How come, when the front door of my house is open, a fly can enter my home easily, but not escape as easily out of the same door?
Lucy Singh
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Wantage, Oxfordshire, UK
My experience is that blue and green bottle flies are perfectly able to escape out of the door when they want to. I suspect they mostly just don’t fancy it – possibly because they like the still air indoors. I find that when they feel threatened, they can zoom straight through two doorways to get to the outside, even if they have spent some time bumbling against the window beforehand.
To make a fly feel threatened, you can wave a cushion at it as though you are trying to swat it in mid-air. If you manage to make contact with it, you are unlikely to harm it (which is important to me), but you will find that it then flies directly out of the house.
If it dodges the cushion, you will have to flap at it a few times so that it can’t settle anywhere. It won’t enjoy this and will eventually decide it has had enough, then zoom straight out of the door.
In the absence of a sentient being controlling door access, flies in your home are a statistical inevitability
No flies were harmed in these experiments, but I am sure they got rather annoyed.
Eric Kvaalen
Les Essarts-le-Roi, France
I would say it is an entropy effect. It is like a situation where you have air outside a room and a vacuum inside. When you open the door, the air comes in and doesn’t immediately go back out. The air molecules are like the flies: there are lots of them outside and when you open the door, one or more happen to come in. They may eventually go back out, just as any air molecule may, but it will take a while before that happens. In the end, you will have the same concentration of flies indoors as outdoors, on average. Unless there is something about your house that they like!
Ron Dippold
San Diego, California, US
I think it is mostly a matter of odds. Imagine a 2D architectural diagram of a house’s ground floor seen from above with an outside door open. There are hundreds of flies (dots) randomly buzzing around outside. Your door is relatively tiny, but eventually one of them will wander through – let’s call him Liam.
Liam doesn’t really grasp the concept of “inside” versus “outside” your house and is just semi-randomly flying around smelling for nasty stuff to eat. Now imagine the dot that is Liam buzzing around your house – most buzzes don’t end up with him going out of your open door before the outside dots that are Noel and Tony also randomly end up inside.
So it is just a consequence of the fact that there are orders of magnitude more flies outside your house who might randomly get in. If you had thousands of flies inside your house (nooooo), some would randomly find their way out of your door as well.
Of course, there are some flies that do eventually try to go somewhere else because there isn’t enough food inside, but they have no idea where the door is and just go for the nearest sky – which is your closed window. So an even worse choice than a random one.
Alan Cooper
Glasgow, UK
Maybe the fly could, but it chooses not to. Perhaps your home is warmer, brighter, smellier or otherwise more attractive than the outside world.
But, even if not, this is simply a manifestation of statistical mechanics, entropy and the second law of thermodynamics at the macroscopic level. Once the front door of your home is open, the population density of randomly flying objects (flies) inside and out will equalise to a thermodynamic equilibrium.
In the absence of a Maxwell’s demon or other sentient being such as yourself controlling door access, flies in your home are a statistical inevitability.
Jeff Stanton
Sydney, Australia
During tests in an office where the front door was often left open, it was found that turning off the fluorescent lights encouraged annoying blow flies to find their way out the door. They flew back and forth, eventually finding the door and going away. This led to a hypothesis that they are attracted to UV light, which is supported by the dead flies often found on window sills. It seems that flies can’t tell the difference between a door and a window.
Sadly, there are no records, as the original hypothesis arose from observation during a morning tea break. Subsequent observations showed that turning off the lights did encourage the flies to go outside. An unfortunate side effect was a decline in office productivity during the lights-off periods.
Spencer Weart
Hastings-on-Hudson, New York, US
Point one concerns statistical mechanics. There are many more flies outside than inside, so if they all move randomly, it is more likely for one of them to come in than for the single one inside to go out (think of your house as a container with low fly pressure).
Point two concerns perception psychology. You are very aware of that buzzing nuisance inside, but you don’t perceive flies outside that may be trying to get in.
And point three concerns biology. Your house smells like food. Why would the fly want to leave?
Gillian Peall
Macclesfield, Cheshire, UK
How do you know it is the same fly?
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