General: The families Dytiscidae and Hydrophilidae include most of the aquatic beetles. Watch for them diving in springs and ponds around Las Vegas.

The Dytiscids (predaceous diving beetles) are mostly large (1-1/2 inches long) beetles with sharp mandibles, dark colors (some have golden highlights), a distinctive shape: elongate-oval, and the hind legs are flattened and fringed. The antennae are threadlike and longer than the maxillary palpi. The larvae, which are also predaceous, are called water tigers. There are some 500 species in North America.

The Hydrophilids (scavenging diving beetles) are smaller, but up to 1-1/2 inches. They are a diverse group with terrestrial and aquatic subfamilies, and their feeding habits range from scavenging to herbivorous and predaceous. Most are predaceous as larvae and herbivorous as adults. The body is oval or elliptical. The antennae are club-like and shorter than the maxillary palpi. There are some 225 species in North America.

According to BugGuide.net, Hydrophilids come up for air head-first, while Dysticids come up for air tail-first. Hind legs move alternately in Hydrophilidae, together, like oars, in Dysticidae.