A Moment of Science

Forensics

Ever wonder how forensic scientists determine how long a corpse has been dead? Find out on this Moment of Science.

A murder has been committed. The scene of the crime is a secluded area. There were no witnesses, and the body is considerably decayed.

As the chief investigator, you have a suspect who was in town a week ago. But to convict him, you’ll need to know the date and time that the victim was murdered. How can you tell? And besides, what’s that annoying buzzing in your ear? Wait, don’t swat that fly! He’s your star witness. In fact, that little fly just told you the date of the crime!

Flies don’t carry pocket calendars, and they can’t testify before a jury. How can a crime-scene insect help a police investigation? By doing what it naturally does–eating and laying eggs.

When someone dies outdoors, the first investigators on the scene are usually bright greenbottle blowflies. Attracted by the smell of decaying flesh, these usually arrive within ten minutes of the death. While this smell might make us retch, for the blowfly it’s as savory as grandma’s apple pie. The blowfly’s scientific name is “Sarcophagi,” which means “corpse eater.”

While they eat, the blowflies lay eggs in the softer parts of the body, and these hatch into maggots twenty-four hours later. These maggots attract other insects, like the predatory rove beetle, who feast on them. And so on. Wave after wave of hungry insects arrive, marking time as accurately as a clock.

Meanwhile, the blowflies follow their natural life cycle. Maggots become pupae, the pupae become adults. By surveying the insects in and around a corpse, forensic scientists can help a police investigator determine how long ago a victim was murdered.

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