Forensic entomology is the use of insects as evidence in legal cases. There are numerous applications for such evidence, for example, as cruelty indicators in animal welfare cases, and perhaps most notably as a method of establishing post-mortem interval for decomposing bodies, the latter of which will be the focus of this article.
So why are insects a useful source of forensic evidence?
The diversity and abundance of insects, their occupation of microhabitats, and their ability to find host material rapidly, makes the group an ideal source of contextual information when gathering evidence from a crime scene. Most frequently, coleoptera (beetles) and diptera (flies) are involved. Existing knowledge of the ecology of a species identified allows inferences to be made about the scene. For example, key questions might include:
- How far can the species fly/walk?
- In what habitat would it usually be found?
- What is the typical development time of this species?
- How do they detect a host?
- What are the typical life stages of the species/seasonality requirements?
The answers to these questions indicate details of post-mortem interval, corpse location and movement, trauma – ante/post mortem wounds, and crime scene characteristics. If, for example, an insect which prefers habitat A has been sampled from a body found in habitat B, it could be suggested that the body has been moved from its original location.
Gathering insect evidence
The most important data to collect are; the physical stage of decomposition (if a corpse is involved), sites of arthropod invasion, specimens from within, around and below the corpse – including flying, stationary or insects in the leaf litter), climactic and weather conditions (e.g. slope, altitude, vegetation cover).
The accuracy of the findings then depends upon the handling of specimens post collection. It is therefore preferable for specimens to be identified at the scene where possible, and those taken to the lab to be transported in sterile conditions. A number of specimens found are reared from immature stages to adults in order to confirm their species, and others are preserved at various life stages. The data collected can then be analysed based on the diversity and proportions of life stages observed.
Sources of uncertainty
Despite refined techniques, it is important to allow room for case by case variation when using insects as evidence. Individual species characteristics, weather and environmental factors are not always predictable or easily measured. For example, the time between death and invasion by insects may vary by context, or the ambient temperature of a location may not correlate with the temperature of a larval mass – which can heat up. For these reasons, estimates of development time will not always be exact.
Forensic entomology is a broad and fast developing field. Other applications/legal contexts not covered here include environmental health (e.g. store product legislation), using bees and wasps to detect explosives or drugs, and sampling insect blood meals to detect human DNA, drugs and toxins (e.g. sampling mosquitos at the scene of a crime).
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