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Why Insects Can't Fly Straight at Night
Internal guidance system can't distinguish between distant stars and nearby artificial light
Nicolas van der Leek (Nick)     Print Article 
Published 2006-12-04 18:19 (KST)   
I remember a "Garfield" cartoon where Garfield dispassionately watches a moth swinging around a candle in smaller and smaller circles. Then its wings catch fire, and with a bloodcurdling wail, the moth falls to Earth and dies. Garfield, without blinking says, "Well, at least it didn't feel any pain."

Why on Earth can't these creatures fly straight? Actually, most of the time they do. We just don't see them winging their way under the stars to a chosen flower.

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The reason we see them swinging around is because their guidance systems are based on a good system that didn't take into account the invention of modern artificial lighting. These creatures have evolved over millions of years, and suddenly, in the space of less than a hundred, the environment changes radically, and now they're ill-equipped to deal with these changes.

The eyes of moths and mantises are geared to steering by the moon or stars, with both objects set at optical infinity. Their basic rule of thumb requires them to fly at a specific angle to an optical stimulus. Science Professor and author Richard Dawkins writes that if this angle is 30 degrees, and the object is not set to infinity, then the rays spoking towards the eyes are no longer parallel, but converge, like the spokes of a wheel. He goes on to point out that one can "produce an elegant logarithmical spiral to the candle" using an angle of 30 degrees.

The science is interesting, but the reality has the proportions of a Greek tragedy. The creature of the night caught into a crazy spiral of light and then death.

A huge thing landed next to my dinner this evening, which I was eating next to a strong bright overhead light. I thought it was a bizarre looking insect, and when I made a movement it sprouted wings, and so I had this big bat like insect swooping around in dizzy circles, flirting first with one light, then the next. It finally landed on my blinds and I got a better look at it.

Looks like a stick insect doesn't it?
©2006 Nick van der Leek
I found a wide-rimmed wine glass and quickly covered the creature, then inserted a piece of paper between it and the open end. From 1 cm away I could see exactly what this thing was: a gray praying mantis. Now its pincers were exposed, with pink edges almost like grasshopper legs, with barbs, in reverse. As my eyes looked over it, its head swiveled around at me, and its two big bug eyes blipped some alien impulse of me to the brain inside the beaked shaped head.

I'm glad I am the larger than life monster in this scenario. Imagine being a moth sized human, held between the pincers of such a horrible dragon? Having its beak dig into your stomach, like the creature in the original "Alien"!

Now I've seen plenty of green mantises, and this one seemed to be pretending to be something else, because not only does it have a Desert Storm type camouflage, but when still, it holds its pincers folded in front of it, so that it really looks like a stick insect. Held like that, the pincers don't look like pincers at all. Mimicry? I wonder.

After snapping a few photos, I set Miss Mantis free, and then wondered whether she'd make it from A to B tonight, or whether she'd glance down from her fairy flight and be seduced by a television glow, or worse by the headlights of car. What is it about their eyes, specifically, that lights dazzle them so, sometimes to death?

A praying mantis in mid-lunge
©2006 Nick van der Leek
Well, it's simple really. The structure of their eyes (moths, mantises and plenty of other bugs) is distinctive. Under the microscope their eyes resemble a bunch of long tubes. So when the moth or mantis encounters an artificial light, suddenly when it swoops by the light slips out of its field of vision, and it swings round to get it to shine back into the tubes, and at a constant angle. Hence the chaotic, circular flying. They're attempting to keep the light coming in at the same angle into the tube structure of their eyes.

Our eyes are different. We have rods and cones, which are good for judging perspective and color. But the structure of bug eyes cause man-made lights to interfere with their apparatus, and make bug-based navigation at night a nightmare.

The next time you see a moth's wings catch fire beside a candle, apparently committing suicide, remember that it's mistaking a candle for a faraway star, or the sun. It can't be a pleasant way to check out, so if at all possible, close the windows on summer nights, and keep non-essential lights off. That way some winged creature can find its way silently through the night to the flower or field that it's aiming for.
*Richard Dawkins, The God Delusion, p172-174
©2006 OhmyNews
Other articles by reporter Nicolas van der Leek

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