Hydrophilidae, also known colloquially as water scavenger beetles, is a family of chiefly aquatic beetles.[1][2] Aquatic hydrophilids are notable for their long maxillary palps, which are longer than their antennae.[3] Several of the former subfamilies of Hydrophilidae have recently been removed and elevated to family rank; Epimetopidae, Georissidae (= Georyssinae), Helophoridae, Hydrochidae, and Spercheidae (= Sphaeridiinae).[1][4] Some of these formerly-included groups are primarily terrestrial or semi-aquatic.[citation needed]
The vernacular name water scavenger beetles is not an accurate description of their habit.[citation needed] With rare exceptions, the larvae are predatory while the adults may be vegetarians or predators in addition to scavenging.[5] Many species are able to produce sounds.[citation needed]
Species of Hydrophilus are reported as pests in fish hatcheries.[5] Other species are voracious consumers of mosquito larvae, and have potential as biological control agents.[5][6]There are 2,835 species in 169 genera.[7]
Hydrophilid beetles are found in various locations throughout the Western Hemisphere. They are usually found in areas of high humidity such as the tropics of Central and South America. They can also be found in rain pools and ponds in the forests of Guatemala and Argentina.[8] They also tend to exist in North America in areas with seasonal wetlands or lakes depending on the state. Some hydrophilids are even found in areas of Europe.[9] Hydrophilus triangularis is found widely throughout the United States and is the biggest water beetle in the country. Maine, Minnesota, New Hampshire, Vermont, and Wyoming are the only US states where hydrophilids haven’t been found.[10]
The oldest known fossils definitively assignable to the family are from the Late Jurassic Solnhofen of Germany and Talbragar Fish Bed in Australia.[11]
A majority of the beetles in the family Hydrophilidae live in aquatic environments in both their larval and adult life stages. Some hydrophilid beetles will lay their eggs in ephemeral ponds and puddles where the larvae will live as they develop.[12] Other beetles such as Derralus angustus and Tropisternus setiger live in permanent ponds. Some beetles such as Tropisternus lateralis will only live in aquatic environments that lack fish because fish prey on their eggs, while others like D. angustus prefer aquatic habitats with a specific species of floating fern.[13] Generally, hydrophilids live in marshy, shallow, and heavily weeded aquatic environments.[14] There are some hydrophilid beetles that make their homes in fresh animal waste, decaying vegetation, or humus-rich soil.[15] They survive in a very wide variety of locations and because of that some types are more adapted to specific environments than others and will often only move to habitats of the same type. The aquatic hydrophilids are less diverse than the terrestrial hydrophilids.[16]