The eye that moves without moving
Most animals scan a scene by moving their heads or rotating their eyeballs. A jumping spider does neither. Its two large principal eyes, the forward-facing pair that give it that unnervingly alert expression, are fixed tubes. The lenses cannot swivel. What moves instead is the retina inside: a thin, boomerang-shaped strip of photoreceptors that sweeps back and forth behind the static lens, sampling the scene in a narrow, high-resolution strip rather than a wide field all at once. Researchers at the University of Massachusetts Amherst used laser interferometry to measure this retinal movement and found it traces a precise scanning path, much like a printer head moving across a page. The spider builds its image of the world from sequential sweeps, not a single snapshot.
