This is because larvae lose mass through exertion (Gomes et al. 2006), dehydration, and the gradual depletion of their crops' contents (Denlinger and Zdarek 1994). Additionally, at least in Calliphora vicina, extended wandering adds disproportionately to duration of the puparial stage (Arnott and Turner 2008).

Larvae in laboratory settings do not generally have to go far to find suitable pupariation sites as they do in the field, and so that laboratory measurements of the duration of this stage are biased relative to natural situations (Arnott and Turner 2008). Migration in the field may be extended if larvae have difficulty finding refuges, and particularly if larvae of predatory species such as C. rufifacies or C. albiceps are also present (Gomes et al. 2006; Greenberg and Kunich 2002). Larvae that have settled down and contracted may resume wandering if they are disturbed, for instance by other larvae, so that laboratory results can be both biased and unrepresentatively precise.

Wandering lasts about 150-400 h °C in C. albiceps (Richards et al. 2008) and 600-900 h °C in C. chloropyga (Richards and Villet 2008; Richards et al. 2009b) under laboratory conditions, a level of imprecision that is a substantial proportion of the mean. Except in a few species like C. vicina, migration is seldom protracted by more than 3 or 4 days (Arnott and Turner 2008; Gomes et al. 2006; Greenberg 1990), after which larvae tend to die. In C. vicina, if the wandering phase exceeded a threshold of 5 h (at 21°C), the physiological duration of the puparial stage increased (i.e. was biased) by roughly 700-1,500 h °C (Arnott and Turner 2008).

Because larvae and puparia of different species and different sizes are found in spatial patterns around a food source, (Gomes et al. 2006) suggested a protocol for sampling them in the area around corpses. Although the shrinkage of the crop as it empties after feeding ends is not a smooth process (Greenberg and Kunich 2002; Reiter and Hajek 1984), the associated decline in body length and mass is (Fig. 7.3), and both are useful sources of estimates of PMI . The slope of the relationship min relating size to age is rather shallow (Fig. 7.3), so that the estimates always have poor estimate precision due to the natural variation amongst larvae, but parts of the curve for crop emptying are steeper and thus provide more precision (Greenberg and Kunich 2002). Nonetheless, these processes do not continue indefinitely, so other signs of additional bias due to extended wandering need to be sought in the natural history of specific cases and taken into account.

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