Estimating the Post-Mortem Interval (PMI)

Estimation of time since death is an integral part of medico-legal investigations. Post-mortem Interval is defined as ‘amount of time that has elapsed since the death of the decedent’. The key goal of estimating time since death at the scene of crime is to have a preliminary idea of the time of assault and for narrowing the field of suspects. A precise estimation of PMI is important for criminal law as it validates the witness’s statement, limits the number of suspects and assesses their alibis (Mathur & Agrawal, 2011).

The estimation time for PMI was based on entomological evidence and in agreement with the PMI obtained by standard means, provided that all evidence from the death scene is taken into consideration, such as the delayed arrival of flies to a corpse when in enclosed environments. The estimate of PMI can be different from the

Eggs Pupae Larvae Adult fly

50 real interval due to physical circumstances in the surroundings of the remains (Oliveira- Costa & Mello-Patiu, 2004). Entomologist then determined the age of the specimens to provide evidence as when the female’s flies first found the dead body and laid their eggs, the minimum estimate of the PMI. This can be taken as the latest time by which death must have occurred. The estimation of maggot age relies on detailed knowledge of the fly life cycle and the factors that influence it (Greenberg, 1990; Singh & Bharti, 2001; Wooldridge et al., 2007; Amendt et al., 2008).

Within a few hours of death, corpses start to have strong odour. Large numbers of adult blow flies, with their finely tuned olfactory senses, are attracted by the odours of decomposition. These odours are mainly created by the action of bacteria on dead tissues. Blow flies often swarm to wounds, open sores and ulcers on living vertebrates as well as dead ones and their targets can include humans. On arriving at a corpse, some flies will feed, others will mate and some females will deposit their heavy loads of eggs. A maggot will develop inside each egg and break through the protective shell, the chorion, and a few days later, to start feeding on the decomposition body (Browne et al., 1969; Erzinclioglu, 1983; Smith, 1986; Wall & Warnes, 1994; Anderson, 2001).

On an uninjured body, blow fly eggs are usually laid at the openings of body orifices and it is in those areas that the emerging maggots start to feed. Blow flies are also attracted to injuries, so if maggots are found at sites other than the body orifices, they might indicate that some traumatic wounding took place before death (Lee, 1989; Hall, 2000). There are two insect-based methods for making PMI estimation. As the body passes through stages of decomposition, the biological and chemical properties of the body change, making it attractive to different types of insects at different times throughout the process. A simple observation on the types of species present on a corpse

51 (immature and adult) can be used to estimate how long the body has been deceased (Marchenko, 2001; Carvalho et al., 2004).

The second method involves estimating the age of immature specimens located on the corpse. Their age, sometimes coupled with an estimation of the time that elapsed between deaths and when the eggs were deposited on the corpse, also serves as a basis for PMI estimation (Catts & Goff, 1992; Carvalho et al., 2004). Regardless of the method used, the process of PMI estimations begins at the crime scene. Often, it is not possible for an entomologist to be called to the scene. Therefore, the crime scene investigator is frequently responsible for the proper collection and preservation of entomological evidence. Insect samples are collected from the corpse and surrounding area and should represent the different species present, as well as the different sizes and stages for each species. The samples are labeled and stored in preservation fluids, such as ethanol to preserve the external features that are required for their identification (Amendt et al., 2007).

Accurate PMI estimations not only rely on proper evidence collection, but also require a precise recording of the crime scene details. Because temperature is a key factor affecting the rate of insect development, specific climatological data must be recorded along with relevant body or maggot mass temperatures. The temperature of the crime scene for the days or weeks before the discovery of the body can be estimated using data collected from nearby weather stations. Any additional details that could affect the decomposition of the corpse and concurrent insect colonization, such as partial burial or wrapping of the body, are also recorded (Amendt et al., 2007).

Once the entomologist receives the insect samples and crime scene details, each specimen is identified to the level of species. Proper identification is important since two species that are similar in appearance may have markedly different growth rates.

52 Improper identification would make the specimen’s age estimation and subsequent PMI estimation invalid. An entomologist’s personal experience and the use of published keys aid in the identification process (Greenberg & Kunich, 2002).

However, immature larvae and pupae of some species are so similar in appearance that they cannot be differentiated by microscopic examination. One method for preventing a misidentification is to keep a portion of the immature insects collected at the crime scene alive and return them to the lab where they can be reared to adults, at which point they can easily be identified (Haskell, 1990). If rearing fails or is not attempted, it may be possible to identify immature specimens using scanning electron microscopy (Liu & Greenberg, 1989) or the insect’s own DNA (Sperling et al., 1994; Wells & Sperling, 2001; Wells et al., 2001, Tan et al., 2009, 2010).

The insects associated with a corpse greatly depend on the biogeoclimatic zone. Ideally, a comparison should be made to succession data collected in the general geographic area of the crime (Nuorteva, 1977; Erzinclioglu, 1983; Marchenko, 2001). In addition to geography, other factor such as burial (Rodriguez & Bass, 1985) or wrapping of the corpse (Goff, 1992) must be taken into account. Although the crime details may not exactly match the parameters of the published data, the data provides an indication of how these factors may affect the decomposition process.

Although most corpses immediately attract flies following death, there are various factors that could delay this process. A corpse located indoors is sheltered from fly activity and could extend the time it takes for a carrion fly to discover the corpse (Goff, 1991). Although it was commonly believed that carrion flies do not lay eggs at night (Nuorteva, 1977), recent publications have shown that it is possible, although insect activity is drastically reduced from that in the daytime (Greenberg, 1990; Singh & Bharti, 2001).

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