MALE HOUSEFLIES CLEARLY SEE BETTER

When a male housefly sets his sights on a female, there's a lot at stake. In a high-speed chase, he's got to get his gal, or loose his chance of genetic immortality. The stakes are high enough for male flies to have developed a specialised eye structure, the `lovespot', that is adapted to improve his chances of capturing a dodging and weaving female. But what is it about the male's eye that gives him the tactical edge? Brian Burton and Simon Laughlin started playing female fly escape sequences to photoreceptors in the male's lovespot to find out what was special about the insects ommatidia, and discovered that while the male has better optics and neural responses, he also has the ability to de-blur his vision as the female flashes past; he simply sees more clearly (p. 3963)!

A male fly's visual perception isn't bad. With his acute photoreceptors,the male can register small targets moving at the high angular speeds that an escaping female reaches. Burton explains that while a lot was already known about the male fly's high-speed tactics to trap a mate, no one had ever recorded the remarkable visual system's response to a fleeing female. Laughlin and Burton knew that if they could simulate the visual input at the male's eye, they could record the resulting neural responses at individual photoreceptors, and reconstruct the neural image on the retina to get the male's perspective.

Laughlin explains that at the photoreceptor level, a passing female is simply a succession of dimming events, as her image moves across the retina. By programming an LED to dim and glow in the same way as the passing image,and shining it on individual photoreceptors, Burton was able to record the neural signals that the LED triggered. Laughlin explains that they played 54 different pursuit trials, ranging from a female that was practically stationary at close range, up to a long distance high speed encounter;10,000° s^-1, 76 cm away! Burton also tested the female's neural responses to the same suite of aeronautical encounters, to see how well her eyes fared compared with the males.

At 40 cm, female eyes could hardly see the moving target even when it was practically stationary. But the males had no problems with their long distance vision at any speed. Laughlin explains that the male's visual responses were much stronger because their neurons had a much larger gain than the females'. The neural signals had a higher amplitude and a shorter sharper duration. As well as having superior neural responses, the male's optics are better too, as the lovespot's ommatidia have larger, higher quality lenses than the female's eyes.

But Burton and Laughlin were really amazed when they reconstructed the neural signals in the male fly's eye; the retina was de-blurring the image!Laughlin admits that this was completely unexpected and he was `knocked out'when he saw the male's sharp view. Using a combination of high gain and rapid damping of the signal, the male's photoreceptor's response lags the visual image on the eye by only 7.8 ms! Which explains why there's no escape once she's in the lovespot's focus.