A comparison of drying patterns between insect stains created by the blue blow fly, Calliphora vicina and human...

We examined the drying characteristics of insect stains produced by the blow fly Calliphora vicina following ingestion of human blood using image analysis of stains deposited or produced on non-porous ceramic tile. The duration of drying and final morphologies of insect stains were influenced by ambient conditions (predominantly temperature), fluid volume, and time. The latter was most apparent with regurgitate stains, which typically required 2-3x longer to dry under ambient conditions than comparable sized bloodstains. As human blood dried on non-porous surfaces, a perimeter ring formed followed by radial cracks, and eventually lateral cracks between the radial lanes. The entire process occurred in > 10 min at 25 °C on ceramic tile when 2 µl of blood was deposited directly onto the tile surface. In contrast, regurgitate stains deposited directly by adult flies varied as to whether radial cracks formed. When radial cracks did form, they lacked symmetry like human bloodstains. Defecatory stains never formed contact lines at the perimeter, nor were radial or lateral cracks ever observed with dried fecal fluids. In experiments in which regurgitate was forcibly exuded from individual adult flies after consumption of human blood, the drying patterns were more consistent with blood than naturally deposited regurgitate in that many stains formed thickened perimeters and radial and lateral cracks during drying on ceramic tile. The results of this investigation indicate that some insect stains can be distinguished from human bloodstains based on morphological differences of dried stains when deposited on non-porous surfaces.