Cases can hinge on maggots being away from their home turf, or the ages of maggots as figured by their sizes and the surrounding temperatures, or even the absence of flies. An interest in controlling disease prompted Greenberg to spend decades studying flies. That led him to consult on about 50 murder cases, testifying in court on about half of those. The primary bit of information is time of death. There are numerous species of flies and maggots.
Different species thrive in different times of the year, which also can vary by geography.
Different species of maggots grow at different rates. But a forensic entomologist needs to have a good handle on temperature records because that influences a maggot's growth. So an urban maggot on a corpse found in the countryside could mean the body was moved after the murder. This knowledge is fine-tuned by some unusual research. Greenberg talked about entomologists hunting for road-kill dogs to study.