Insects have hairy eyes that reduce particle deposition

An insect's eyes may make up to 40% of its body's surface, and are in danger of being coated by foreign particles such as dust and pollen. To protect them, several insect species possess an array of ocular hairs evenly spaced between each photoreceptor unit. Although these hairs have been observed for over 50 years, their purpose remains a mystery. In this study, we elucidate the function of ocular hairs using a combination of experiments, numerical simulation and micro-fabrication. We measure the eyes of 18 species of insects and find that the length of their ocular hairs is equal to their spacing. We conduct wind tunnel experiments using both an insect eye mimic and an at-scale fabricated micro-pillar array of the same dimensions as the insect eye. Our experiments and simulations show that ocular hairs reduce airflow at the eye surface by up to 90%. We conclude that ocular hairs act similarly to mammalian eyelashes: as insects fly, ocular hairs deflect incoming air and create a zone of stagnant air. Airflow and particle deposition are reduced dramatically, while light is only minimally occluded. Micro-scale ocular hairs may find application in the deployment of sensors outdoors, for which accumulation of airborne dust and pollen has no current solution.