Researchers at the Hebrew University of Jerusalem set out to explain the phenomenon after a video of hundreds of pill bugs spiraling around a single lamp went viral. The answer, published in the journal Ecology and Evolution, turned out to be both simple and telling: it is the shape of the light, not just its color, that does the trapping.
Woodlice naturally avoid bright light and dry surfaces, moving toward the damp shade of leaves and stones. Under a white streetlamp, however, they encounter a bright, roughly circular pool of light with a sharp boundary against the surrounding darkness. Each animal drifts inward while trying to follow a dimmer edge, and the whole group begins to rotate around the lamp, faster and tighter, until they collapse from exhaustion. In one observation, more than five thousand individuals were drawn into a single spiral.
Crucially, the effect depends on the light's profile. White light consistently triggered the spiral behavior, but only when it was directed straight down at the ground, producing that clear circular pool and crisp edge. Lamps aimed in other directions, or light of other colors, did not have the same hypnotic effect. The spiral is not random wandering — it is a predictable outcome of how these tiny animals read their surroundings.
The finding is a small but sharp window into light pollution. Streetlights are meant to illuminate human paths, but their glow spills outward in patterns that wildlife have not evolved to navigate. Animals that rely on dim, diffuse cues can be pulled into unnatural loops, wasting energy and exposing themselves to predators. Understanding the rule — bright pool, sharp edge, downward aim — gives city planners a concrete lever: fixtures that scatter light more gently or avoid creating hard-edged pools would be far less disruptive.
Knowledge takeaway: pill bugs (woodlice) are decomposing crustaceans that naturally avoid light; white streetlamps pointed straight down create a bright circular pool with a sharp boundary that pulls them into spirals; more than five thousand can be trapped in one spiral; softer, more diffuse lighting would break the effect.