Why do moths come out at night?

04 October 2016

Question

Why do moths come out at night?

Answer

We put this question to zoologist Max Gray...

Max - A lot of moths do come out at nighttime but, in fact, not all moths do. There are plenty of moths that are active during the day. So, if you've ever had clothes moths in your wardrobes or eating through your jumpers, they're often active during the day. You'll see them flying around the house and they'll get into your carpets and eat.

Chris - So they're not exclusively active at night?

Max - No they're not, no. And, by and large, a lot of moths are nocturnal and a lot of butterflies are diurnal (active during the day), but you get overlap. There are night butterflies and there are day moths.

Chris - And just in case Mark is picky about the answer and he's saying why are they coming out in the day and night - how do they know it's day and night and what makes them do that behaviour?

Max - It's dark at night is essentially the answer.

Chris - But biologically how do they know it's day and night and to do the behaviour they do - why do they decide I'm going to go out at night and not during the day?

Max - There'll be some mechanism in the brain that gets activated when their eyes are receiving less light and there'll be less active when there's lots of light.

Chris - But why do they fly around a candle and singe their wings - do we know?

Max - Yes. That's to do with how insects navigate at night. So if you imagine starlight and moonlight, because it's coming from a source that's very far away to all intents and purposes the rays of light can be considered parallel, so all light is coming from the same direction. And so a moth wanting to fly in a certain direction will just fly at a set angle, it will decide a direction and set I'm going to fly that way and keep the light coming from the left, or right, or wherever they chose to.

But if you've got a point source of light like a candle or a light bulb hanging from the roof, you can imagine using the same means of navigation, you keep the light bulb to your left. You just slowly spiral in and eventually hit the light. It's not just moths - anybody who's sat on a porch with a candle or a lantern, pretty much all night insects will do that.

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