Blow Flies Help Forensic Entomologists During Murder Investigations

Posted on November 1, 2011

Blow flies are usually the first insect to arrive after a person has died. Blow flies can arrive at a body in less than 10 minutes after death, and lay their eggs within an hour.

The larvae (maggots) that hatch from these eggs go through three stages (called instars) before they turn into adults. Near the end of the third instar, if the body is above ground, the larvae crawl up to 40 yards away from the remains and burrow as much as six inches into the ground to pupate. The pupa is similar to cocoons used by moths. A fully developed fly emerges from the pupa three to five days later.

Forensic entomologists, such as Dr. West Watson Watson, a professor of entomology at NC State and co-author of a paper describing the research, can determine approximately when insects found the corpse based on the blow flies' stage of development and the size of the maggots. Watson says this is different from estimating time of death, because multiple factors can influence the time of insect arrival. For example, a dead body could be stored in an airtight cooler for several days before being dumped in a field.

A great deal about how blow flies develop when bodies are exposed above ground was already known. Watson and his team wanted to know how their development would be affected if a body was buried after the blow flies had laid their eggs. To find out, the researchers buried pupae, second instar larvae and third instar larvae from two different blow fly species. The insects were buried at depths of 5 cm, 25 cm and 50 cm. The insects were not buried very deep because murder victims are usually buried in shallow graves. The researchers also gave the insects beef and pork as a food source. The researchers found that the buried fly larvae did well underground.

Watson says, "We discovered that these insects do survive. Soil, it appears, is not much of a barrier to flies. This gives investigators additional information. We can now tell that a body was on the surface long enough for blow flies to arrive and lay eggs, or that the body may have been moved - exposing it to blow flies."

The research conducted by Watson and his team stemmed directly from questions that arose during a murder investigation, in which a body had been moved from one burial site to another. The paper, "Blow flies (Diptera: Calliphoridae) survive burial: Evidence of ascending vertical dispersal," is published online by the journal Forensic Science International.

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