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How Crime Scene Insects Reveal the Time of Death of a Corpse

Forensic Entomology - Calculating the Postmortem Interval

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When a suspicious death occurs, a forensic entomologist may be called to assist in processing the crime scene. Insects found on or near the body may reveal important clues about the crime, including the victim's time of death.

Insects colonize cadavers in a predictable sequence, also known as insect succession. The first to arrive are the necrophagous species, drawn by the strong scent of decomposition. Blow flies can invade a corpse within minutes of death, and flesh flies follow close behind. Soon after come the dermestid beetles, the same beetles used by taxidermists to clean skulls of their flesh. More flies gather, including house flies. Predatory and parasitic insects arrive to feed on the maggots and beetle larvae. Eventually, as the corpse dries, hide beetles and clothes moths find the remains.

Forensic entomologists collect samples of crime scene insects, making sure to take representatives of every species at their latest stage of development. Because arthropod development is linked directly to temperature, she also gathers daily temperature data from the nearest available weather station. In the lab, the scientist identifies each insect to species, and determines their exact developmental stage. Since identification of maggots can be difficult, the entomologist usually raises some of the maggots to adulthood to confirm their species.

Blow flies and flesh flies are the most useful crime scene insects for determining the postmortem interval, or time of death. Through laboratory studies, scientists have established the developmental rates of necrophagous species, based on constant temperatures in a laboratory environment. These databases relate a species' life stage to its age when developing at a constant temperature, and provide the entomologist with a measurement called accumulated degree days, or ADD. ADD represents physiological time.

Using the known ADD, she can then calculate the likely age of a specimen from the corpse, adjusting for the temperatures and other environmental conditions at the crime scene. Working backwards through physiological time, the forensic entomologist can provide investigators with a specific time period when the body was first colonized by necrophagous insects. Since these insects almost always find the corpse within minutes or hours of the person's death, this calculation reveals the postmortem interval with good accuracy.

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