Effect of massing on larval growth rate.

Estimation of minimum postmortem interval commonly relies on predicting the age of blowfly larvae based on their size and an estimate of the temperatures to which they have been exposed throughout their development. The majority of larval growth rate data have been developed using small larval masses in order to avoid excess heat generation. The current study collected growth rate data for larvae at different mass volumes, and assessed the temperature production of these masses, for two forensically important blow fly species, Chrysomya rufifacies and Calliphora vicina. The growth rate of larvae in a small mass, exposed to the higher temperatures equivalent to those experienced by large masses, was also assessed to determine if observed differences were due to the known temperature effects of maggot masses. The results showed that temperature production increased with increasing mass volume, with temperature increases of 11 °C observed in the large Ch. rufifacies masses and increases of 5 °C in the large C. vicina masses. Similarly, the growth rate of the larvae was affected by mass size. The larvae from small masses grown at the higher temperatures experienced by large masses displayed an initial delay in growth, but then grew at a similar rate to those larvae at a constant 23 °C. Since these larvae from masses of equivalent sizes displayed similar patterns of growth rate, despite differing temperatures, and these growth rates differed from larger masses exposed to the same temperatures, it can be concluded that larval growth rate within a mass may be affected by additional factors other than temperature. Overall, this study highlights the importance of understanding the role of massing in larval development and provides initial developmental data for mass sizes of two forensically important blowfly species commonly encountered in Australian forensic casework.

Tracking movement and temperature selection of larvae of two forensically important blow fly species within a "maggot mass".

The current study responds to the lack of understanding about the temperatures experienced by individual blow fly larvae within "maggot masses." The temperature selection of both aggregating (in a mass) and nonaggregating larvae was compared and their pattern of movement assessed. Infrared imaging determined the temperatures within a mass and in the vicinity of the constituent individual larvae, whose movements were tracked by dyeing their tissues red. Individual Chrysomya rufifacies larvae selected temperatures above 27°C, significantly higher than the temperature selected by Calliphora vicina larvae (24.5°C). However, this same difference was not seen within a mass, with both species selecting temperatures around 28°C. Larval movement in a mass was nonrandom, indicating that larvae actively select their position in a mass. Furthermore, larvae have a strong tendency to select the hottest part of a mass; therefore, maximum mass temperatures might provide a reliable proxy for the actual temperatures experienced by larvae.

Thermogenesis in decomposing carcasses.

It is of fundamental importance in forensic entomology that the factors controlling carcass temperatures during decomposition are thoroughly understood. The thermal environment to which fly larvae are exposed is the primary influence on their growth rate, and hence affects any estimate of minimum time since death using such specimens in homicide investigations. To date, much of the entomological research on maggot masses has focused on their elevation of carcass temperatures, with very little focus on the bacteria associated with larval activity. The aim of this study was to investigate the heat associated with decay and the types of bacteria present during the decomposition of a carcass, both in the presence and in the absence of maggots. Three treatments were imposed: fresh, frozen and maggot-infested, each consisting of five replicate pig carcasses. Temperature measurements and bacterial swabs were taken from the gastro-intestinal region of each pig and temperatures and bacterial communities compared between treatments. All carcasses reached average maximum temperatures above 32 °C in a temperature controlled room set at 23 °C. Treatment had no statistically significant effect on the temperatures recorded in each carcass but did significantly affect the community structure of the bacteria. However, bacterial community structure varied across time. This study suggests that bacterial metabolism plays a significant role in carcass thermogenesis, and that maggot masses, while contributing to localised heating within the carcass, may have less of a role in elevating carcass temperatures than previously assumed.

Experimental and casework validation of ambient temperature corrections in forensic entomology.

This paper expands on Archer (J Forensic Sci 49, 2004, 553), examining additional factors affecting ambient temperature correction of weather station data in forensic entomology. Sixteen hypothetical body discovery sites (BDSs) in Victoria and New South Wales (Australia), both in autumn and in summer, were compared to test whether the accuracy of correlation was affected by (i) length of correlation period; (ii) distance between BDS and weather station; and (iii) periodicity of ambient temperature measurements. The accuracy of correlations in data sets from real Victorian and NSW forensic entomology cases was also examined. Correlations increased weather data accuracy in all experiments, but significant differences in accuracy were found only between periodicity treatments. We found that a >5°C difference between average values of body in situ and correlation period weather station data was predictive of correlations that decreased the accuracy of ambient temperatures estimated using correlation. Practitioners should inspect their weather data sets for such differences.