Blow flies - does forensics really use them to tell time of death?

Main:  A blow fly feeding on a dead Steller's jay chick.  Inset:  Maggot showing a pair of mouth hooks.

Blow flies - if you watch forensic shows like Bones, you are probably familiar with the idea that the flies' larvae are used to help determine how long someone has been dead for.  The TOD (time of death) is determined in days, not hours, as the corpse has to be old enough that blow flies would find the body, lay eggs on it, and the maggots would hatch, grow, and moult.  The time between moults is an instar; each instar (3 in all) takes a certain length of time, with growth occurring along the way.  You can tell how long a body has been in place by the presence of blow fly maggots and how big they are.

However, it is a bit more difficult than that.  There are different varieties of blow fly, each of them reproduces at specific rates relative to that species.  Another factor is the temperature associated with the body.  Flies develop more slowly when it is cool and faster when it is warm.  Under ideal conditions, eggs may develop through all 3 instars, pupate, and become adults in as little as 7 days.  

Although blow flies are notoriously fast at finding and laying eggs on a corpse, many may visit the body over the entire time of its decay.  Any larvae present could have hatched mere hours ago or days, so it is important to find the most mature individuals present.  

The eggs are laid in open wounds or cavities.  The larvae burrow into the flesh using their mouth hooks, but have to be near the surface to extract oxygen that dissolves through the decaying tissues.  When it is time to pupate, the maggots crawl to a dry area and encase themselves in their last moult, where they will eventually emerge as an adult.  In ideal conditions, the adult can emerge within 3 or 4 days.

♫ Blow fly, don't bother me. ♫

Thanks for reading.

Eric Svendsen     www.ericspix.com