Estimating Postmortem Intervals: Forensic Entomology
Forensic entomology is the study of insects in relation to the law.
Under favourable conditions, some species of insects are attracted to decomposing carrion almost immediately following death (Amendt et al., 2004). Due to their predictable colonization patterns, insects recovered from a corpse can provide insight into the time of death, also known as the postmortem interval (PMI). In a forensic context, the most useful insects are those which use the corpse as a food source during larval development (Rivers & Dahlem, 2014). Amongst the most forensically important insects are members of the Order Diptera, or true flies, as well as beetles (Coleoptera) (Amendt et al., 2004). Commonly, blow flies are the first colonizers as they are drawn to remains by odours produced during early stages of decomposition. Once they have detected the body they oviposit on the corpse’s ammonia-rich compounds (Amendt et al., 2004). At optimum temperatures their development can be understood as a linear function of ambient temperatures, which can be used to estimate PMI.
In order to estimate the PMI, forensic entomologists must have access to four main critical pieces of information. This includes the oldest insects present at the crime scene, the temperature of the scene, the species of the insect(s) present, and lastly, peer reviewed data on how long it would take that given species to reach the stage they were found in in accordance with the temperature(s) at the crime scene (Anderson, 2000). The oldest stage of insect present along with the predictable colonization and development rates allow forensic entomologists to estimate the time of death. The insects rate of development depends on ambient temperatures which is why temperature data is needed. The linear relationship between temperature and development exists because the metabolic rate increases as temperature increases resulting in faster development within optimum ranges (Anderson, 2000). In order to account for the various temperatures the insects may have been exposed to over a given period of time the data produced at a specific temperature is converted to thermal units. These thermal units can be in either accumulated degree days (ADD) or accumulated degree hours (ADH) (Amendt et al., 2006).
Aside from fluctuating ambient temperatures, many other factors can affect insect development that also need to be taken into consideration. One of these is the naturally occurring formation of aggregations or maggot masses during larval development. Maggot masses can generate a considerable amount of heat due to concentrated larval metabolisms (Anderson, 2015; Rivers & Dahlem, 2014). Many studies have shown that this increased heat allows for rapid growth under certain conditions (Heaton et al., 2014).
Morgue storage is another factor that can impact developmental rates. Although insects are generally collected at the scene, it is not uncommon for insects to remain on the corpse until the autopsy (Huntington et al., 2007). Forensic entomologists need to know how long the body was stored in the morgue cooler and the temperature of the cooler in order to estimate PMI. Some studies have indicated as much as a 24-hour delay in adult emergence after being stored in coolers (Johl & Anderson, 1996).
Many other factors can affect developmental rates including environment, crime scene location, genetic differences, moisture content in food sources, overall diet, as well as the presence of drugs in the tissue being consumed (Bauer et al., 2019; Harnden & Tomberlin 2016; Tarone & Foran, 2011). All of these aforementioned factors can complicate an entomologist’s ability to estimate time of death which is a vital consideration for homicide investigations as well as other criminal and civil cases. An accurate PMI estimate is needed to avoid wrongful convictions in many cases which is why research on these factors as well as the reliability of forensic entomology as a whole is so important.
Amendt, J., Campobasso, C., Gaudry, E., Reiter, C., LeBlanc, H, & Hall, M. (2006). Best
practice in forensic entomology – standards and guidelines. International Journal
of Legal Medicine, 121(2), 90-104. https://doi.org/10.1007/s00414-006-0086-x
Amendt, J., Krettek, R., & Zehner, R. (2004). Forensic Entomology. Springer-Verlag,
Anderson, G. (2000). Minimum and Maximum Development Rates of Some Forensically
Important Calliphoridae (Diptera). Journal of Forensic Sciences.
Anderson, G. (2015). Human Decomposition and Forensics. In Carrion Ecology,
Evolution, and Their Applications.
Bauer, A., Bauer, A., & Tomberlin, J. (2019). Impact of diet moisture on the development
of forensically important blow fly Cochliomyia macellaria (Fabricius) (Diptera:
Calliphoridae). Forensic Science International.
Harnden, L.M., & Tomberlin, J.K. (2016). Effects of temperature and diet on Black
Soldier Fly, hermetia illucens (L.) (Diptera: Stratiomyidae), Development.
Forensic Science International. http://dx.doi.org/10.1016/j.forsciint.2016.05.007
Heaton, V., Moffatt, C., & Simmons, T. (2014). Quantifying the Temperature of Maggot
Mass and its Relationship to Decomposition. Journal of Forensic Sciences. 59
Huntington, T.., Higley, L., & Baxendale, F. (2007). Maggot Development During Morgue
Storage and Its Effect on Estimating the Post-Mortem Interval. Journal of
Forensic Sciences. 52, 453-8. https://doi.org10.1111/j.1556-4029.2007.00385
Johl, H.K. & Anderson, G.S. (1996). Effects of refrigeration on development of the blow
fly, Calliphora vicina (Diptera: Calliphoridae) and their relationship to time of
death. J Entomology Society of British Columbia.
Rivers, D.B., & Dahlem, G.A. (2014). The Science of Forensic Entomology. Wiley
Blackwell, West Sussex, UK.
Tarone, A., & Foran, D. (2011). Gene Expression During Blow Fly Development:
Improving the Precision of Age Estimates in Forensic Entomology. Journal of
Forensic Science. doi: 10.1111/j.1556-4029.2010.01632.x