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Ranging in length from 11 to 16 mm, they are generally a dull black, with a squarish head and a pronotum significantly narrower than the elytra. The larvae also live underwater, but breathe through the eighth abdominal segment and must therefore stay close to the surface. They cling to rocks or floating debris. Both adults and larvae are predators, but will also scavenge dead insects.
It comprises six species, three from western North America and three from China.
When disturbed, adults exude a yellowish fluid from the anus, with an odor described as that of cantaloupe or decaying wood, probably as a defense mechanism against hungry frogs and toads. http://bugguide.net/node/view/498214/bgp...