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Insects found at a crime scene can produce traces referred to as fly artifacts (FA) due to their movement over the corpse and the manner in which they feed upon it. These can be detrimental for carrying out criminal investigations. Confusing a FA with a genuine bloodspot can lead to misinterpretations, also taking into consideration that FA may contain a human DNA profile. The aim of the present study was to employ scanning electron microscopy (SEM) for the analysis of FA produced by Calliphora vomitoria on hard surfaces and fabrics that are commonly present at crime scenes. FA and control bloodstains were produced under experimental conditions on metal, glass, plaster, cotton, and polyester. After macroscopic analysis, FA were examined at standard low (20–40 ×), medium low (300–600 ×), and high ultrastructural (1200 ×) magnification through a SEM Stereoscan 360, Leica, Cambridge. SEM analysis enabled the identification of distinctive features of FA on hard surfaces, namely, amorphous crystals, micro-crystals with a morphology similar to those of uric or micro-crystals with a comparable morphology to cholesterol, absent in controls. Moreover, red blood cells (RBC) were absent in FA but were always present in controls. On cotton, for both FA and controls, the drop was almost completely absorbed and thus indistinguishable from the underlying fabric texture. On polyester, FA showed amorphous/crystal-like deposits and no RBC, as observed on hard surfaces, except for those showing a completely flat surface. SEM analysis appeared to be suitable for differential diagnosis between FA and genuine bloodstains on hard surfaces, although the results may be inconclusive on tested fabrics.

Fly artifacts resulting from insect activity could act as confounding factors on a crime scene and interfere with bloodstain pattern analysis interpretation. Several techniques have been proposed to distinguish fly artifacts from human bloodstains based on morphological approach and immunological assay, but a DNA-based method has not been developed so far. Even if in forensic genetic investigations the detection of human DNA is generally the primary goal, fly artifacts can provide useful information on the dynamics of crime events. The present study provides a molecular method to detect fly DNA from artifacts deposited by Calliphora vomitoria after feeding on human blood through the analysis of the mitochondrial cytochrome oxidase gene subunit I (COI). Fly artifacts originated from digestive process and of different morphology spanning from red and brownish/light brown, circular and elliptical stains to artifacts with sperm-like tail or a tear-shaped body were collected. The COI amplification was successfully obtained in 94% of fly artifact samples. The method showed high sensitivity and reproducibility, and no human DNA contamination was observed, offering specificity for use in confirmatory test. This molecular approach permits the distinction of fly artifacts from genuine bloodstains and the identification of fly’s species through the COI region sequencing by protocols usually applied in forensic genetic laboratories. Supplementary Information The online version contains supplementary material available at 10.1007/s00414-021-02643-7.

Bloodstain pattern analysis has a key role in crime scene reconstruction; however, it can be hampered by diverse confounding factors, such as insect activity which may lead to the production of small artifactual bloodstains, commonly referred to as fly artifacts (FA). Although several techniques aimed at distinguishing human bloodstains and FA have been developed, actually, no standardized and reproducible methodology is available. The aim of our study was to test the use of scanning electron microscopy (SEM) to distinguish human bloodstains from FA produced by Sarcophaga carnaria. FA and bloodstains have been produced on five different deposition surfaces under experimental conditions. After visual analysis, bloodstains and FA were analyzed at standard low (× 40–× 300) and high (× 600–× 1200) magnification through a Philips SEM 515. Although differential diagnosis between bloodstains and FA resulted often inconclusive at visual analysis, SEM analysis allowed the identification of additional key distinctive morphological features. In particular, on the surface of FA, small crystal-like and/or amorphous material deposits were observed. Such deposits were absent on bloodstains which, on the other hand, displayed red blood cells stacked in “rouleaux.” Basing on these results and under our experimental conditions, SEM analysis resulted suitable to perform a differential diagnosis between bloodstains and FA produced from the insect activity of Sarcophaga carnaria.