[3] Several of the former subfamilies of Hydrophilidae have recently been removed and elevated to family rank; Epimetopidae, Georissidae (= Georyssidae), Helophoridae, Hydrochidae, and Spercheidae.
[11] They also tend to exist in North America in areas with seasonal wetlands or lakes depending on the state.
[12] Hydrophilus triangularis is found widely throughout the United States and is the biggest water beetle in the country.
[13] The oldest known fossils definitively assignable to the family are from the Late Jurassic Solnhofen of Germany and Talbragar Fish Bed in Australia.
[14] 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.
[17] There are some hydrophilid beetles that make their homes in fresh animal waste, decaying vegetation, or humus-rich soil.
The ability to consume oxygen at deeper water levels helps them avoid being preyed upon from surface predators.
[17] Larvae use two main types of feeding mechanism, chewing and piercing-sucking, with the latter evolving at least three times independently in Hydrophilidae and once in Epimetopidae and allowing underwater extra-oral digestion, decreasing dependence on aerial environments.
[21] Adult hydrophilid beetles can survive in deeper areas of water and stay under for longer due to their special abilities in acquiring oxygen.
They can pull oxygen from the water into these areas on their body so that they can avoid returning to the surface for long periods of time.
[23] Some species of hydrophilid beetles in the genus Tropisternus have complex methods of signaling and communication including chirps, clicks, buzzing, and various body postures.
Courtship behaviors have been observed in Tropisternus ellipticus in which the females produced audible buzzing and shaking to rebuff potential suitors.
Males may remember these encounters for up to 39 minutes when properly reinforced, which suggests that they have some capability for recording short-term memory.
[25] In Enochrus quadripunctatus, a species of hydrophilid beetle, it takes an average of 43 days for a newly hatched larva to reach its adult form.
Larval forms are carnivorous and may exhibit cannibalism among the individuals in a single egg case before hatching.
The third instar will last an average of 8.4 days, during which the larvae will continue feeding and become progressively slower as they reach their pupation stage.
Generally adult hydrophilids feed on various vegetation, alive or decaying, and will occasionally eat dead animal tissue.
[29] Larval hydrophilids hunt a wide variety of prey such as copepods, mosquito larvae, snails, and conspecifics.