Insects and dead bodies: what is forensic entomology and can you work out a time of death?

Emma Brisdion

Once a body has been found, three of the first questions that are asked are: ‘who is this person?’, ‘how did they die?’ and ‘when did they die?’. Forensic pathology helps to answer these questions by examining the body, and in our Silent Witness episode, we ask Forensic Pathologist Dr Stuart Hamilton about the last of the three: determining the time of death.

Decomposition

The body goes through several predictable stages of decomposition in the hours after death; livor mortis, algor mortis and rigor mortis. By examining the stage that the body is currently experiencing when it is found, you can work backwards and work out an estimated time of death. This can help investigators piece together a timeline of events and ultimately help them work out what could have happened, and who the suspects are.

Livor mortis occurs first. This is the pooling of the blood on the bottom of the body as it is no longer being pumped by the heart and gravity takes hold. This causes dark bruising and discolouration on the underside of the body and a pale, drained colouration on the upper side. By examining this, you can tell how a body was left in the immediate hours after death, and whether the body had been moved and the position of it had been changed after this time.

Algor mortis. This is the cooling down of the body from normal body temperature, and it usually doesn’t start to occur until 30 minutes after death and can take several hours. Internal body temperature can be tricky to rely on, as it is dependent also on the room or environment in which the body is left; a warm room with the heating left on will slow this process down, while the internal body temperature will drop faster if a corpse is left outside on a cold night.

Rigor mortis. This is the stiffening of the body as the muscles contract. Normal chemical reactions within muscles still occur for a while after death, which causes the muscles to continue to contract. ATP, produced by respiration, is needed for the muscles to relax again. But, as the muscles are no longer receiving oxygen and the body is not aspiring aerobically to produce ATP, the muscles don’t relax. Cue: rigor mortis. It starts to occur a few hours after death, there are six different stages, and the whole process can last for up to 24 hours.

What about insects?

So where do forensic entomologists come in? Entomology is the study of insects, and forensic entomology is the study of insects as part of a criminal investigation.

Paying close attention to the insects on or in a body can be useful in helping to detect poisons or the location of an incident, but they’re also particularly useful when it comes to estimating the time of death. Much like the three post-mortem mortis’, the different stages in the lifecycle of many insects happen fairly quickly and very predictably. In essence, this means that by taking a sample of any insects that have colonised the body, and by working out what life stage they’re at, again you can get a bit of an idea about how long that body has been dead.

Many different insect species can be used during the investigation of a suspicious death, as they each tend to inhabit the body at different times during decomposition. One such species, is the common blowfly.

Blowflies (Calliphoridae) have a relatively predictable lifecycle and are one of the most studied flies involved in forensic entomology. They are often one of the first to arrive at the scene, laying their eggs in the natural orifices of the body which then normally hatch into maggots within 24 hours. The age of the eggs, larvae or larval remains present on the body can help inform investigators how long a body may have been decomposing and when death is likely to have occurred.

The environment has an impact on this too, though; a colder environment around the corpse may slow down both the body’s rate of decomposition and also the insect development for example. But of course, forensic entomologists and forensic pathologists account for temperature during their calculations.

As Stuart says in the episode, when presented with a body which is expected to have died under suspicious circumstances, you can never confidently give an exact time of death (unless it’s caught on CCTV, for example).