Insects tune in to the speed of their world.

Although people can augment their eyesight with prescription glasses for special tasks, such as driving or working at a computer, insects must perform their day-to-day work with only their natural optics. Fortunately, however, insects come fitted with visual systems that are well suited to their particular lifestyles, a new study demonstrates.

Insects depend on their ability to detect motion, either their own or that of an object speeding by them, to fly successfully and to detect fellow insects. Hawkmoths, the so-called hummingbirds of the insect world, and other bugs that frequently hover in the air must track slow motion well. Insects like butterflies and bumblebees tend to dart around quickly and need to monitor fast movement accurately.

Scientists tested the sensitivity of a variety of insects to patterns moving at different rates. Nonhovering insects, on average, responded most successfully to patterns that moved 5 to 10 times faster than the speed at which hovering insects process patterns best.

In insects, as in people, the neural mechanisms for motion detection are matched as closely as possible to motion experienced during their normal activities, assert David C. O'Carroll of the University of Cambridge in England and his colleagues in the July 4 Nature.

Moreover, the optimally processed speed "varies over an enormous range" among species, says O'Carroll. Hawkmoths respond best to patterns that repeat at a frequency of 1 to 2 hertz (Hz), while bumblebees are most sensitive to frequencies of 25 Hz. Insects can generally detect fast-moving objects more clearly than slow ones, he notes.

While not surprising, the findings "give us another handle on eyes and visual systems" by showing that what an insect sees depends on how it moves, says Mandyam Srinivasan of the Australian National University in Canberra. The authors use an elegant approach for depicting the sensitivity of insects to motion, he adds.

The investigators studied two species each of bees, flies, butterflies, hoverflies, and hawkmoths. They measured the firing rates of motion-sensitive neurons in the insects' brains while pictures of black, white, and gray stripes moved by the bugs. Previous research on flies suggests that these neurons code motion information during flight and help stabilize the insect. Mutant flies that lack these neurons can't fly.

By presenting the insects with patterns of fine and broad stripes at slow and fast speeds, O'Carroll's group tested the insects' responses to both different temporal frequencies-the rate at which each stripe passes by-and various spatial frequencies-the number of stripes in a pattern. Insects that could see a fine pattern of stripes were able to detect a wider array of slow-moving objects, the study shows.

"When you plot temporal frequency against spatial frequency, you can see [the] optimum velocity for the neuron," O'Carroll explains.

The hoverflies stood out from their eight colleagues. With their very large eyes, they detected fine stripes much more readily than the other insects and were unusually sensitive to both fast- and slow-moving patterns. Most insects can't afford to carry around such big specs. However, these flies require them because they hover while watching out for females and possible invaders, then dart quickly after them.