The foraging activity of necrophagous flies has the potential to be counterproductive to criminal investigations, largely as a result of the way they feed. For most species of flies tht frequent human remains, adults land on or beside a corpse, walking across the surfaces or through wet body fluids.
Gustatory receptors located at the tips of tarsi and on the sponging mouthparts are used to assess the nutritional value of the fluids and tissues. Applying Locard’s Exchange Principle to the interaction between necrophagous flies and a corpse, evidence of this association will be left behind at the crime scene.
Foraging activity is known to cause mechanical disruption of pooled blood and body fluid stains that have not dried. It can also lead to transfer patterns, created by tarsi or the abdomen leaving impressions after passing through wet fluids, either at the primary scene or at other sites.
As adult flies consume body fluids, they regurgitate and defecate some of the ingested food onto surfaces at or near the crime scene, creating an intermixing of fly artifacts with bloodstains and other human body fluids.
Fly contaminates are not restricted to the primary crime scene, as adults display positive phototaxis, and thus are attracted to windows and lights, locations in which wet blood may be transferred or artifacts deposited. In essence, false secondary crime scenes are established as a direct consequence of foraging activity on a corpse.
The problems with fly artifacts are magnified by the fact that regurgitate and defecate are virtually indistinguishable from human bloodstains. Fly stains are morphologically very similar to impact (i.e., forward, back, and mist-like spatter), projected, sneezed, and expirated bloodstains, and cannot be reliably distinguished using presumptive or confirmatory tests available for identification of human blood.
The use of molecular methods, namely DNA typing, for person’s identification does not overcome these limitations since DNA profiles can be obtained of an individual from blood consumed by flies. A few methods have been reported to be useful in differentiating fly artifacts from human bloodstains, but all have limitations that prevent each from being consistently reliable for use in crime scene investigations.