Post-feeding larvae dispersal of forensically important UK blow flies

Abstract

The post-feeding larval dispersal stage of forensically important UK blow flies is generally underrepresented in the literature. However, in order to give an accurate estimation of the minimum post mortem interval it is necessary to locate the oldest entomological specimens associated with the body. Often these are not on the body but have dispersed, consequently it is essential to determine how far from the body to search for post-feeding larvae, puparia and/or empty puparial cases and also how deep in the substrate to excavate. Therefore, this study examined the horizontal and vertical dispersal of post-feeding larvae, how dispersal is affected by different dispersal substrates, and whether post-feeding larvae, and the subsequent puparia, exhibit aggregation behaviour. This study determined that substrate had a significant effect on the horizontal distance dispersed by post-feeding larvae. The majority (> 50 %) of Calliphora vicina, C. vomitoria and Lucilia sericata larvae dispersed less than 4 m if the dispersal substrate allowed larval penetration, such as soil, whereas an even, impenetrable surface (e.g. smooth plastic) forced the larvae to disperse the full length of the experimental apparatus, producing concentrations of puparia recovered from either end of the apparatus (i.e. where contact with the apparatus was greatest). In contrast, Protophormia terraenovae, a species not generally known to disperse far from the feeding source, were recovered within 50 cm of the origin in all but one of the experimental runs, in which the larvae dispersed over 5 m. This result shows that under certain circumstances P. terraenovae is capable of dispersing much further than has been reported in the literature. In experiments that examined vertical dispersal, C. vicina larvae were capable of successfully dispersing up to 56 cm deep in the substrate where horizontal dispersal was impeded, and over 75 % of puparia were recovered within the top 20 cm of the substrate. However, if horizontal dispersal was possible then over 80 % of puparia were recovered from just the top 5 cm of the substrate. Under most conditions, larvae appeared to aggregate prior to pupariation, and this was more defined when an impenetrable, smooth plastic substrate was utilised, resulting in puparia being recovered only from either end of the experimental apparatus, clumped together. This study also reports the novel use of a servosphere to examine apodous insects. A servosphere was used to record the speed of C. vicina and P. terraenovae post-feeding larvae on a smooth plastic surface over time. During 5 minute runs the speed of both species remained almost constant over 4 days and Calliphora vicina larvae were approximately twice as fast as P. terraenovae larvae. However, during one experiment that tracked the changes in speed of C. vicina over one hour, speed initially decreased at a rate that reduced over time, until speed was almost constant for the final 20 minutes of the experiment. The servosphere was also used to examine phototaxis of C. vicina post-feeding larvae and, as expected, a negative phototactic response was recorded, within 5 seconds of the introduction of a light source. The data produced during this study was consolidated to produce an addition to the current guides on collecting and preserving entomological evidence. This could be used by forensic entomologists, SOCOs and CSIs at crime scenes to optimise the collection effort.

Date of Award	Aug 2018
Original language	English
Awarding Institution	University of Winchester
Supervisor	Amoret Whitaker (Supervisor), Keith Wilkinson (Supervisor) & Martin Hall (Supervisor)