Abstract
Minimum post-mortem interval (minPMI) estimates often rely on the use of developmental data from blow flies (Diptera: Calliphoridae), which are generally the first colonisers of cadavers and, therefore, exemplar forensic indicators. Developmental data of the intra-puparial period are of particular importance, as it can account for more than half of the developmental duration of the blow fly life cycle. During this period, the insect undergoes metamorphosis inside the opaque, barrel-shaped puparium, formed by the hardening and darkening of the third instar larval cuticle, which shows virtually no external changes until adult emergence. Regrettably, estimates based on the intra-puparial period are severely limited due to the lack of reliable, non-destructive ageing methods and are frequently based solely on qualitative developmental markers. In this study, we use non-destructive micro-computed tomography (micro-CT) for (i) performing qualitative and quantitative analyses of the morphological changes taking place during the intra-puparial period of two forensically relevant blow fly species, Calliphora vicina and Lucilia sericata, and (ii) developing a novel and reliable method for estimating insect age in forensic practice. We show that micro-CT provides age-diagnostic qualitative characters for most 10% time intervals of the total intra-puparial period, which can be used over a range of temperatures and with a resolution comparable to more invasive and time-consuming traditional imaging techniques. Moreover, micro-CT can be used to yield a quantitative measure of the development of selected organ systems to be used in combination with qualitative markers. Our results confirm micro-CT as an emerging, powerful tool in medico-legal investigations.
Highlights
Developed for its use in clinical practice [1], computed tomography (CT) was soon after applied to legal medicine [2]
In practical terms, ageing blow flies to within 10% of the intrapuparial period is probably as much as could be realistically expected in a criminal investigation, bearing in mind the natural variation in developmental rates within any generational cohort of larvae and the conservative confidence intervals that would need to be placed on any estimation of age due to difficulties in estimating the many variables at a scene, for example the temperatures to which the insects were exposed during their development [43]
Iodine is not a permanent stain, so after being scanned in micro-CT, specimens may be suitable for a subsequent analysis of external morphological characters [8, 19, 20] if they are first properly washed
Summary
Developed for its use in clinical practice [1], computed tomography (CT) was soon after applied to legal medicine [2]. Post-mortem CT in combination with conventional autopsy is currently recommended as the diagnostic gold standard for determining the cause of death [4]. Through these ‘virtual autopsies’ using post-mortem CT, even the entomological evidence associated with a cadaver can be analysed by locating and estimating the volume of maggot masses on the body [5]. Still less available than CT to forensic practitioners, micro-computed tomography (micro-CT) can be a powerful tool in medico-legal investigations [6]. In a forensic context, micro-CT has a wide range of potential roles, e.g. the examination of gunshot residue particles, the analysis of tool marks on bones or the analysis of the age of entomological evidence for the estimation of a minimum post-mortem interval (minPMI), among others [6, 8]
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