Why do bugs circle lights?

It’s 9pm and you’re heading for town. You look good, you’re keen for a biggie, and you’re a prionoplus reticularis or huhu beetle like this handsome brown fella here. Coco’s, you think – I’ll go to Coco’s for a drink and some kai – so you point your nose in the right direction, set out, and – wham – smack right into a streetlamp – wham – you do it again, but no worries, I’ll just head over there and… shit, damn! You smack right into that light again and… fuck it. This is just embarrassing. You limp home and give up.

It sucks being a beetle these days – and being a moth is all shaaame – but it didn’t always used to be that way. Once upon a time moths and beetles were the masters of navigation – until people came along and messed things up.  I was thinking about this on New Years Eve.

I was at a party in Northland, everyone was out on the deck, and first a puriri moth and then our friend the huhu beetle above came blundering around the light, time and again, smacking himself into guests and generally making a nuisance. Have a look at the video:

Thanks by the way to Renee (from Coco’s Cantina restaurant – nums) for being an excellent hand model and beetle wrangler.

So why do beetles and moths get messed up by lights? When you think about it, it’s quite simple.

Go back to the scene where our beetle pal is heading out – but think about how it happened before people came along with artificial lights. Our beetle picks his destination – the top beetle hangout spot – and he knows it’s over there, so he makes sure that he’s plotting a straight course in his journey by finding the brightest light in the sky – ie, the moon – and keeping it in the same corner of his eye for the whole journey. It’s a bit like if you saw a distant mountain in the south. If you walked along with it on your left hand side you’d know you were going west – but if suddenly it was behind you you’d know you’d drifted north, and correct your path: that’s what beetles do.

BUT that only works for objects, like the moon, that are a long way away. Imagine you walk down the street keeping an eye on one lamppost. Because the lamppost is close to you, you soon pass it, and your relative position changes – to keep watching it, you’d have to turn your head as you walk along. It’s just something that happens with things nearby, but less so with far away objects, right? So near by = useless for navigating and far away = better.

So when something’s as far away as the moon is, it means you can pretty reliably use it for navigation: to get your beetle ass to the beetle party you find the moon – which is easy because it’s the brightest light in the sky – you make sure that it’s in the same corner of your eye as you fly, and hey presto – you’re flying in a straight line no matter how much beetlejuice you drink, or how many diversions you make to hide from hungry owls, or whatever: because moon over there equals party over there, and that works fine until some a-hole gets an ever brighter light than the moon and puts it really really close to you.

BASTARDS. Now when you use your old trick of keeping the brightest light in one corner of your eye you find that it pretty quickly shifts behind you. What’s that all about? You must have strayed off that straight line – but no worries, just adjust your flight path: okay that’s better… except, well… it’s behind you again – you better make a sharp turn, and another, and another until you’re endlessly circling that light in the poor, deluded belief that you’ve been flying in a straight line all night.

So that’s the simple reason why moths and beetles go crazy for lights, circling round and round: they really, honestly think they’re going straight, because, in short, we’ve screwed their system. I think that casts a kinder light on insect intelligence – they’re pretty sophisticated really (in a dumb kind of way); I also think that the explanation above has a kind of neat parallel to the rubber sheet analogy which is often used to explain gravity. But that one, yeesh, yeah it’s late and if you’re interested you’ll look it up…

Huhu beetles – as an aside for those who’ve never heard of them – are harmless and the largest beetle in New Zealand. They’re the adult version of huhu grubs, which are a large white worm that eats soft rotten logs on the forest floor, and which are kind of delicious fried. My brother and I picked a couple out of a log when were kids and cooked them and I can report that they’ve got a nice peanut butter sort of taste – though the head’s a bit crunchy. ie, they’re not really the sort of thing you want to look at too closely while you eat them – by artificial light, or otherwise.

Leave a Reply

Your email address will not be published. Required fields are marked *