Hydraenidae is a family of very small aquatic beetles, sometimes called "Minute moss beetles", with a worldwide distribution. They are around 0.8 to 3.3 mm in length. The adults store air on the underside of the body as well as beneath the elytra which allows them to crawl underwater, often on the underside of the water surface tension, though they cannot swim. Some species have gills that effectively allow them to stay underwater indefinitely. Larvae vary from being fully terrestrial, to being aquatic at least in their earliest instars. The diet of hydraenid larvae and adults is thought to consist of algae, spores and other plant matter. At least some hydraenid adults use stridulation to communicate. There are around 1,300 species in 42 genera.

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