Relevant Approaches in estimating PMI:Thermal summation and Degree of Development

  Estimating PMI: historical perception

The first case which utilized entomological evidence in estimating PMI was reported by Bergeret in 1855, who analyzed the insect assemblage in an infant corpse and concluded, perhaps incorrectly, that the infant died two years ago leading to the prosecution of the murderer (Smith, 1986; Catts & Goff, 1992; Hall, 2001; Greenberg & Kunich, 2002; Amendt et al., 2004; Gennard, 2007).  Later, Megnin (1894) characterized eight stages of human decomposition and the insect taxa that were associated with them and this knowledge about insect succession became the basis for estimating PMI (Lee, 1989; Hall, 2001; Gennard, 2007).  During the first 72 hours after death, estimation of PMI based on pathological changes is reasonably accurate (Gennard, 2007).  However, as the time since death increases, the pathological changes become less useful for estimating PMI, while necrophagous insects recovered from the corpse may provide more accurate PMI estimate (Wells & LaMotte, 2001; Amendt et al., 2004; Gennard, 2007).  Estimation of PMI depends on the accuracy in estimating the age of the larvae and for that purpose, various approaches have been suggested (Wells & LaMotte, 2001; Amendt et al., 2004).  Pertinently, all these attempts are based on the fact that insects are poikilothermics and therefore, their developments depend on the prevailing ambient temperature (Wells & LaMotte, 2001; Amendt et al., 2004).  The relevant approaches for estimating PMI are discussed below.

(a) Thermal summation approach 
Thermal summation approach for estimating PMI, introduced by Wigglesworth (1972), is based on the assumption that the relationship between the developmental rate and the ambient temperature is linear within the mid-range of the sigmoidal developmental curve (Greenberg & Kunich, 2002).  It relates to the fact that insects are poikilothermics, using environment as the source for heat for their growths and the thermal units are known as the accumulated degree days (ADD) or the accumulated degree hours (ADH) (Gennard, 2007).  In this aspect, the ambient temperature data covering the period when the deceased was last seen alive until the time the corpse was recovered must be obtained from the nearest meteorological station (Amendt et al., 2004; Gennard, 2007).  The obtained ambient temperature data must be corrected using a ‘correction factor’ obtained from the regression equation between the ambient temperature data from the meteorological station versus the half-hourly ambient temperature data recorded at the crime scene for three to five consecutive days after the corpse was discovered (Amendt et al., 2004; Gennard, 2007).  Upon obtaining all the relevant climatological data and the base temperature for the observed insect species, the ADH or ADD can be calculated and ultimately, PMI can be estimated.  Base temperature is the temperature threshold that is species specific below which growth and development will not take place.  Ultimately, PMI can be estimated by back-calculating the ADH or ADD from the time the body was discovered.  It is worth mentioning here that although ADH can be converted easily into ADD by dividing ADH with 24, the same cannot be accurately applied for converting ADD to ADH. 
ADH=Time (in hours) X (temperature – base temperature) 
ADD=Time (in days) X (temperature – base temperature)

(b) Degree of development approach

This approach for estimating PMI is based on the assumption that larvae of the same age hatch and molt in a relative synchrony (Davies & Ratcliffe, 1994; Wells & Kurahashi, 1994; Wells & LaMotte, 1995).  Hence, the age of a larva can be estimated from its size and dry weight using the appropriate growth curve (Wells & LaMotte, 2001) and this approach may be useful in countries where the observed fluctuation in ambient temperature is less.  Since most of the development data were obtained under constant temperatures, while temperature at crime scene is fluctuating, the time period under consideration must be divided into shorter intervals (e.g. 12 hours) (Williams, 1984) and the closest developmental model to the mean temperature recorded during each period can be applied for estimating PMI (Wells & LaMotte, 2001).  Interestingly, Schoenly (1992) prescribed estimation of PMI using the computer model that they developed.  In this aspect, it is pertinent to indicate that the data obtained in laboratory need to be compared with the data gathered under field conditions for utilization in human death investigation (Byrd & Allen, 2001).