Conclusion and Final Remarks

In conclusion, there is ample evidence that hydrocarbons in general, but also cuticular hydrocarbons, can assist many different research areas. Many hydrocarbons have been identified as compounds to be essential in insect development and that these compounds are under strict genetic control. The main problem right now is the lack of basic research in how these compounds are regulated and how they will change over time. If these processes are known and the relation between the cuticular hydrocarbon compositions and e.g. larval age is known, this would greatly enhance the precision in determining the larval age of larvae found on corpses. Extracting the hydrocarbon content on eggs, larvae and or pupae is fairly simple and has been carried in many other disciplines. The consequent analysis of these extracts with either GC-FID or GC-MS is a quick and sensitive method of obtaining information on the composition of the hydrocarbons present. This method is quick and requires minimal work, certainly if this is compared to rearing eggs or larvae to adulthood. Added to this is that morphology is sometimes not clear enough to distinguish either the species or age of larvae.

Although the process of weathering of hydrocarbons on the cuticle of insects is less well understood, it has certainly the potential to become a powerful tool in establishing the time of death, if the changes that will occur under certain conditions are known. This research area is fairly new and much more is needed to see whether the change in hydrocarbon composition due to e.g. larval development and weathering is independent from each other or not. Can we treat each of these processes independently, or do we need to develop a more complex model in which all of these factors are taken into account? All studies on the change of hydrocarbons in necrophagous flies and on their empty puparia (Zhu et al. 2006, 2007; Roux et al. 2008) were carried out under strict climate conditions. However a change in temperature or humidity in nature will affect the weathering process of hydrocarbons on empty puparia as well as the development of larvae. A change in temperature could e.g. result in the insect entering the diapause stage. Will this have any effect on the hydrocarbon profile? To my knowledge no research has been done on what effect the diapause stage has on the production and maintenance of hydrocarbons. Nelson and Lee (2004) reported that overwintering larvae of the goldenrod gall fly, Eurosta solidaginis, increase their total hydrocarbons from 122 ng/larva collected in autumn (September) to 4,900 ng/larva collected in winter (January). Although there was no indication that certain individual hydrocarbons increased more than others, the percent composition of the hydrocarbons changed from larvae collected in autumn compare to those collected in winter. It is however important to note that larvae collected in autumn were young larvae and these collected in winter were old larvae, hence this change could be due to larval development (and hence useful in age determination of larvae) rather than due to a decrease in temperature. From this it is clear that there are at least two classes of factors that may influence the hydrocarbon patterns of larvae or adult flies (Zhu et al. 2007). The first class is related to the larvae/pupae/puparia in unweathered conditions; factors include e.g. age, sex, and geographic population. The second class of factors are those related to the weathering of hydrocarbons; factors include sunlight wind or rainfall. Therefore further research is needed to determine if the results obtained so far in the lab will be similar to those obtained in the field. The ultimate experiment will be to have eggs hatch in the field and compare their change in hydrocarbon patterns with those that have hatched and grown in the lab.

These early studies clearly indicate the forensic importance of these cuticular hydrocarbons. It is clear that cuticular hydrocarbons can be used to identify insect species found on bodies if the cuticular hydrocarbons composition are known for the more well known species found on corpses. In addition these compounds have a great potential in establishing the time of death, as their cuticular hydrocarbons change with age. However, factors other than age may influence the hydrocarbon profiles and therefore much more research is needed to exploit the full potential of hydrocarbons in establishing PMI.

Acknowledgements The author thanks Dr S. Martin and Prof. E.D. Morgan for their help during the preparation of the manuscript.

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