Modelling the Thermodynamics of Maggot Masses during Decomposition

Abstract

Estimating the minimum PMI (mPMI) based on larval age involves identifying the species, reconstructing the thermal history at a crime scene, and modelling the rate of development. However, few studies take into consideration the mass-generated heat produced by larvae co-existing in an aggregation. These localized increases in temperature are often highlighted in the literature as having an influence on larval development, but there are ongoing difficulties with incorporating this concept into mPMI estimates. This is mostly due to a lack of research on the topic, particularly with controlled laboratory experiments or in natural conditions simulations. The aim of this research was to determine whether heat generation varied in different sized aggregations and, if so, did it influence larval development and behaviour. Various sized aggregations (50-2500 larvae) composed solely of Lucilia sericata (Meigen, 1826)(Diptera: Calliphoridae) larvae were reared in the laboratory at a constant ambient temperature of 22 °C (±1 °C). Data loggers and a thermal imaging camera were used to record mass temperatures throughout the feeding stage of development. Larvae were sampled from these different sized aggregations at set times and had their instar determined and/or their lengths and fresh weights recorded so that developmental rates could be monitored. To investigate the movement of larvae as they fed in an aggregation, individuals were tagged with a fluorescing elastomer. These larvae were easily distinguished from the rest of the cohort, which allowed their positions within the mass to be recorded at regular intervals. The results showed a strong positive relationship between mass size and the amount of heat generated by the aggregation (p<0.001), with temperatures rising as masses increased in size. A minimum mass size of 1200 larvae was required for the local temperature to increase significantly above ambient, with aggregations of 2500 larvae producing temperatures that exceeded ambient by up to 14 °C (± 1.2 °C). Larvae sampled from increasingly large masses showed an accelerated rate of development during the 2nd and 3rd instar. This coincided with when masses were at their warmest. These faster growth rates resulted in larger aggregations entering the post-feeding phase of development an average of 13 hours earlier than smaller, cooler masses. Physical measurements taken from larvae at 70 hours development demonstrated that individuals sampled from larger masses were significantly longer and heavier than those sampled from smaller aggregations (p<0.001). This provided further evidence of faster growth rates. However, when compared to solitary larvae, all mass-reared larvae, regardless of the size of the aggregation, appeared to benefit from a faster rate of development, reduced mortality and larger body sizes at dispersal. Larvae were observed to be in a constant state of motion and continually repositioned themselves within the mass, rotating between the periphery and the centre where they presumably fed. The thesis highlights the need to incorporate mass temperatures into forensic casework when using larval development to estimate the time of death. Larvae sampled from large masses, particularly during the 3rd instar, could appear older than they actually are due to the accelerated rates of development experienced under warmer conditions. If this isn’t taken into consideration then it could result in an overestimation of the mPMI. Future research should focus on identifying how other variables influence heat generation in masses, as well as finding ways to estimate the size of a mass, and hence its thermal history, at a crime scene