A forensic toolkit for the "cheese skipper": Larval identification and thermal requirements of the immature stages of Piophila casei (L.) (Diptera: Piophilidae)

The 'cheese skipper', Piophila casei (L.) (Diptera: Piophilidae), is a cosmopolitan and synanthropic species that frequently colonises cadavers in advanced stages of decomposition. Despite its potential as a forensic indicator, the use of P. casei in casework is hampered by (i) some contradictory descriptions of its larval morphology, which limit the reliability of the identification of specimens collected at the forensic scene, and (ii) the lack of reference data and temperature-dependent models, which hinders the accuracy of minimum postmortem interval (minPMI) estimates based on the developmental rates of this species. Here we determine and illustrate a series of diagnostic morphological characters that allow for the unequivocal identification of P. casei larval specimens. The shape and degree of sclerotization of the larval cephalopharyngeal skeleton and the number and arrangement of the rows of spines of the ventral creeping welts are particularly useful allowing not only for species identification, but also for larval instar determination. In addition, we analyse the influence of temperature on the development of the different immature stages of P. casei, providing an isomorphen diagram and determining the thermal requirements (lower developmental threshold and thermal summation constant) for five developmental events, enabling accurate minPMI estimates in forensic casework. In addition, within the intra-puparial period, we determine a series of age-specific morphological markers that are easily observable through the translucent puparium of P. casei, thus increasing the accuracy of minPMI estimates and avoiding the potential risk of sample damaging during puparium dissection when using puparial samples.