How Important Are Hydrocarbons in the Area of Forensic Entomology

From the information given above, it is quite clear that hydrocarbons are very important, and as such the time and research spend on these compounds can easily be justified. Hydrocarbons can give us some crucial information with regards to understanding communication system in insects, such as in ants. Furthermore, due to the many different hydrocarbons that exists on the cuticle of insects, each insect generally has a very distinctive hydrocarbon profile. This profile, in combination with powerful statistics, such as Principle Component Analysis (PCA), is a valuable tool in identifying species, a research area referred to as chemo-taxonomy. This use of chemicals to identify a species is used in plant taxonomy as well as insect taxonomy and hydrocarbons have been proven to be very useful within chemo-taxonomy (Page et al. 1997; Urech et al. 2005).

Within the context of forensic entomology, there are two possible areas where hydrocarbons could be very useful if not essential. The first area would be in the species identification of either larvae or eggs. Currently any larvae or eggs found on a body, when it is not obvious to which species they belong, needs to be reared to full adulthood before a positive identification can be obtained. DNA-Barcoding can be used to identify the species when it concerns eggs, larvae or adults, however it can be a time consuming process and will be very difficult in the case of empty puparia. Yet, due to the species specificity of a hydrocarbon profile, the hydrocarbons can be extracted from eggs or larvae, and the obtained hydrocarbon profile can be compared to a database with known hydrocarbon profiles from different species. If a match is found the egg or larvae can be identified to its species within a couple of hours. It is possible to extract hydrocarbons from eggs, larvae, adults and even exuviae. The latter was shown by Ye and co-workers (Ye et al. 2007) where the authors extracted hydrocarbons from the pupal exuviae of six necrophagous flies. After analysing the extracts with GC-MS and performing discriminant analysis on all the observed peaks, all exuviae from the six different flies could be separated from each other. Both linear alkanes (such as tricosane and octacosane) and branched alkanes (such as methylpentacosane, dimethyl hexacosane) were important hydrocarbons in order to discriminate between the six different extracts. This study emphasised the forensic importance of hydrocarbons and how these can be used in conjunction with DNA-Barcoding. Up till now no other papers have been published using hydrocarbons in the context of combining forensic entomology and chemot-axonomy, but this may only be due to the fact that this is a very new research area. In addition, there is evidence (Rouault et al. 2001; Ugelvig et al. 2008; Cremer et al. 2008) that insects could have varying cuticular hydrocarbon profiles, depending on the geographical region they are found in. If this variation is significant in those insects frequently found on corpses, the hydrocarbon profile could reveal potential transport activities of the body if the profile of the hydrocarbons is different from those insects found in a certain region.

The second area in which cuticular hydrocarbons can be a valuable tool in aiding forensic entomology is in establishing the post-mortem interval (PMI). Two recent papers have been published whereby the authors state that a significant change of the hydrocarbon profile on the blowfly, Chrysomya rufifacies has been observed. This changed was correlated either with larval age or with weathering conditions created in the lab. In the first study the hydrocarbon composition of C. rufifacies larvae were correlated with age (Zhu et al. 2006). It was found that the ratio of the peak area of a linear alkane, nonacosane (n-C29) divided by the combined peak areas of another eight selected peaks increased with larval age. In the second study Zhu et al. (2007) used the puparial cases of C. megacephala and placed them in incubators to simulate weathering conditions. The relative abundance of several linear alkanes (especially the low molecular weight even numbered n-alkanes) increased over time, whereas almost all branched alkanes seem to decrease over time. In contrast the abundance of the high molecular weight hydrocarbons with chain length of more than 31 carbons all increased over time. Up till now this is the only paper dealing with the change of cuticular hydrocarbons on puparia over time, yet it showed that there is a real potential to use the puparia in the PMI estimation. However, there are also still some unexplained results e.g. why the abundance of 5MeC29 decreased whereas the abundance of 7MeC31 increased? Is this observation related to the fact that the abundance of most hydrocarbons >C31 increased as well? And is therefore chain length more important than branching? All this indicates that further investigation is necessary, but if these results are consisted, they could very well be used in estimating the PMI even though we do not quite fully understand the process behind these changes. In the most recent study, Roux et al. (2008) provided even further evidence for the importance of hydrocarbons as an alternative method for evaluating the postmortem interval through a complete onto-genetical study revealing the changes in hydrocarbon profiles in three calliphorid Diptera of forensic interest, Calliphora vomitoria, C. vicina and Protophormia terraenovae. Results enabled them to obtain good resolution of larvae within 1 day's precision. Some early results indicate that there is ontogenetic variation of the hydrocarbon profiles in Lucilia sericata as well (Drijfhout to be published). Although mosquitoes may not be directly of forensic importance, research showed that female Aedes aegypti of different ages also have different hydrocarbon profiles (Desena et al. 1999a). As with C. rufifacies it was the linear alkanes (pentacosane and nonacosane) that corresponded to different ages. After refining the method, it is now even possible to reveal information about the age of a female A. aegypti only using the cuticular hydrocarbons found on the leg of mosquitoes (Desena et al. 1999b). Results form all these studies showed that there is a big difference in the hydrocarbon profiles from eggs, to larvae to adults. Adults seem to have an increase in hydrocarbons with a higher molecular weight (Roux et al. 2008; Drijfhout to be published). Furthermore, the ratio of odd linear alkanes versus even linear alkanes also changed from eggs to larvae to pupae. The rational behind these changes are still not fully understood, although events such as moulting and pupation can in some cases be linked to a change in the hydrocarbon profile.

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