The Beginning of the End: How Blow Flies find Corpses

In the spirit of Halloween…

“I see dead people”, whispers  Haley Joel Osment in M. Night Shyamalan movie “The Sixth Sense”… and he is very convincing. That’s just a movie, but for blow flies, seeing and smelling dead people, or any decomposing corpse for that matter, is what adult life is all about. Unlike Osment, blow flies want and need to see and smell dead things! They have to be able to find decomposing corpses quickly in order to lay their eggs and propagate their kind. In fact, they are so good at finding dead things that we use their progeny (read maggots and pupae) in forensic sciences for determining time of death (TOD), and ultimately putting criminals behind bars.

Haley Joel Osment in M. Night Shyamalan movie “The Sixth Sense” (Photo: Huffington Post) and blow fly (Photo: Mike Hrabar)

In a recent article published in Entomologia Experimentalis et Applicata, my collegues and I explain how fertile blow flies rapidly locate a recently deceased corpse.  Reproductively mature female blow flies use very low concentrations of dimethyl trisulfide (DMTS) in combination with dark animal pelt mimicking colours (black and reddish brown) to rapidly locate the corpse.

Blow flies lay their eggs on recently deceased animal corpses.  The eggs quickly hatch into maggots which consume and break down the corpse. After approximately 1 week of consuming the rotting flesh, they will leave the corpse and pupate in the soil nearby.  But blow flies aren’t the only organism scavenging the corpse; they face a lot of competition with other insects, bacteria, fungi, and vertebrates.  In order to reduce competition with these organisms, blow flies need to get there first, and they do!  Often, they get there within the first few hours after death!  This means that they can smell a corpse long before our noses can; very intriguing!

Working with one of the first species of blow fly to arrive on the scene, Lucilia sericata, we show that blow flies can detect ‘death’ volatiles, and respond faster to a recently dead and wounded rat carcass than they do to an intact rat carcass.    Our next step was to identify the odour using a variety of lab equipment including a gas chromatograph electro-antennal detector (GC-EAD) which is a fancy name for a process with a easy explanation… the antenna acts as a filter for all the smells and we only identify the odours that excite the antenna.  Using this process we identified 9 compounds that excited the antenna.

Using a series of laboratory and field experiments, we concluded that DMTS was the key compound that attracted flies, but not just any flies… female flies laden with eggs!

Like most insects blow flies use antenna to smell odours and locate resources, like the corpse, but unlike many insects blow flies have huge eyes that take up 70% of their head.  So we paired visual cues with DMTS and found that dark animal pelt mimicking colours accentuate the response of blow flies.

Ultimately these findings will be developed into a lure for trapping blow flies, both industrially and residentially.  But more importantly, the lure can be used to monitor blow flies for the impending Zombie Apocalypse.  Due to the fact that the rotting flesh of zombies is likely similar to the rotting flesh of a recently deceased corpse (although, arguably, my dead experimental rats were far from being undead), Metro Vancouver (one of the safest Canadian cities in case zombies decide to finally take down us humans) will be able to use our lure in a trapping and monitoring system, part of their “Zombie Preparedness Campaign“.

…No, but really, BC really does have an emergency zombie preparedness Campaign!  Deal is: If you are ready for zombies, you are ready for the inevitable Megathrust Earthquake, which is due every 70 years or so in the Pacific Rim. Anyway, zombie preparedness is probably one of the things that makes Vancouver one of the best places to live in the Solar System.  I swear I didn’t make any of this stuff up!

Read the full article:

Brodie, B.S., R. Gries, A. Martins, S. Vanlaerhoven, and G. Gries. 2014. Bimodal cue complex signifies suitable oviposition sites to gravid females of the common green bottle fly. Entomologia Experimentalis et applicata. 153(2) 114-127

Citations:

McCann, S. feeding and ovipositing blow flies. [Cover Photo] 2012. Vancouver, BC, Canada.

4 thoughts on “The Beginning of the End: How Blow Flies find Corpses

  1. Interesting. I’m a little confused about how the female blowflies “use” animal pelt-mimicking colors to locate a carcass. That’s visual identification? Does that mean that the blowflies are more likely to quickly locate a dead brown rat than an albino?

  2. So my cat just died within an hour of last checking on her. My Windows are closed yet she already had tiny little flies on her. Where did they come from so fast?

  3. I saw a wounded possum near my home and watched it because there were flies buzzing around it. Within 10-15 min, the possum lie still and it was loaded with these flies.
    Can these flies actually know when an animal is about to die. That seemed to be the case here.

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