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Forensic Entomology
Tools and Clues


Tools and Clues | Insects and Decompositonal Stages of the Body | Two main ways of using insects to determine elapsed time since death | Photo Gallery | Favorite Links


Culture of Sarcophagidae, or flesh flies,
in the laboratory.

Only a few types of insects eat rotting human flesh. A couple of dozen species are likely to colonize a corpse during the full decay, but at any one time, only half a dozen or so are likely to be present. The two main players are the carrion feeders or blowflies (Calliphoridae), and the flesh flies (Sarcophagidae). The occasional beetle participates too, and bloodsuckers such as mosquitoes and lice can carry evidence away.

One of the toughest challenges for forensic entomologists is to distinguish the three larval stages, or instars, of important species. Whereas blowfly larvae tend to look alike, beetle larvae are very diverse, ranging from white to brown, skinny to fat, and hairless to hairy. Investigators look to mouth parts, spiracles, and genitalia to identify species, but even the best forensic entomologist must sometimes rear a maggot to adulthood on beef liver to see what it is. The pupae, or cocoons, are valuable too, although police often ignore blowfly pupae, which look like rat excrement.

Ecology is part of crime scene analysis. Factors that affect the rate of decomposition in water, for example, include temperature, bacterial content, salinity, and algal blooms. A corpse submerged in a highly eutrophic, shallow lake will decompose much more rapidly than a corpse in a cooler, deeper lake with a lower bacterial count.

Forensic entomologists use data to estimate the time elapsed since insect activity began--called the postmortem interval, or PMI--that could be the time of death. This is done in either of two ways. The accumulated degree model, commonly used in agriculture, assumes that within a certain range, maggot development is a linear function of temperature, measured as the degrees above a critical temperature where development stops. Alternatively, some people chop the temperature/time curve thought to have occurred at the scene into short intervals, such as six hours. The average temperature during that time is then used to estimate the growth based on a lab model at a similar constant temperature.

As a corpse begins to decay it gives off an odor. This may smell foul to the living, but to many insects it's a chow call. Insects of the Diptera order (which includes mosquitoes, flies and gnats) usually arrive first in the form of blowflies (Calliphoridae). Flesh flies (Sarcophagidae) come as well.

When they arrive at a corpse, the flesh flies deposit their larvae. Meanwhile, the blowflies lay eggs in the body's orifices and open wounds. Thus, a new arthropod life cycle begins within the cadaver.

Sometimes within hours, the blowfly eggs hatch into larvae, which live on the dead tissue. Over time, the larvae molt and transform, going through several stages, or instars, until finally reaching maturity. The precise time of these changes depends on the species, as well as the temperature of the surrounding environment.

But when biologists know the life cycle of these carrion bugs, they can tell how long the insects have been in the body. Determining this length of time can further indicate the estimated time of death. For instance, blowflies generally start laying eggs within 48 hours of a death. Their eggs become larvae within 16 and 25 hours, and the larvae become pupae within six to 12 days, depending on several variables.

Other species become important as time passes. Eggs of the cheese skipper (Piophila casei) show up three to six months after a victim's death. Beetles that thrive on bones arrive after the bone is exposed. Other beetles begin preying on the maggots feeding on the flesh. In all, it's a complex ecosystem that will generously betray its secrets to the informed entomologist.
Insects can also help investigators establish if a body has been moved after death. Entomologists will check the type of insects within the corpse to see if they are local to the area. If the bugs are foreign to the place in which the body is found, investigators know that someone moved the body after the victim's death.