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The Elmidae, commonly known as riffle beetles, are found in freshwater streams all around the world. There are about 1400 species known world-wide, but probably many more have not yet been discovered. About 100 species in 27 genera have been found in North America. These are small (1-8 mm long) aquatic beetles that are most often found crawling on stones and other solid debris in fast-moving streams. A few species are found in slow streams or still water. They have relatively long legs and both adults and larvae are well-sclerotized. Both larvae and adults are fully-aquatic, extracting oxygen from the water around them.