Coupled GCElectrophysiological Recording

ANTENNAL DETECTOR GC DETECTOR

Fig. 11.4 System designed used for coupled gas chromatography and electroantennogram experiments
Fig. 11.5 Coupled GC-EAG trace. The upper trace shows the chromatogram of volatiles from the sample extract. The lower trace shows the corresponding EAG response of the insect preparation

possess the capabilities of also detecting responses of single olfactory cells (GC-SCR) (Wadhams et al. 1982). Although behavioural studies, have previously shown that C. vomitoria respond odours from bodies, to liver (Woolbridge et al. 2007) and to a compound, dimethyl trisulfide (Nilssen et al. 1996), no study had identified specific compounds associated with dead bodies which elicit a response from blowflies. However, recently the above chemical ecology methods have been used, for the first time in forensic entomology research, to identify semiochemicals from decomposing bodies that could be involved in the attraction of carrion insects (LeBlanc 2008). The identifications of chemicals from this study will be subsequently published.

While there are five recognisable stages in the physical decomposition of the carcass (Anderson and VanLaerhoven 1996), there is often no clearly defined beginning or end to a decomposition stage, especially during the later stages, making the decision of stage change somewhat subjective (Bornemissza 1957; Early and Goff 1986; Shoenly and Reed 1987; Archer 2004; Grassberger and Frank 2004). However, if the chemicals that are characteristic of specific stages of decomposition (i.e. "chemical fingerprinting") can be identified, this could aid confirmation of decomposition stage.

LeBlanc (2008) describes the daily collection of volatiles from decomposing pigs using an air entrainment method and the analysis of these compounds using GC-EAG with blowfly C. vomitoria. In these studies it was found that specific compounds, which included mainly sulphur compounds, within the volatile collections triggered an electrophysiological reaction from the blowfly. Most importantly differences were found between the different stages of decomposition. While the five stages of decomposition were determined through physical characteristics, the chemical composition and concentration of the volatiles sampled changed in a manner that closely followed these five stages. The Fresh and Dry stages exhibited the lowest concentrations of volatiles, however, the composition of volatile were different between these two stages. The EAG-active compounds, or semiochemicals, were at their highest concentration during the active decays stage. Visual observations supported these findings as the greatest number of Calliphoridae, adults and larvae, were recorded during this stage of decomposition.

Semiochemicals play a considerable role in mediating insect behaviour (Birkett et al. 2004; Pickett et al. 1998). Identifying these specific compounds and investigating the responses they elicit could provide a better understanding of the insects and, in some cases, allow manipulation of their behaviour. The period between death and the arrival of the first ovipositing blowflies is of great interest to the forensic entomologist, yet not fully understood.

Although some semiochemicals are likely to be attractants, others could have a negative effect on the insects behaviour. For example, Birkett et al. (2004) and Logan et al. (2008, 2009) found that at times when vertebrate hosts are not attractive to pests, there are elevated levels of certain semiochemicals which may act as active repellents or more passively by 'masking' attractants, thereby reducing the sensitivity of flies to these attractants. This could be relevant to decomposing bodies. Semiochemicals could be present at certain stages and may explain why certain carrion insects, such as Calliphora spp, are prevalent during the early stages of decomposition while others only appear later during decomposition. Competition and availability of food play an important role in insect succession on a corpse and therefore, repellents or 'masking' compounds could be used to prevent attraction of certain insects at particular times. For example, late colonizers could be repelled by early decomposition volatiles or by those produced by the immature stages (larvae) of the early colonizers to help avoid competition. However, the alternative is that the flies are simply not attracted to the odours present on a body at a particular time.

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