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By John Roach

When detectives find a corpse lying in a ditch or anywhere else, determining how long it has been there is one of the first tasks. A good estimate comes from the age of the flies found swarming the dead body, but this technique may be complicated in the Midwest by a fly found newly buzzing there, according to a forensic entomologist.

"The composition of what (flies are) around is changing as the climate changes," Christine Picard, a biologist with the Forensic and Investigative Sciences program at Indiana University — Purdue University Indianapolis, explained to NBC News.

To determine how long a person has been dead, insect specialists collect flies found swarming in and around corpses, determine the fly species and its age. "From how old it is, you can then work backward and say, 'okay, the body had to have been here a minimum of this time,'" she said.

These flies, she added, only lay eggs on dead individuals, so the development stage of the fly serves as a time stamp.

The introduction of a new fly to the Midwest mix is a problem for crime scene investigators because different flies go through their life stages at different speeds, Picard explained. Other factors governing development include what other species are swarming the body and the temperature in the vicinity of the corpse.

The forensic entomologist discovered the first instance of a species of fly not native to the Midwest in Indiana in September 2012 during routine collection of samples. Until now, the species — Chrysomya megacephala — had never been documented further north than New Mexico.

"It is not a species that is ever in the northern half of the country," she said. "So it showing up and potentially being misidentified can definitely be a problem." The fly, which is native to Africa and Asia, is closely related to another fly species already in the Midwest.

"This can have an impact on forensic casework because their development times could be very different," she said, adding that, for now, researchers are unclear what the invasive fly's development times are in the Midwest and whether its presence has an influence on the development of native flies.

Research with other flies has shown, however, that the presence of one fly species can alter the development rate of another fly species, Picard explained. As well, populations of the same species but in different parts of the world are known to develop at different rates.

Picard said Chrysomya megacephala, the fly new to Indiana, appears to have taken advantage of the record-breaking warmth in summer 2012 to buzz north from New Mexico. As temperatures continue to rise in a warming world, she expects the fly to be in Indiana more frequently.

"As the temperatures change, new species are showing up that have previously not been present (and) older species are possibly moving further north following this temperature gradient," she said.

Piccard's findings will be published in the July edition of the Proceedings of the Entomological Society of Washington.

John Roach is a contributing writer for NBC News. To learn more about him, visit his website.