Estimating the Postmortem Interval: Principles

Every forensic entomologist's report should state clearly whether the estimate given is for the true PMI or for the minimum PMI, because the two can be very different numbers. The true PMI ends at the moment of death. The entomological clock does not start at death. It starts at the moment flies first laid eggs on the body, and anything that delayed that moment extends the gap between the two figures.

Consider a body stored in a domestic freezer for three days before being moved outdoors. Blow flies reach it within an hour of exposure. An entomologist examining the body two days after discovery would estimate a mPMI of roughly three days from the larval development. The true PMI is six days. The access interval, the three days of refrigeration, is invisible to the insects and to the entomologist unless an independent witness or physical evidence reveals it.

Blow flies of the family Calliphoridae are, in temperate and tropical regions alike, usually the first insects to arrive at a fresh body. A gravid female detects volatile compounds released from the body and deposits eggs or, in some species, first-instar larvae directly. Those offspring develop through three larval instars and a pupal stage at a rate that depends almost entirely on temperature. The larval age method exploits this predictability.

1. Identify the oldest stage present
   Collect larvae and puparia from multiple sites on the body. The oldest, most developed specimens set the minimum colonisation age. Collecting only the largest larvae from one site risks missing older material elsewhere.
2. Assign accumulated thermal units
   Each instar and the pupal stage has a known thermal requirement expressed as accumulated degree days (ADD) or accumulated degree hours (ADH) above a species-specific base temperature. Match the recovered stage to the relevant developmental dataset for the identified species.
3. Apply the local temperature record
   Retrieve temperature data from the nearest weather station and, where possible, from on-scene data loggers. Correct for any systematic difference between the station and the actual scene microhabitat (shade, sun, indoor vs outdoor, larval mass heating).
4. Back-calculate to oviposition
   Sum the daily or hourly thermal units backward from the recovery date until the total matches the ADD or ADH required for the stage found. The resulting date and time is the estimated earliest oviposition, which sets the mPMI floor.

The method's accuracy rests on three main assumptions. First, the oldest stage present really does come from the first oviposition event; if early egg masses were eaten by predators or washed away, later arrivals are now the oldest. Second, the developmental dataset used matches the species actually collected; misidentification of blow fly species is a known source of error, especially at the larval stage when morphological characters are fewest. Third, the temperature record accurately represents conditions at the scene; a body in direct sun on a concrete surface in summer may experience temperatures 10 to 15 degrees Celsius higher than the meteorological station reading.

Insect succession on a cadaver follows a broadly repeatable ecological sequence. Pioneer blow flies and flesh flies arrive within hours of death under permissive conditions. Specialist beetles of the families Dermestidae and Cleridae arrive during advanced decay. Hide beetles and mites dominate the dry stage. The succession method uses this sequence as a calendar: the species assemblage found at examination is matched to a known succession chart for that geographic region and season, and the decomposition stage implied by that assemblage sets the time window.

The succession method is coarser than the larval age method. It gives a window measured in days or weeks rather than hours. Its great advantage is that it works when larval data are unavailable: skeletonised remains, heavily mummified tissue, or material that has been treated with insecticides may all destroy larval evidence while leaving an identifiable suite of adult beetles and flies. The method is also less sensitive to temperature fluctuations, since species turnover is driven more by the chemical signature of each decomposition stage than by day-to-day temperature variance.

The key assumption is that a published succession dataset for the relevant region and season exists and is reliable. Succession sequences differ significantly between biogeographic regions, between urban and rural environments, and between seasons within the same location. Using a North American succession table for a case in sub-Saharan Africa, or a summer table for a body found in spring, will produce the wrong answer. Where no local data exist, the analyst must say so explicitly and widen the stated uncertainty interval accordingly.

In practice the two methods are not rivals. A good entomological report uses both when the evidence permits. The larval age method provides the precise floor: the oldest larvae say "colonisation happened at least this long ago." The succession method provides a broad contextual check: the species assemblage says "a body in this ecological state is typically at least X weeks old." When both agree, the estimate gains credibility. When they disagree, the analyst has an obligation to explain why, rather than simply picking the more convenient figure.

Several scene conditions systematically distort both methods and need to be documented and accounted for in the report.

• Wrapping or concealment: Plastic sheeting, clothing, or burial can delay colonisation by days or prevent it entirely. An unwrapped body in open woodland in summer may be colonised within minutes of death; the same body in a sealed container will not be colonised until it is exposed. The access interval here can dwarf the entomological estimate.
• Indoor versus outdoor: Buildings exclude some species and admit others through gaps and vents. Indoor succession assemblages differ from outdoor ones, and temperature regulation inside a building can slow or accelerate development compared to ambient conditions.
• Insecticide or drug exposure: Insecticides at the scene delay colonisation. Drugs in the body, particularly narcotics and cocaine metabolites, have been shown to alter larval development rates. If toxicological findings are available, developmental datasets corrected for drug effects should be used where they exist.
• Season and time of day: Blow flies do not oviposit at night or in cold temperatures. A death in winter at night will not be colonised until the following day when temperatures rise and daylight allows fly activity, creating a built-in access interval of hours regardless of any concealment.
• Submersion or saturation: Aquatic and semiaquatic entomological succession sequences differ substantially from terrestrial ones. A body that was submerged and then recovered carries a fundamentally different insect record.