Examining external morphological characteristics of Lucilia sericata pupae for age estimation in medico-legal investigations

Master Thesis


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Insects play an essential role in the resolution of medico-legal investigations. For various insects, like necrophagous flies, it is vital for their survival to find and inhabit a food source such as a decomposing body. This makes it possible to use these insects as evidence in medico-legal investigations. A crucial part of any medico-legal investigation is estimating the time that has elapsed since death, otherwise known as the post-mortem interval (PMI). The age of the oldest insects can provide the most precise estimation of the PMI. Flies undergo different stages of development. All the immature stages can be utilized as evidence at death scenes. Out of all the immature stages, pupae represent the oldest specimens, which makes them valuable in establishing a minimum time since death. Identification and aging of pupae is currently a challenging process since they all look similar in appearance. Few studies have been done on pupae for PMI estimation. This study aims to identify reliable morphological markers to aid in a more accurate age estimation of Lucilia sericata during the pupal stage. A total of 145 pupae were collected and examined for external morphological changes over time. Five pupae were collected at each time point. A total of six external morphological characteristics of the puparium and twenty external morphological characteristics of the pupae were initially examined. These characteristics were linked to age in accumulated degree hours (ADH), with the aim of creating a timeline that can aid in the estimation of pupal age. The pupal ADH ranges from the youngest being 6550 ADH and the oldest being 11300 ADH. The timelines identified several characteristics that develop during the early or late stages of development. Multiple linear regression analysis was performed to assess characteristics which were useful for estimating the age of the pupae (ADH) and develop a regression equation based on the data collected. The regression analysis identified 10 characteristics that are the most significant in aging pupae. They were the colour and shape of the labellum, leg length, leg width at full length, thoracic setae, facial setae, abdominal macrosetae, palp shape, genal setae and labrum colour. Some of these characteristics like the leg length and width and abdominal macrosetae did provide important time-breaks on their respective timelines. However, the development and the pigmentation of the compound eye also provided valuable time-breaks it's timeline.