Decomposition - Forensic Evidence
The presence of animals on a corpse can provide information for investigators on some of the circumstances surrounding death. They can help pinpoint the time of death, the location of wounds, whether a body has been moved, and whether drugs were present in the body at the time of death. The material in this section focuses on determining the time of death.
Determining time of death
The time of death is a critical piece of information for investigators attempting to understand the cause of suspicious deaths.
The temperature of a body can be used to estimate time of death during the first 24 hours. Core temperature falls gradually with time since death, and depends on body mass, fat distribution and ambient temperature. If the body is discovered before the body temperature has come into equilibrium with the ambient temperature, forensic scientists can estimate the time of death by measuring core temperature of the body.
The presence of rigor mortis also assists forensic scientists in determining the time of death. The body muscles will normally be in a relaxed state for the first three hours after death, stiffening between 3 hours and 36 hours, and then becoming relaxed again. However, there is considerable uncertainty in estimates derived from rigor mortis, because the time of onset is highly dependent on the amount of work the muscles had done immediately before death.
The presence of insects in a corpse is a critical clue towards estimating the time of death for bodies dead for longer periods of time. Because flies rapidly discover a body and their development times are predictable under particular environmental conditions, the time of death can be calculated by counting back the days from the state of development of insects living on the corpse.
For example, if a body was found in an air-conditioned building (21°C) with second instar larvae (stage of development between moulting) of Lucilia sericata feeding on the corpse, a forensic entomologist could calculate that those larvae had moulted from their first instar sometime in the last 12 hours. Because the eggs take 18 hours to hatch and the first instar takes 20 hours to develop, the most recent time the eggs could have been laid was 38 hours earlier, if the larvae had just moulted. If they were old larvae, about to moult into their third instar, the most recent time of death would be 50 hours prior to discovery of the body.
Usually, time determinations would not be so easy because weather conditions are more variable, and identification of most maggots to species level is difficult. Forensic scientists usually undertake more detailed entomological work to determine time of death.
Detailed entomological calculations
Step 1. Determine temperature history at crime scene
- Extract weather bureau records of maximum and minimum daily temperatures at the weather station nearest to crime scene, over the general period the body has been exposed.
- Set up weather station at crime scene (after body has been found) and compare temperature changes with those at the nearest weather station. Calibrate the weather bureau data for the period preceding discovery of the body, accounting for differences between crime scene and weather station.
- Calculate the average temperature that the body has been exposed to.
Step 2. Rear maggots to adulthood to identify species
- Collect a range of maggots, (particularly those that might be the oldest) from the body and rear them (on ox-liver) at constant temperature.
- Record time taken until larvae pupate.
- Keep pupae until adults emerge.
- Identify fly species from adult characteristics. (For some species, identification from larval features may be possible or they can be identified from DNA samples, if a DNA library is available.)
Step 3. Estimate time of egg laying
- Using knowledge of development rate of the particular species at rearing temperature, count back to estimate age of maggots when body found.
- Using knowledge of development rate* of the particular species at the average crime scene temperature, count back to determine date of egg laying.
- This is the latest time at which the body died. (It may have died earlier if there was a delay between death and egg laying. This depends on weather conditions and accessibility of the body to insects).
*If the development rate of the particular species at a range of temperatures is unknown (which is the case for most Australian flies), the emergent flies can be used as breeding stock. Eggs can be reared at temperature conditions corresponding to those estimated for the crime scene, to determine development rate.
Step 4. What other insect evidence is available?
- Do steps 1-3 for all the different insect types found at the crime scene to improve accuracy of determination.
Criminal case history
In rural New South Wales, in the early 1980s, a man and a woman were shot dead in their home. The time of death was first arrived at by determining, with the help of the local telephone exchange, the time of the most recent phone call made from the house (a Saturday night). The fact that the victims had last been alive on the Saturday was corroborated by a witness who claimed to have seen the woman and her two children that morning.
A suspect was interviewed about the double murder, but he had a strong alibi for the Saturday night. At this point the police turned to maggot evidence that had been collected from the dead bodies.
The forensic entomologist estimated the minimum possible age of the oldest maggots among those presented to him to be four days. This placed the time of death at least one day earlier than that arrived at using the information from the telephone exchange and the witness. As a result, both lots of evidence were checked and found to be in error.
The suspect lacked a valid alibi for Friday night, the revised time of death. Confronted with this and other evidence against him, he confessed and was subsequently found guilty and sentenced to life imprisonment.
A curious aspect to this case was that the body of the woman, which was found in bed, was much more decomposed and contained much better-developed maggots than the corpse of the man, which was found on the kitchen floor. Initially this perplexed the investigators, until they realised that the woman had gone to bed with the electric blanket on!