The Case of the Casings: Application of Field Desorption-Mass Spectrometry for Species Identification of Blow Fly Puparial Casings for Postmortem Interval Determination

Medicolegal forensic entomology employs insects to estimate postmortem interval (PMI), or the time since death. Accurate PMI estimation relies on accurate identification of the species of the collected insect evidence. However, this process is particularly challenging when the only available evidence consists of empty puparial casings, as they exhibit limited and highly similar morphological features across species. Thus, there remains a critical need for rapid approaches for the accurate identification of the species of insect evidence retrieved from decomposing remains to reveal critical information about the timing of the death in forensic investigations.
Reported here is a novel, rapid approach in which the soft ionization technique of field desorption-mass spectrometry (FD-MS) coupled with multivariate statistical analysis enables species identification of empty puparial casings with 100% accuracy for 6 blow fly spp.: Calliphora latifrons, C. livida, C. vicina, Lucilia cuprina, L. sericata and Phormia regina. The chemical basis for the high accuracy results was the presence of differentiating masses associated with alkanes as large as C55H102, and sterols and esters as large as C58H118O2, many of which have yet to be reported in the literature in association with insects. Other detected compounds were unique to particular species, such as 3-methyltricosane (C. vicina), 11,11-dimethylhenicosane (C. latifrons), 2-methylhexacosane (C. livida) and 11-decyltetracosane (L. sericata). There were 31 m/z values which were important for discrimination that were detected by FD-MS, but which could not be detected by GC due to their nonvolatility. The presence of these compounds could facilitate the development of hydrocarbon profile-based species prediction models that feature higher molecular weight molecules that are more stable to weathering under field conditions, and which could serve to enhance species determination accuracy for aged and/or weathered samples.