Tools and Clues
Culture of Sarcophagidae, or flesh flies,
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.