Introduction

The cuticle of all insects is covered with a very thin epicuticular layer of wax. This layer consists of free lipids, a class of compounds that includes hydrocarbons, alcohols, fatty acids, waxes, acylglycerides, phospholipids and glycolipids (Gibbs and Crockett 1998), although the presence of the last three groups may reflect aggressive extraction techniques which remove both internal as well as external lipids. This waxy layer prevents desiccation and penetration of micro-organisms (Gullan and Cranston 1994) as well as encoding various chemical signals. In the majority of the insects, and nearly all social insects so far studied, the free lipids are dominated by hydrocarbons (Lockey 1988). Cuticular hydrocarbons are found in all life stages of insects and are biologically very stable. Their biosynthesis is genetically based and modulated by factors such as reproductive status (Monnin 2006), developmental stage (Martin et al. 2001), diet (Buczkowski et al. 2005) or temperature (Toolson 1982; Savarit and Ferveur 2002; Rouault et al. 2004). The link found between diet and hydrocarbon production in social insects by Liang and Silverman (2000, 2001) is disputable, since behavioural changes occur in <2 min suggesting direct hydrocarbon transfer via contact with the prey rather than via diet, which explains why the host acquires the entire cuticular hydrocarbon profile of the prey (Liang and Silverman 2000).

Necrophagous insects are important insects for the determination of the postmortem interval. In establishing the postmortem interval a forensic entomologist will try to establish the age of the oldest colonising species; for which various methods exists (see review by Amendt et al. 2004). One of these methods could involve the identification of hydrocarbons present on the cuticle of the insects or even on their pupae and puparia as these contain hydrocarbons as well (Gilby and McKellar 1970) as some recent studies (Zhu et al. 2006, 2007; Ye et al. 2007; Roux et al. 2008) have shown that the composition of the hydrocarbon profile found on either the cuticle

F.P. Drijfhout

School of Physical and Geographical Sciences, Keele University, United Kingdom

J. Amendt et al. (eds.), Current Concepts in Forensic Entomology, 179

DOI 10.1007/978-1-4020-9684-6_10, © Springer Science + Business Media B.V. 2010

of the larvae or the pupae is not static but changes over time. If these changes occur as part of the development of larvae into adults and if this can be incorporated into a model, these hydrocarbons could be a very useful tool in estimating the age of a larvae or pupae and hence could increase the accuracy of the PMI.

The chapter aims to discuss the possible use of hydrocarbons in forensic entomology through a general overview of the structure of hydrocarbons as well as how they can be chemically analysed and identified. The second part focuses on the physiological function of hydrocarbons in insects and possible changes that may occur either related to their developmental stage or different climate conditions that may exist.

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