How do flys know that someone is dead and not sleeping when they lay eggs dead people?

Question

How do flys know that someone is dead and not sleeping when they lay eggs dead people?

Answer 1

The putrid smell would be a "dead" giveaway.

Answer 2

They have an antenna to do it =]

I hope it helps!

--Mae Chan

Answer 3

Smell

Answer 4

Many species of fly are attracted by the smell of carrion; that is, the decomposing flesh of a dead animal. A living person does not emit that smell.

As an interesting aside, some flowers take advantage of flies' attraction to the smell of decomposing flesh my mimicing the odour, tricking the flies into visiting the flower and carrying away pollen to help the flower reproduce.

Answer 5

I don't think that flies do always know (or care) whether a person/animal is dead when they lay their eggs on them. I think the deciding factor is whether or not the fly is brushed away or not. If it lands on a person/animal that doesn't move or brush it away, it will lay it's eggs on that person or animal if it can find a suitable spot to do so, eg. eyes, etc..

I grew up in the country, and remember three instances of seeing children (who hadn't brushed away a fly) get fly eggs laid in their eye. It happens very quickly, and you can actually wipe the eggs out of the eye fairly easily.

Flies will also lay eggs in sheep wool sometimes, particularly if it's damp. This is known as the sheep becoming fly-blown, sometimes also called fly-strike, and will usually kill the sheep if it isn't treated. I grew up on a sheep and cattle property/ranch, and we regularly had to catch fly blown sheep and bring them to the shearing shed to treat them. The wool needs to be shorn off that area and a type of disinfectant applied. If the sheep isn't found and treated, the maggots actually eat away the flesh under the wool, and the sheep gets sick and dies.

Flies will also lay eggs on animals that have bad cuts on them, even if they aren't dead. I have seen this quite a few times, and it's a really unpleasant sight.

On a dead animal/person, when flies lay their eggs, they will do so in cuts and other places where the body is easy to penetrate, eg. eyes, mouth, etc. They won't just lay eggs on flat skin, so I believe if a live person stayed perfectly still when flies landed on them, and didn't brush them away, the flies would soon find their eyes and mouth, and start laying eggs on them as well.

When an animal dies, it doesn't give off any bad smell until after it's been dead for quite a while, so I don't think that smell is what attracts the flies, initially (or no more than smell attracts them when the animal is alive). I think it's just that the flies are always around, landing on animals and people, and if the animal doesn't move or brush the fly away, the fly will lay it's eggs. Certainly, the smell will become a factor after a day or so, and attract more flies. Flies do lay eggs on people right from the moment they die though, and that is actually one of the ways they determine a person's time of death. They do this by finding out the size of the largest maggots on a body, and also have to take into account other factors like what type of fly it is, air temperature and growth rate of maggots in that temperature etc..

So, in a nutshell, flies will lay eggs on live people and animals, as well as dead ones, under the right circumstances.

Hope that helps.