Species and age determination of blow fly pupae based upon headspace analysis

As submitted by the proposer: The study of insects found on a body can yield valuable information about the time and circumstances of death. In forensic entomological investigations, the oldest of such insects are traditionally collected from the corpse, their species and age determined, and a postmortem interval (PMI) can be estimated. In most cases, these insects are flies of the Dipteran family Calliphoridae. Females will lay eggs on a body shortly after death, the eggs will hatch and larvae will feed on the decomposing tissue through three larval life stages, reach a developmental threshold and leave the body to pupate and later eclose as an adult to start the life cycle again. The determination of the PMI is based on the assumption that the oldest specimen from the body is collected, and this is often pupae. Pupae are notoriously difficult to use as casework samples, as it is generally difficult to identify their species, and determining the age of a pupa either requires expertise not generally available. We propose a simple, fast, and cost-effective technique for identifying the species and age of fly pupae via the analysis of volatile organic compounds (VOCs). These compounds are given off by the cuticle - the protective "skin" of the insect. Previous research has shown these compounds are capable of identifying flies with a high degree of discrimination. In addition, the analysis of VOCs is highly amenable to the existing infrastructure of forensic laboratories, as it relies chiefly upon the use of gas chromatography-mass spectrometry systems. Because little is known about variations in VOC profiles over time and under a varying set of conditions, this proposal seeks to investigate the effects of several biotic and abiotic variables on the suite of VOCs synthesized and emitted by flies. The influence of diet, pupation substrate, temperature, light level, and humidity will be independently evaluated, and the VOCs observed for each variable will be subject to rigorous statistical treatment via chemometric methods. This will allow for the determination of a subset of VOCs that are relatively unaffected by changes in environment, which can be used for identification in entomological investigations regardless of region or climate. In short, we will determine which compounds given off by pupae are the least variable, and therefore, the most useful for species and age identification, and we will recommend those compounds for consideration in future forensic contexts. ca/ncf