Katherine Brown viewing Calliphora vicina blowfly under a microscope. Credit: University of Potsmouth
Experts have long studied and used blowflies as forensic tools given their ability to help estimate time of death. In fact, blowflies are usually the first to arrive at an open-air crime scene—typically just 10 minutes after a murder. They are drawn by body fluids and gasses associated with the decomposition of a corpse.
Over the years, much research has been done on blowflies, including decomposition patterns, the effect of different weather/environments, the impact of clothing, and more. And now, a researcher in England is studying blowflies for an entirely new purpose—to help solve rape cases.
Blowflies and semen
A new study published in the Journal of Forensic Entomology by Katherine Brown and Alexine Clarke has shown that blowflies can inadvertently collect and retain DNA evidence from sexual assault crime scenes.
“I wanted to question whether eggs being laid on a body with semen present could act as a vessel for DNA, which would theoretically allow us to link an offender to the crime,” explained Clarke. “Although forensic entomology can be used in death investigations to estimate time since death, recovering male offender DNA, attributed to sperm cells, from the gut content of blowflies is relatively new, and this is the first time research in England has looked specifically at sperm cells.”
For the study, Clarke and Brown placed three piglet cadavers outside in temperatures ranging from 42◦ to 57◦ F. They then placed 6 mL—the upper level of average human ejaculation—of boar semen in three locations on the cadavers: behind the ears, in-between the rear legs and across the surface of the trunks.
Naturally, blowflies were immediately attracted to the decomposing remains, laying hundreds of eggs and larvae.
The scientists then recovered the eggs at different intervals: first instar larvae recovered from the three specific locations; second instar larvae from no specific area; and third instar larvae from no specific area.
Employing differential extraction, Clarke and Brown found intact spermatozoa and/or sperm heads in at least 50% of the samples.
“This is an exciting finding because it shows that the blowfly eggs and larvae can retain evidence such as sperm, potentially linking perpetrators to crime scenes,” said Clark.
The samples from the first instar larvae has the highest presence of sperm. They also had the more intact spermatozoa compared with the third instar larvae samples, which had a higher percentage of sperm heads with no attached flagellum.
“This research builds on existing knowledge of host DNA detection in blowfly and other insect larvae, with the important distinction that we can attribute the DNA to the cell-of-origin—sperm,” said Brown, a principal lecturer of forensic science at the university of Potsmouth. “Not only does this indicate the source of the DNA, but also potential activity, contributing significantly to serious crime and tackling violence against women and girls.”
Clarke said she has plans to develop the research further, as this was a scoping study. She is already exploring the possibility of generating a full DNA profile from the samples. Further research is also necessary to explore how environmental factors and time affect the preservation of such evidence on and within blowflies.