Forensic identification techniques
Forensic entomologist, Dr James Wallman, with a batch of maggots.
(Image by Silvia Liber, 2010) |
In order to develop an accurate post-mortem interval (PMI), the forensic entomologists need to know two things: how long after the death the insects arrived, and how long the insects have been there for. By the addition of these two lengths of time, an accurate PMI can be calculated. The first question is not hard to answer; the entomologist simply has to look at the ecology of the insects involved to determine at what stage in the decomposition of a body do they prefer to lay their eggs. Some species, like the blow flies, arrive almost instantly, whereas some, like the house fly, arrive 4-5 months after decomposition begins (Gennard, 2007). The second question, however, is much more difficult to answer, mostly because it is difficult to identify the species of an immature insect, especially an egg. Forensic entomologists however have a number of techniques to accurately determine the species, and thus age, of eggs and maggots.
Firstly, the entomologists can use a scanning electron microscope to differentiate between species based on morphological characteristics of eggs or maggots (Gennard, 2007). For example, thick posterior spiracular hairs found on a maggot would identify it as a first-instar larvae of an Oriental latrine fly (Chrysomya megacephala), a species found mainly in the Australasian and Oceanic regions (Sukontason et al, 2002).Before an egg or maggot is observed under the microscope, they are usually fixed, stained with a dye such as potassium permangenate and dried (Sukontason et al, 2002). However, the larvae of some species are so morphologically indistinguishable that identification is difficult, even for experts (Budowle et al, 2003). In these cases, a molecular approach is needed. Scientists can use mitochondrian DNA from maggots to identify the sample down to a species level (Wallman & Donnellan, 2001). Mitochondria are sub-cellular organelles that contain a genome separate from the genome of the nucleus (Budowle et al, 2003). In most cases, the locus of importance is the region including genes COI and COII (cytochrome oxidase subunits one and two) (Wallman & Donnellan, 2001). The process involves extracting the DNA, amplification of the COI or COII gene by the PCR (polymerase chain reaction) technique, sequencing of nucleotides then comparing these sequenced with a database of known species sequences(Wallman & Donnellan, 2001). Once the species has been identified, the age of the specimen can be calculated, and using knowledge of the decomposition stage that is most preferable to that particular species, the PMI of the corpse can thus be determined. |