A newly documented isopod species has been filmed forming mass, night-time circles under artificial light in northern Israel — a behaviour never recorded in woodlice before, on evidence from just five nights.
Isopods — the woodlice and pill bugs familiar from garden borders — are not known for choreography. A peer-reviewed study from the Hebrew University of Jerusalem has now documented Armadillo sordidus, a little-studied isopod species, forming dense circular processions of thousands of individuals on five separate nights between 2021 and 2025, in two sites in northern Israel and always beside an active streetlight.1 First noticed by chance by an amateur naturalist, it is the first formal record of mass circular movement in any isopod.1
To find the trigger, the researchers tested two candidate explanations: the Golan Heights' unusual geomagnetic anomalies, and different light wavelengths. Strong magnets placed near an aggregation had no effect on the isopods' movement, and ultraviolet light drew only a small fraction of the population without inducing circling. A vertical white lamp, by contrast, reliably triggered the mill.1 The isopods appear to be following the sharp-edged circle of light it casts on the ground, not the light source itself — once enough of them gathered along that edge, their individual movements resolved into a self-sustaining collective circle.1
The aggregations were mostly female, many carrying eggs, which the authors say argues against this being a mating swarm.1 In one recorded instance, a centipede (Scolopendra cingulata) was seen preying on isopods drawn from the swirling mass — so far the only direct evidence that pulling the animals out of shelter and into the open carries a cost.1
This is a single study built on five recorded nights, and only one of the five aggregations was precisely counted by image analysis — the other four are described only as "thousands to tens of thousands", so the true scale of most sightings is imprecise.1 Three of the five nights fall inside one ten-day window in August 2025, which may reflect closer observation after the first report as much as any real change in the isopods' behaviour. Whether the circling itself costs isopods energy or survival — the "death" in the ant-mill comparison the authors draw — has not been measured; the single centipede attack is suggestive, not proof. And the study covers one species at two sites in Israel: whether UK woodlice do anything similar under British streetlights is untested.