Alteration of bloodstain patterns by Diptera: objective analysis based on Wonder’s 2001 criteria.

Abstract

The blood feeding activity of flies at crime scenes can be confounding. Where a high number are present at a crime scene, a forensic investigator could mistakenly sample a fly artefact with the assumption that it is blood spatter, thus, making errors in the reconstruction of the event, or even recovering DNA from an individual who has not attended the scene. Three experiments were conducted to investigate the blood feeding activity, and blood artefact patterns created by flies following a blood meal. The respective trials were undertaken in a staged environment that can be considered operationally relevant to the field of bloodstain pattern analysis (BPA); in which approximately 500 flies per experiment were exposed to tray of 500ml of horse blood in a sealed 3m x 3m gazebo for a period of approximately 72 hours. The resulting patterns, consisting of a total of 539,507 individual fly blood artefacts, were objectively compared to recreated impact and expirated bloodstain patterns. These comparisons focused on overall pattern shape, total artefact/stain numbers, artefact/stain density per cm2 and the distance from the blood source in which they were deposited. By utilising the bloodstain size classification proposed by Laber in 1985 as a model, individual artefact/stain size was also examined. Informal observations and recordings were also made regarding similarities in individual artefact/stain colour, artefact/stain alignment and artefact/stain morphology but these were not quantified. Lastly, the present-day suitability of Wonders 2001 objective criteria for BPA was assessed against its suitability to include the feeding/deposition activity of flies, which is currently not accounted for. Results found highly significant differences between genera in the zones which their artefacts were deposited, the size of these artefacts (χ2 .05, 4 = 314093, p < 0.01), their count (F0.05, 4, 476025 = 137082, p < 0.01) and their densities (F0.05, 2, 538278 = 77680, p < 0.01). Furthermore, these results indicate that Diptera have a high affinity for window areas. From a total of 170,898 within this area, where fly artefacts were considerably more abundant than in other areas, Calliphora deposited 33.9% of its total artefacts, Hydrotaea 31.1% and Lucilia 29.1%, suggesting that artefact deposition /bloodstain pattern alteration by Diptera may occur significantly more in areas of light ingress. Moreover, in the tested window area, all five artefact categories (Calliphora, Lucilia, Hydrotaea, impact spatter and expirated spatter) displayed significantly different count totals (F0.05, 4, 310417 = 364774, p <0.01) thus, significantly different respective densities (F0.05, 4, 284227 = 360217, p <0.01). Additionally, impact spatter, expirated spatter and the fly artefacts from the three genera of flies exhibited significant differences in their respective zonal distribution (A-E) in both artefact count (F0.05, 4, 120369 = 38787, p < 0.01) and artefact density (F0.05, 4, 107509 = 4597, p < 0.01). These findings suggest that within fly artefact deposition patterns, and generated impact and expirated bloodstain patterns, distinctive areas in which artefacts/bloodstains will be of higher or lower count and density are present. As shown here, this could be confounding for a BPA analyst when fly artefact patterns are overlaid on top of these spatter patterns, creating a uniformly dense mixture of fly artefacts and bloodstains. As a result of these studies, three additional criteria are proposed for addition to the current objective criteria: 1.) External factors impacting on overall pattern shape 2.) Morphology (shape) of individual bloodstains 3.) Colour of individual bloodstains.