In forensic entomology, post-mortem interval (the time since the victim’s death) is typically estimated based on the types of insect present at the scene and, most importantly, their stage of development. It is probably of no surprise that flies (Order: Diptera) play a big part in insect colonisation of a cadaver, thus have been subject to a lot of research in forensic entomology. The life cycles of certain flies are relatively well known in terms of the different stages of development and when those stages are likely to be reached.
The lifecycle of a fly consists of a number of stages: egg, 1st instar, 2nd instar, 3rd instar, pre-pupa, pupa, and finally the adult fly (instar refers to stages of moulting as larvae). The time taken to reach each phase can vary between species. And there are of course factors which affect these development times, some that have been greatly studied, including environmental temperature, sun exposure, food availability, and even drugs taken by the deceased prior to death.
Now researchers are branching out into the study of other affecting factors, in this case typical household products. It isn’t uncommon for certain products to be spilled or in some other way present at the scene of a death (whether criminal or otherwise). Maybe a victim was smothered in acid in vain attempts to dispose of the body (good luck with that one) or perhaps the deceased happened to have slathered on some insect repellent immediately before his or her untimely death. Regardless, the ways in which chemicals appear at a death scene are plentiful, and they need to be taken into account if we’re relying on a somewhat environmentally-dependent factor to determine the post-mortem interval.
Enter researchers at the University of Lille Nord de France.
These guys and girls aimed to figure out how some common chemicals might affect the development of a particular species of common fly (in this case Lucilia sericata, the green bottle fly) by allowing the first instar larvae to feast on beef liver laced with different chemicals – in particular bleach, perfume, hydrochloric acid, caustic soda, insecticide, mosquito repellent, and gasoline. Specimens were subjected to either low concentrations of the chemicals (supposedly the equivalent to a realistic quantity being splashed on or otherwise applied to the body) or high concentrations. The development of the different groups of Lucilia sericata were then studied, allowing researchers to establish whether the chemicals present delayed, accelerated or had no effect on larval development, as well as possible effects on insect size, survival rate and sex ratio.
The results were interesting even if they were not wildly significant (ignoring the chemicals which just outright killed all subjects, not making them terribly useful post-mortem indicators). Low concentrations of mosquito repellent and caustic soda extended the development time of the larvae (361 hours and 352 hours respectively, in comparison to the control of 333 hours), as did high concentrations of perfume (342 hours). These figures I’ve listed are hours taken to reach adulthood, the mean values being used. At first glance these may not seem like such large differences, but what a difference a day makes when trying to pinpoint time since death (though the confidence interval typically used by entomologists is about 25 hours). There were also certain size variations noticed between adults fed on different sources, though this was not a particular focus of the study so no conclusions can really be made. The research also looked at survival rates and sex ratio, but I will skim over this (with the exception of pointing out that perfumed meat resulted in the survival of more females over males – we ladies do enjoy a good perfume!). Despite the limitations of the work and the relatively small differences caused, differences were observed, which suggests this path of research could be a fascinating and relevant one.
Realistically this was a pretty limited study, looking at a single species of laboratory-reared larvae and examining a small handful of household products, but the results are interesting nonetheless, indicating the impact of household chemicals on necrophagous fly development. We know conclusively that certain factors can have a renowned effect on the development of insects, thus affecting what we know about figuring out time since death. However taking into account what else might be on the victim’s body is something that may be overlooked, or at least not considered by the lay person perhaps.
So next time you’re slapping on some repellent to keep the mosquitos at bay, give a thought to the forensic entomologist whose day you might be making a little more tricky.
Source: Locard’s Lab