HYDROPHILIDAE Latreille, 1802
Water Scavenger Beetles
Includes many common wetland and dung pasture species. A good variety of species can be found by collecting in these habitats throughout the year.
The Hydrophilidae has variously included several other groups that are now generally accepted as distinct families, Helophoridae Leach, 1815, Epimetopidae Zaitzev, 1908, Georissidae Laporte, 1840 and Hydrochidae Thomson, 1859. In this widest sense the group included more than 2800 species and about 170 genera in 8 subfamilies. A currently accepted definition of the family includes 4 subfamilies of which 2, the Hydrophilinae Latreille, 1802 and the Sphaeridiinae Latreille, 1802 are Holarctic and well represented in Europe.
The Horelophinae Hansen, 1991 includes the single genus Horelophus Orchymont, 1913 of which one species is known. Although distinctly different, it was originally placed in the Helophoridae and then later in the Hydraenidae, the species is recognizably hydrophilid. A small, flat and elongate beetle, 2-3mm in length. Entirely dark, but for pale margins to the pronotum and the elytra, the upper surface is shiny and moderately punctured. The eyes are large and convex and the anterior margin of the clypeus broad and straight, the palps much shorter than the antennae. The antennae are dimorphic; 9-segmented with a 3-segmented club, the last of which is much longer in the male. The pronotum is transverse, broadest in front of the middle and sinuate laterally, with two depressions either side of the middle. The elytra are randomly punctured and depressed across the middle. The legs and elytral apices are pale. The species is known only from the South Island of New Zealand.
The Horelophopsinae Hansen, 1997 includes 2 species of Horelophopsis Hansen, 1997, one from New Guinea and one from Japan. They are dark, oval and convex species, weakly constricted between the pronotum and the elytra. The head is smooth, punctured and has a Y-shaped impression on the frons, the palps are as long as the antennae. The pronotum is convex and smooth with a central and two basal depressions, rounded laterally and broadest at the middle. The elytra have regular longitudinal puncture series.
POLYPHAGA Emery, 1886
HYDROPHILOIDEA Latreille, 1802
The two remaining subfamilies are readily separated:
Basal segment of the meso- and meta-tarsi shorter than the second segment. Tarsal formula 5-4-4. Aquatic species.
Basal segment of the meso- and meta-tarsi longer than the second segment. Tarsal formula generally 5-5-5. Aquatic and (mostly) terrestrial species.
The Hydrophilinae is a large and cosmopolitan group of more than 1600 species divided among 6 tribes, 5 of which are represented in the U.K. All are aquatic insects typified by the various groups constituting the U.K. fauna. Size varies widely, 1-50mm, although most are <10mm. Many superficially resemble the Dytiscidae with a streamlined appearance; oval and more or less smoothly continuous in outline, convex above but, unlike the dytiscids, flat below. The upperside is glabrous. Head usually prominent with the eyes either convex and protruding e.g. in Berosus species, or smoothly continuous with the outline e.g. Enochrus species. A feature characterizing the group is the palps which are as long as, and often longer than, the antennae. The antennae are 7-9 segmented with a 3-segmented and pubescent club. The pronotum is smooth and variously punctured, lacks depressions and is broadest at the base. The elytra are smooth, rounded in outline and glabrous although the punctures may bear setae or bristles, and variously punctured and striate. The legs sometimes have long swimming hairs. Tarsi generally 5-5-5; in Cymbiodyta species 5-4-4 and Berosus species are dimorphic; 5-5-5 in the female and 4-5-5 in the male. The only non-British tribe, the Sperchopsini Hansen, 1991, includes about 25 Asian and Nearctic species.
Many species live in marginal situations, especially in well vegetated areas and even more so where this is patchy; marshes , ponds and slow-moving water, shallow environments are preferred because of the respiratory behaviour. In the absence of permanent water bodies many species can be found in temporary pools. Because many species inhabit shallow waters they can be heavily affected by avian and fish predation; in seasonal wetlands in California they are an important food source for waterfowl. All species swim with alternate movements of the mid- and hind-legs and renew their air supply head first (unlike dytiscids) at the water surface, in most cases the antennae being modified in some way to facilitate this. In many cases the adults are long lived, 2-3 years, and produce a single generation each year. Mating and oviposition generally occur in water; the females of some species carry the eggs beneath the abdomen but most enclose them in a densely woven silk cocoon which has one end elongated into a ‘mast’ which helps with respiration. Cocoons are usually attached to emergent vegetation just above the water or in marginal areas near the water’s edge. The eggs generally hatch within a week or two. Larvae are usually entirely carnivorous and spend most of their time searching for prey; they consume a wide range of organisms although some are specialist feeders and those of Helochares Mulsant, 1844 are herbivores. They are free swimming and have well-developed and serrated mandibles, sometimes prey is taken out of the water to immobilize it. Most breath atmospheric oxygen through spiracles on the apex of the abdomen but some have also developed gills e.g. Berosus species have thoracic gills which allow them to remain submerged longer and live in deeper water. Pupation occurs out of the water, generally among marginal vegetation or in cells below the soil surface. Adults of most species can fly and will often disperse by flight after eclosion. They are mostly vegetarian but will occasionally take invertebrates or scavenge carrion etc.
Hydrophilinae are distinctive; they lack the pronotal grooves of the Helophoridae and the base of the pronotum is as wide as that of the elytra so separating them from the Hydrochidae and the Georissidae. Some, or even many, superficially resemble dytiscids but are distinguished by, among other things, the form of the palps and the antennae. The only tribe not represented in Europe is the Sperchopsini Hansen, 1991 which contains 5 genera and about 25 species. They occur in North America, Canada, Alaska and Asia. The following few points relating to the various tribes are intended to put the U.K. fauna into some context but it must be remembered that a familiarity of our fauna will allow the majority of the world fauna to be recognized and appreciated. Classification is not settled and many other systems can be found, more especially as the group seems to be the subject of extensive worldwide study.
Berosini Mulsant, 1844 A large group with about 350 species in 5 genera. Cosmopolitan with many Neotropical species. The genus Berosus Leach, 1817 includes >260 species worldwide with 17 from North America and 6 from Canada. All are aquatic and very distinct species.
Chaetarthriini Bedel, 1881 contains both Holarctic and tropical species. Includes the U.K. genus Chaetarthria Stephens, 1833; 2 U.K. species, total >40. Aquatic or marginal species, adults are often found buried in the substrate. Oval and convex with the first abdominal sternite lined with long golden pubescence which covers the second sternite.
Anacaenini Hansen, 1991 is a cosmopolitan group containing >250 species. The genus Anacaena Thomson, 1859 contains about 110 species of which only 4 are Nearctic including 2 of the U.K. species, A. limbata (Fabricius, 1792) and A. lutescens (Stephens, 1829).
Laccobiini Bertrand, 1954 species are distinctive; small, up to 5mm with numerous lines of punctures on the elytra and without spines on the metasternum. Palps robust and about as long as the antennae, last segment longer than the penultimate. 12 genera. Laccobius Erichson, 1837 is a widespread genus of about 250 species included in 10 subgenera, 8 species are British and many occur in Central America. The genus Oocyclus Sharp, 1882 includes a single Central American species.
Hydrophilini Latreille, 1802 is sometimes divided into 2 tribes; the hydrophilini and the Acidocerini Zaitzev, 1908. This tribe contains up to 22 genera worldwide including several that occur in the U.K. Enochrus Thomson, 1859 is the third largest genus of the family with about 220 species in 6 subgenera worldwide, Cymbiodyta Bedel, 1881 contains 31 Palaearctic and Nearctic species, and Helochares Mulsant, 1884 is cosmopolitan with>180 species in 5 subgenera. Larger species are included in the genera Hydrochara Berthold, 1827 with 23 Holarctic and African species, and Hydrophilus Geoffroy, 1762 with about 50 species in 3 subgenera.
HYDROPHILINAE Latreille, 1802
SPHAERIDIINAE Latreille, 1802
Most members of this subfamily are terrestrial, inhabiting various types of decaying organic matter e.g. dung, compost and litter etc. Only a few genera are aquatic, in the U.K. the genus Coelostoma Brullé, 1835 and some species of Cercyon Leach, 1817. The majority of species are small, <5mm and rather nondescript but nonetheless typically hydrophilid with the palps as long as the clubbed antennae. Typical colouration is drab, black or various shades of brown, but red or yellow markings, especially to the elytra, are not uncommon. The elytra are often striate or with rows of punctures and are generally glabrous. The tibiae often have short spines or teeth. Five tribes are generally recognized and our 6 U.K. genera are representative of the group.
Protosternini Hansen,1991 includes 4 genera of mostly Oriental species, none of which occur in the U.K. Includes the myrmecophilous genus Sphaerocetum Fikácek, 2010 with a single Malaysian species S. malaganum Fikácek, 2010.
Coelostomatini Heyden, 1891 comprises >200 species in 17 genera. Includes Dactylosternum Wollaston, 1854 with a worldwide distribution and about 65 species, and the large Holarctic genus Coelostoma Brullé, 1835.
Omicrini Smetana, 1975 includes 15 genera of tiny Holarctic and New World species, also some native and introduced to Hawaii. Some Brazilian species of Omicrus Sharp, 1879 inhabit Bromeliads. Not represented in the U.K.
Megasternini Hansen, 1991 is a large tribe including at least 36 genera. A cosmopolitan group with many European species and well represented in the U.K. by the genera Cercyon Leach, 1817, a cosmopolitan genus of >250 species, Cryptopleurum Mulsant, 1844, a Holarctic group of >20 species, and Megasternum Mulsant, 1844, a Holarctic group of 6 species.
Sphaeridiini Latreille, 1802 contains the single genus Sphaeridium Fabricius, 1775 with about 20 Palaearctic species some of which have been introduced to North America. They are relatively large insects, 4-7.5mm that inhabit very wet, almost liquid, dung, moving through the medium rather more like swimming than burrowing. Both the adults and the larvae are predatory on other dung inhabiting insects and their larvae. The antennae are 8-segmented which is unique among the U.K. fauna.