Hydrophilidae is the second largest family of North American aquatic beetles, including more than 250 species. The larvae and adults are aquatic or semiaquatic; the pupae are terrestrial. They prefer lentic waters such as ponds and shallow lakes, or sometimes pools at edges of streams. Unlike most beetles, the adults of this family tend to swim by alternating strokes with their hind legs, rather than moving them in unison. Also atypical, they have a habit of surfacing for air head first, breaking the water surface tension and accessing air with their specially formed antennae. The adults have a well-developed plastron to hold air underwater; in well-oxygenated water this thin layer of air serves as a gill. Larvae frequently surface to take in air from the end of the abdomen. Larvae are usually predators of other invertebrates, and can even break snail shells with their strong jaws. However, only a small number of adults are predators, most are omnivores or detritivores.
Mid-Atlantic: 5 - 10
Midwest: 1.9 - 6.7
Southeast: 6.5 - 9.8
Engulfer / Predator
+ Expanded Character List
Order: Larvae: Usually without lateral abdominal gills. If gills present, then 4 hooks clustered on segment 10. Thoracic legs each usually with 4 or 5 segments and with 1 or 2 claws; if without legs, head distinctly sclerotized and posterior body (thorax and abdomen) simple, without gills, hair brushes, suckers, or breathing tube. Eye spots usually present, but compound eyes absent.
Family: Each mandible with 1 to 3 teeth along inner margin. Labrum and clypeus not separated by distinct suture. Antennae each 2- or 3-segmented. Head with simple-eyes in groups of 5. Legs each 3- or 4-segmented, with single claw; legs adapted for walking, sometimes small. Thoracic tergites clearly defined. Abdominal tergites without dorsal lobes. Abdomen usually 8-segmented. Body roughly cylindrical or spindle-shaped, not markedly thick. Head and legs visible in dorsal view.