Investigating potential information obtained from blowfly artefacts deposited by Lucilia sericata flies (Diptera: Calliphoridae)

Abstract

Blowflies are amongst the first organisms to arrive at a corpse, their olfactory system attracts these flies to decomposing organic material, where they can feed and deposit their young. It has been accepted that blowflies will regurgitate and defecate as a response to normal feeding behaviours, the resulting stains have been termed fly artefacts. Artefacts have become of interest with the development of techniques to extract DNA from these stains, which have the potential to determine what the blowflies have been feeding on. This information could be of use to forensic casework in cases where the human remains have been removed and only the fly artefacts are available for analysis. The aim of this study was to determine whether artefacts could be differentiated into colour groups for sampling, then to establish whether there is a difference in DNA (food source) quantity between the different groups. Flies were observed depositing artefacts both by eye and through filming, showing that defecated material can be observed in a range of colours (creamy white to deep brown). An artefact sampling colour key was produced to enable a more objective separation of the artefacts into the five colour groups (colourless, very light, light, dark, and very dark) for further analysis. Findings of this study display a correlation between fly artefact colour and the time passed since blowflies ingested food; the longer blowflies were left without porcine liver the lighter the colour of their artefacts became, eventually becoming colourless after a period of 15 days. Adult flies could therefore be collected from a crime scene to enable observation of their artefact, indicating an approximate time since the fly had last ingested food. To determine if there was any advantage in separating the samples for DNA analysis, two real-time PCR DNA assays were developed, one to detect and quantify the food source and blowfly DNA, and the other to detect inhibitors within samples. Very light and light (grouped) artefacts contained the highest mean quantities of DNA (0.62 ng/L and 0.52 ng/L respectively) with no inhibitors detected in any of the colour groups. As the artefacts that gave the best DNA quantities were very light and light, this could make differentiation from blood spatter easier as these are often darker in colour. Based on the findings of this study, a standard operating protocol has been started that can provide guidance for the sampling of artefacts at the crime scene, with scope for further developments identified.