Same, same but different!-matching entomological traces to a human food source by stable isotope analysis.

Blow flies are important tools in forensic entomology, as they feed and develop on human bodies. This fact can be used to estimate the minimum post mortem interval (PMImin), by classifying the age of the immature stages found on corpses. The capacity to also be able to classify the age of adult flies, or even empty puparia, could increase the timeframe for a PMImin. However, the association of older developmental stages, such as adult flies or their empty cases, with a human cadaver may be challenged, as such specimens could stem from another food source. Analyzing the stable carbon (δ13C) and nitrogen (δ15N) isotopes in such specimens could be helpful here, as the isotope signatures reflect those of the food sources. We compared the δ15N and δ13C signatures of tissue from humans and 12 additional species by elemental analyzer-isotope ratio mass spectrometry (EA-IRMS). We did the same for adult flies and empty puparia of the blow fly Lucilia sericata, which were derived from juvenile stages developed on tissues from the mentioned species. The isotope signatures for the adult flies and their empty puparia linearly mirrored the isotope signatures for the respective tissues on which they developed as larvae. Results such as these are useful to indicate the flies' nutritional history, but they do not provide the same kind of evidence that a DNA analysis would. As individual dietary habits, global distribution and environmental effects can influence isotope ratios, we strongly recommend to analyzing entomological traces and muscle tissues of potential donors or for other food sources.