Precocious Development

Precocious egg development refers to the starting of embryonic development in a single egg in the common oviduct of a gravid female fly, before it is laid. This phenomenon, first mentioned by (Smith 1986), occurs when a fertilized female has sufficient time to convert a protein meal into mature eggs but, for several days, lacks opportunity to lay them. During this period, one egg will be pushed into the common oviduct, become fertilised as it passes the opening of the spermathecal duct, and begin developing to the point where it may even hatch almost as soon as it is laid (Wells and King 2001). Because it is commonly advised to use the largest larvae available (Fig. 7.3) to estimate the PMImin (Amendt et al. 2007), precocious development will lead to overestimates.

This phenomenon is very common in sarcophagids (where many eggs may be involved, which is often misinterpreted as viviparity), but has also been reported in the calliphorids Aldrichina grahami (Aldrich), Calliphora terraenovae Macquart, Calliphora nigribarbis Vollenhoven, C. vicina, and perhaps also Lucilia sericata (Meigen) (Erzinglioglu 1990; Wells and King 2001). It is not known if precocious eggs occur in beetles. (Wells and King 2001) reported that 34 (62%) of 55 wild, gravid females of C. terraenovae had a virtually mature embryo in their common oviduct, but this rate certainly varies with the availability of protein meals, the speed at which meals are metabolised to eggs, and oviposition opportunity, which in turn probably all vary seasonally.

Precociously developing larvae would lead to an overestimate of PMImin that may be as long as the entire length of embryonic development in calliphorids, and perhaps longer in sarcophagids. This can be 7-323 h (0.5 - 13.5 days), depending on species and temperature (Davies and Ratcliffe 1994; Higley and Haskell 2001; Melvin 1934). (Wells and King 2001) suspected that calliphorid larvae that hatch within their mother are expelled, but sarcophagids appear to retain such larvae for some time.

Age (Days)

Fig. 7.3 Growth curve of Chrysomya chloropyga at 20°C, showing the typical decelerations in growth rate at ecdysis (*) and the decrease in size during the wandering phase. The two lines enclose the area where the largest larvae occur. Using larvae selected by such a narrow criterion to estimate the PMI . allows more temporal precision in the estimate. Their measurements will have min r r smaller relative errors, and they are less likely to be influenced by competition, thermoregulation, parasitoids or other disturbances (cf. Tarone and Foran 2006), although they may be precocious (Erzinglioglu 1990; Wells and King 2001). Examination of the crop size will distinguish wandering larvae (to the right of the peak) from feeding larvae of the same size (to the left of the peak) (Greenberg and Kunich 2002; Reiter and Hajek 1984). ♦ - first instar larva; o - second instar larva; ▲ - third instar larva; □ - puparium

Age (Days)

Fig. 7.3 Growth curve of Chrysomya chloropyga at 20°C, showing the typical decelerations in growth rate at ecdysis (*) and the decrease in size during the wandering phase. The two lines enclose the area where the largest larvae occur. Using larvae selected by such a narrow criterion to estimate the PMI . allows more temporal precision in the estimate. Their measurements will have min r r smaller relative errors, and they are less likely to be influenced by competition, thermoregulation, parasitoids or other disturbances (cf. Tarone and Foran 2006), although they may be precocious (Erzinglioglu 1990; Wells and King 2001). Examination of the crop size will distinguish wandering larvae (to the right of the peak) from feeding larvae of the same size (to the left of the peak) (Greenberg and Kunich 2002; Reiter and Hajek 1984). ♦ - first instar larva; o - second instar larva; ▲ - third instar larva; □ - puparium

Managing this source of inaccuracy must take contingencies into account. (Erzinglioglu 1990) pointed out that if several hundred female flies laid on a carcass, there may be substantial numbers of precociously developed larvae. He suggested that, '[i]nstead of using the age of the largest larva as the basis for time of death estimation, it would be more accurate to base the estimate on the age of the larval stage that is present in the largest numbers'. This approach assumes that female flies oviposit within a few hours of each other, and may be applicable when only one or two flies are involved. When larger numbers contribute to the maggot population, they may lay over a few days, and the modal age of the resulting larvae will underestimate the PMI . In these situations, an interim solution is to err conservatively min J

and 'to subtract the time required for embryonic development when calculating the minimum possible age of a bluebottle larva' (Wells and King 2001). The assumption here is that the very largest larvae are likely to be precocious. The reference to a 'minimum possible age' refers to the lower bound of the window of prediction (Fig. 7.1); the upper bound would assume that the larvae were not precocious. The strategy that is appropriate to a particular case can be refined by estimating how many females contributed to the pool of larvae and empirically examining the occurrence (and maturity) of precocious eggs in a sample of gravid, conspecific, wild females trapped under comparable seasonal conditions.

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