Hydrophiloidea

Spercheus is a genus of aquatic beetles which are placed in a family of their own, Spercheidae within the Hydrophiloidea. About 20 species are known from around the world except the Nearctic with the majority being from the Oriental and Afrotropical Realms.

The Cupedidae are a small family of beetles, notable for the square pattern of "windows" on their elytra, which give the family their common name of reticulated beetles.

The whirligig beetles are water beetles, comprising the family Gyrinidae that usually swim on the surface of the water if undisturbed, though they swim underwater when threatened. They get their common name from their habit of swimming rapidly in circles when alarmed, and are also notable for their divided eyes which are believed to enable them to see both above and below water. The family includes some 700 extant species worldwide, in 15 genera, plus a few fossil species. Most species are very similar in general appearance, though they vary in size from perhaps 3 mm to 18 mm in length. They tend to be flattened and rounded in cross section, in plain view as seen from above, and in longitudinal section. In fact their shape is a good first approximation to an ellipsoid, with legs and other appendages fitting closely into a streamlined surface. Whirligig beetles belong to the beetle suborder Adephaga, which also includes ground beetles and diving beetles.

The Trachypachidae are a family of beetles that generally resemble small ground beetles, but that are distinguished by the large coxae of their rearmost legs. There are only six known extant species in the family, with four species of Trachypachus found in northern Eurasia and northern North America, and two species of Systolosoma in Chile and Argentina. They were much more diverse in the past, with dozens of described species from the Mesozoic.

The Dytiscidae – based on the Greek dytikos (δυτικός), "able to dive" – are the predaceous diving beetles, a family of water beetles. They occur in virtually any freshwater habitat around the world, but a few species live among leaf litter. The adults of most are between 1 and 2.5 cm (0.4–1.0 in) long, though much variation is seen between species. The European Dytiscus latissimus and Brazilian Megadytes ducalis are the largest, reaching up to 4.5 cm (1.8 in) and 4.75 cm (1.9 in) respectively. In contrast, the smallest is likely the Australian Limbodessus atypicali of subterranean waters, which only is about 0.9 mm (0.035 in) long. Most are dark brown, blackish, or dark olive in color with golden highlights in some subfamilies. The larvae are commonly known as water tigers due to their voracious appetite. They have short, but sharp mandibles and immediately upon biting, they deliver digestive enzymes into prey to suck their liquefied remains. The family includes more than 4,000 described species in numerous genera.

Hydrophilidae, also known colloquially as water scavenger beetles, is a family of beetles. Aquatic hydrophilids are notable for their long maxillary palps, which are longer than their antennae. Several of the former subfamilies of Hydrophilidae have recently been removed and elevated to family rank; Epimetopidae, Georissidae, Helophoridae, Hydrochidae, and Spercheidae. While the majority of hydrophilids are aquatic, around a third of described species are terrestrial, mostly belonging to the subfamily Sphaeridiinae.

The Cerophytidae are a family of beetles belonging to Elateroidea. Larvae are associated with rotting wood, on which they are presumed to feed. The family contains over 20 species in five genera, primarily distributed in the New World, but also in Eurasia and Africa. 17 fossil species in 7 genera are known extending to the Early Jurassic. Like some other elateroids, the adults are capable of clicking.

Anaxyelidae is a family of incense cedar wood wasps in the order Hymenoptera. It contains only one living genus, Syntexis, which has only a single species, native to Western North America. Fossils of the family extend back to the Middle Jurassic, belonging to over a dozen extinct genera, with a particularly high diversity during the Early Cretaceous. Syntexis lay eggs in the sapwood of conifers, preferring recently burnt wood.

Pelecinidae is a family of parasitic wasps in the Proctotrupoidea. It contains only one living genus, Pelecinus, with three species known from the Americas. The earliest fossil species are known from the Jurassic, and the group was highly diverse during the Cretaceous. Members of Pelecinus are parasitic on larval beetles, flies, green lacewings, and sawflies.

Histeroidea is a superfamily of beetles in the infraorder Staphyliniformia.

Helophorus ís the only genus in the beetle family Helophoridae within the Hydrophiloidea. They are small insects, found mainly in the Holarctic region, but two or three species also live in the Afrotropical region, Central America and one in the Indomalayan region.

Nymphidae, sometimes called split-footed lacewings, are a family of winged insects of the order Neuroptera. There are 35 extant species native to Australia and New Guinea.

Coptoclavidae is an extinct family of aquatic beetles in the suborder Adephaga. The Coptoclavidae lived from the Late Triassic to the Early Cretaceous. Coptoclavidae is a member of the adephagan clade Dytiscoidea., which contains other aquatic beetles. Suggested reasons for their extinction to include the rise of teleost fish, or competition with Gyrinidae and Dytiscidae, which possess defensive secretions and sucking channels in the mandibles of larvae, which coptoclavids likely lacked. It has been suggested that the genus Timarchopsis and the subfamily Timarchopsinae are only distantly related to other coptoclavids based on cladistic analysis, with Timarchopsis being more closely related to geadephagans like carabids and trachypachids instead. Another study also suggested similarly for Coptoclavisca and possibly other coptoclaviscines.

Hydrophilus is a genus of beetles in the family Hydrophilidae, the water scavenger beetles. There are 53 species in three subgenera in the genus: Hydrophilus, Dibolocelus, and Temnopterus.

Omma is a genus of beetles in the family Ommatidae. Omma is an example of a living fossil. The oldest species known, O. liassicum, lived during the final stage of the Triassic (Rhaetian), over 200 million years ago, though the placement of this species in Omma has been questioned. Numerous other fossil species are known from the Jurassic and Cretaceous of Europe and Asia. The only living species is Omma stanleyi, which is endemic to Australia. Three other extant species endemic to Australia that were formerly part of this genus were moved to the separate genus Beutelius in 2020.Omma stanleyi is strongly associated with wood, being found under Eucalyptus bark and exhibiting thanatosis when disturbed. Its larval stage and many other life details are unknown due to its rarity. Males are typically 14–20 mm in length, while females are 14.4-27.5 mm. Omma stanleyi occurs throughout eastern Australia from Victoria to Central Queensland.

Mesoraphidiidae is an extinct family of snakeflies in the suborder Raphidiomorpha. The family lived from the Late Jurassic through the Late Cretaceous and is known from twenty-five genera. Mesoraphidiids have been found as both compression fossils and as inclusions in amber. The family was first proposed in 1925 by the Russian paleoentomologist Andrey Vasilyevich Martynov based on Upper Jurassic fossils recovered in Kazakhstan. The family was expanded in 2002 by the synonymizing of several other proposed snakefly families. The family was divided into three subfamilies and one tribe in a 2011 paper, further clarifying the relationships of the included genera.

Helophorus sibiricus is a species of water scavenger beetle in the family Hydrophilidae. It is found in Northern Europe, Northern Asia, and Alaska. Fossils from Early Miocene sediments of the Irtysh River in western Siberia have been reliably assigned to this species, which means that Helophorus sibiricus existed already 16-23 million years ago.