Flies can provide important clues in determining the post-mortem interval – that is, the time that has elapsed since a person or animal has died. Forensic entomologists, scientists who study insects in a legal context, use the presence, absence, and development stage of various fly species on or around a corpse to help estimate when death occurred. This information can be critical in criminal investigations.
How flies are involved in decomposition
When an animal or human dies, chemicals produced during decomposition begin to attract blow flies. Blow flies are typically the first insects to arrive at remains, sometimes within minutes of death. The female flies are attracted to lay their eggs on or near the body. These eggs hatch into larvae (maggots) which begin to feed on the corpse. As the maggots grow, they go through developmental stages known as instars. By examining the age and stage of the maggots present, forensic entomologists can determine a minimum post-mortem interval.
Other flies and insects are also involved in decomposition. After blow flies arrive, flesh flies, house flies, and bottle flies may also lay eggs on remains. Beetles and mites feed on the corpse in later stages of decay. The presence and stage of these insects along with the blow fly larvae provide more clues about the post-mortem interval.
Factors that affect insect development
To accurately calculate a time of death from insects, forensic entomologists must consider several factors that can impact insect development and presence:
Temperature
Temperature has a major effect on the rate of insect growth. Warmer temperatures speed up the development, while colder temperatures slow it down. Entomologists use accumulated degree hours or days to model insect development – each insect species has an ideal temperature range and expected development time under those conditions. By accounting for the temperatures the insects experienced, a more precise post-mortem interval can be determined.
Trauma
Any trauma to the body that exposes fluids or flesh can alter insect colonization and development. Gunshot or stab wounds provide entry points for flies, while burned remains attract a different set of insects. The forensic entomologist has to factor any trauma into their analysis.
Burial, wrapping, or submersion
If remains were buried, wrapped, or submerged in water at any point, insect activity may have been delayed or halted until the body was exposed again. This can complicate determining when colonization first began.
Indoor vs outdoor location
Accessibility to the body can affect insect activity. A body located outdoors is more accessible to flies than one inside a home or other built structure. Indoor remains may have fewer insects present or different species than expected on an outdoor body.
Toxicology
Any toxins or drugs present in the body can alter the insects’ development in some cases by speeding it up or slowing it down. The entomologist has to take into account any toxins found during analysis.
Season and geographic location
The local insects present depend on geographic location and season. Different fly species have different temperature requirements and seasonal population cycles. Determining the post-mortem interval relies on knowledge of the normal blow flies and insects for that time of year in that specific area.
Collecting insect evidence from a corpse
To accurately use insects to determine time of death, the entomologist has to properly collect and preserve insect evidence from the scene. Some of the steps include:
– Collect samples of all life stages of insects present – eggs, larvae, pupae, and adults should be gathered.
– Note what insects are present on different areas of the body.
– Collect control samples away from the body to compare insect fauna.
– Preserve the samples – usually in jars of alcohol.
– Note weather conditions, temperature, location of body, and any other relevant factors.
– Transport the collected samples to the lab for analysis.
Estimating the post-mortem interval
Once in the lab, the forensic entomologist uses all the collected insect evidence and factors such as temperature data to estimate a post-mortem interval. Some of the analysis they perform includes:
– Identifying all insects present to species using taxonomic keys and microscopes. Different species have different development times.
– Aging the blow fly larvae based on length and developmental stage. Larvae go through predictable growth patterns.
– Extracting the larvae and examining their developmental stage under a microscope. This allows aging down to the hour in some cases.
– Comparing the growth of collected specimens to experimental growth data from colonies of the same species.
– Consulting published data on development rates for various insects under different temperatures.
– Factoring in temperatures – both experimental data at various temperatures and actual temperatures from the death scene.
– Considering any trauma, toxins, or other complicating factors from the scene.
With all this data, they can determine a minimum and maximum post-mortem interval. While there are many variables to consider, forensic entomology provides an important estimate of time of death that can help narrow down when a death occurred.
Case study examples
Forensic entomology has been used successfully to determine post-mortem intervals in many investigations and court cases. Here are a few examples:
William Tessier Murder
In 1979, William Tessier murdered and dumped his girlfriend Louise Chaput’s body in a field in Canada. Forensic entomologists examined maggot specimens from the body and determined they were at least 5 days old, based on length and developmental stage. This information contradicted Tessier’s timeline that she had died only a few days before during a camping trip. The entomology evidence showed she died 5 or more days before he claimed. This was a key piece of evidence used to eventually convict Tessier of murder.
Gerry Parker Murder
In 1982, Gerry Parker disappeared from Indiana. Months later, two hunters found her body in a woods 15 miles away from where she was last seen. The forensic entomologist examined maggots from the body and determined she had been dead for approximately 8 weeks based on the stage of their development. This contradicted the timeline presented by the prime suspect, who claimed Parker was seen alive just 3-4 weeks before she was found. Based in part on the entomology evidence, the suspect was convicted of her murder.
Allison Feldman Murder
In 2015, the body of Allison Feldman was found in Arizona 4 days after her disappearance. The medical examiner initially estimated she had been dead for 48-72 hours based on body condition. However, the forensic entomologist identified blow fly eggs and first stage maggots present, indicating she had been dead for at least 5 days. This extended timeline was key evidence used to arrest a suspect who claimed to have been with Feldman just 2 days before she was found. The entomology evidence showed she died earlier than the suspect stated.
Limitations and challenges
While forensic entomology can provide an estimate of post-mortem interval, there are some limitations and challenges:
– Temperature data may be limited or estimates, introducing some uncertainty.
– Insect evidence can be lost due to scavenging, improper collection, or decomposition.
– Different insect species can colonize at different rates, complicating estimates.
– Estimates provide a minimum post-mortem interval – time of death is generally earlier but not precisely known.
– Extrinsic factors like toxins and trauma may or may not be known, affecting insect evidence.
– There are relatively few experts – only around 60 board certified forensic entomologists in the U.S. as of 2022.
Despite these limitations, forensic entomology remains one of the most reliable methods for estimating post-mortem interval when used by a trained expert. Estimates may range from a couple hours to several weeks or more after death. Additional forensic evidence is needed to narrow the time of death further in most cases.
Conclusion
Determining time since death is a crucial and often challenging part of forensic investigations. The evidence that flies and other insects leave behind on a corpse can provide important clues about when someone died. By analyzing the species present, their life stages and developmental timetables, and accounting for environmental factors, forensic entomologists can calculate a post-mortem interval estimate that narrows down the timeframe of death. While entomology alone cannot pinpoint an exact time of death, it can give investigators a critical window of time to focus their investigation and build a more solid case. With proper evidence collection and expert analysis, the tiny flies present on human remains can speak volumes about how long someone has been deceased.