Hydrophilidae is the second largest family of North American aquatic beetles, including roughly 200 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 in 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 with a clubbed antenna, the apical segments of which are covered with dense water-repelling (hydrophobic) hairs. The adults have a well-developed plastron to hold air underwater; in well-oxygenated water, it 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
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+ Expanded Character List
Order: Adults with hardened forewings (elytra) covering the hind wings.
Family: Maxillary palpi usually longer than antennae. Antennae about as long as head, with 2nd segment attached at apex of 1st (basal-most) segment; usually with 5 segments before cupule (cup-shaped, hairless, segment 6); segments 7–9 clubbed. Pronotum usually not narrower than elytra; if narrower, then scutellum long and triangular. Elytra concealing entire abdomen or exposing only part of 1 segment. Hind coxae not extending in the posterior direction to divide abdominal sternite 1. Metacoxae nearly touching. Ventrally, longitudinal keel narrow between coxae. Tarsal formula: 5-5-5 or 5-4-4. Ventrally, abdomen with 5 visible segments.