http://www.newscientist.com/article/dn21774-zoologger-jesus-bugs-evolved-hooks-for-grappling-eyes.html

19:00 03 May 2012 by Michael Marshall
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Video: Watch water striders pin down females for sex

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Species: Rheumatobates rileyi
Habitat: Skittering around on water throughout North America

Our eyes are precious and vulnerable. We rely on them to understand and navigate the world, but they are soft and easily damaged.

Simply having a stray eyelash is enough to stop us in our tracks while we get the thing out. Faced with blown dust or sand, we instinctively turn away or fling up our hands. Many people can't bear to watch the opening of Buñuel's Un Chien Andalou – you know, the bit with the razor.

So female Rheumatobates rileyi are hardy little bugs. When males want to force them to mate, they grab them by the eyes. The females have compound eyes, so there's less risk of damage than there would be for us, but it's still thoroughly inconvenient. Worse, the males have evolved elaborate grasping apparatus to help them cling on.

Jesus bugs

R. rileyi belongs to a family of bugs called Gerridae – otherwise known as water striders, or Jesus bugs.

They're famous for their habit of skating around on the surface of still water, particularly ponds and lakes. Their legs are unusually long, spreading their weight across a wide area. Tension in the water surface helps support them.

In common with many other organisms, male striders want to mate as often as possible, perhaps 30 times a day, to inseminate lots of females. But each female only wants to mate about once a week: she can store sperm so she doesn't need any more sex than that.

This conflict has affected water striders' evolution, with males evolving ways to restrain the females and females developing ways to resist. In one species, the males have to spend 15 minutes tapping rhythms on the water surfacebefore the females put out.

Don't hold me

In R. rileyi, the conflict has produced a similarly extreme result. The males have massively modified antennae which, as well as sensing the environment, are used for grabbing hold of females. Locke Rowe of the University of Toronto, Canada, used electron microscopes and high-speed videos to find out how the antennae work.

Instead of being straight, the antennae bend back on themselves to form a hook that fits neatly around the female's eye. Each hook-like antenna has a small spike and hook protruding from the main length, which lock into the female's body. They also have a pad covered with hair-like setae, which creates friction and helps maintain the grip.

Rowe looked at developing R. rileyi and found that a gene called dll was expressed in their antennae. He used a genetic trick called RNA interference to reduce the gene's activity in developing males, and found that doing so simplified the antennae. When the effect was strong, all of the adaptations disappeared.

Gene for grabbing

Other than their simplified antennae, the modified males were pretty normal. So Rowe introduced them to females to see how they did. Compared to normal males, with their elaborate antennae, the modified males were less successful at mating with females, and produced fewer offspring.

That means a single gene, dll, seems to be responsible for the males' hugely elaborate antennae. It acts late in development, by which time its other functions are done and dusted. That may explain why it has changed so much in R. rileyi: it only affects one thing, so it's easy for evolution to modify it without causing harmful effects.

One thing's clear, though: it hasn't worked that well. Even the fully-evolved males don't mate very often. "They're only successful 10 per cent of the time," Rowe says. "The females are so effective at dislodging males."That's because unwanted matings increase the risk of a predator attacking, force the female to carry a male on her back, and increase her risk of sexually transmitted infections.

Journal reference: Science DOI: 10.1126/science.1217258

  
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Male Rheumatobates rileyi bugs (left) use hooks on their antennae (shown in purple) to grab females' eyes and mate with them (Image: Science/AAAS)