Crime Library: Criminal Minds and Methods

Kevin Neal, Convicted of Murder by Forensic Entomology

How Flies Tell Time

When he received the evidence at his home in Rensselaer, Indiana, where he was on staff as a professor of forensic entomology and biology at St. Joseph's College, Dr. Neal Haskell began his analysis by taking inventory of the shipment. At his direction following a conversation with detectives in Champaign County, Haskell was sent both live and preserved specimens of the insect fauna found at the body site, soil samples gathered at the scene and, equally important, from the body bags in which the cadavers were delivered to the medical examiner.

He further instructed the detectives to send climatological data for the nearest National Weather Service station, which in this case was in Dayton, about 45 miles away from Nettle Creek Cemetery. Each of these components was necessary for Haskell to make an estimate of the minimum and maximum post-mortem interval.

Blow Fly
Blow Fly

Entomologists use a technique called Accumulated Degree Days to measure fly development on a corpse. One of the most common necrophilic fly species is the blow fly, known formally as Phormia regina. Over the years, P. regina has been studied under carefully controlled environmental conditions, so scientists know, when temperature is controlled, just how long the species takes to go from newly deposited egg to first instar, to second and third instar and through the pupal stage to adult.

"After the medicocriminal entomologist has calculated the degree-hours or degree-days available, this information can be applied to the template of known development of the species in question at a temperature regime similar to that recorded from the field," writes Larkin. "The result will give an estimate of the time required for that fly to progress from the stage produced by the female (and deposited on the corpse) to the stage collected.

This, then, is the minimum PMI — all else being equal, the corpse must have been dead for at least that period or the developing flies would not have been able to get to the noted point of maturity."

At a mean temperature of 20 degrees centigrade (68 degrees Fahrenheit), P. regina takes between 19 and 25 hours to hatch from a fresh egg. The average time for what entomologists call egg eclosion is 21.2 hours.

To calculate Accumulated Degree Hours, an entomologist will multiply the number of degrees by the number of hours needed for eclosion. In other words, based on laboratory studies of P. regina, Haskell knew that on average, it takes 424 ADH (20 degrees C x 21.2 hours) for a blow fly to emerge from its egg. If the mean temperature at a crime scene is 25 degrees Celsius, the time for eclosion is simply calculated at 424 ADH divided by 25, or approximately 17 hours.

Each stage of the blow fly's life cycle has been calculated and tabulated for entomologists, so when Haskell examined his preserved specimens and found 40 or 50 puparia that had hatched, he knew he could calculate the maximum post-mortem interval — the earliest time the children had been dead.

The fact that no P. regina were still present on the bodies told Haskell that "we had passed well beyond the time that the black blow fly would be active," he would later testify. "It had done its thing at the early stage of decomposition, fed. They have done their thing and have long since been gone."