With 250,000 described species, beetles are the largest insect order. Though only 3% to 4% of the species have an aquatic stage, beetles comprise a substantial portion of the aquatic insect fauna, with over 1,000 species in North America. We have collected about 120 aquatic species in Iowa, primarily from rivers and streams, though they are generally even more numerous and diverse in standing waters (lakes, ponds, wetlands). The Iowa DNR has initiated a wetland monitoring program that includes macroinvertebrate sampling, so our knowledge of the aquatic beetles (and other aquatic insects) present in the state will likely increase considerably in the next few years. Adult aquatic beetles look like "beetles," often very similar to their terrestrial relatives. The body is hard in general, and there is a line or indentation down the middle of the back, which is where the hardened fore-wings (elytra) meet. Water beetle larvae are quite variable in body form and might be confused with other groups of insect larvae. Those that are more likely to be confused may resemble caddisflies, fishflies, dobsonflies, or alderflies at first glance. However, these water beetle larvae have conspicuous antennae (which are very small on caddisflies); lack paired, hook-bearing prolegs at the end of the abdomen (displayed by caddisflies, fishflies, dobsonflies); and never have the long single filament at the end of the abdomen like alderflies. The name Coleoptera refers to the hardened protective cover ("koleo" = sheath) of the fore wings ("ptera").