|Latreille , 1802|
The water beetles (Hydrophilidae), also called piston water beetles or water friends, are a family of beetles. There are different views about their delimitation. Some researchers prefer a narrow demarcation that essentially includes the subfamilies Hydrophilinae and Sphaeridiinae. The subfamilies Helophorinae, Georissinae, Hydrochinae and Spercheinae would be independent families. According to the more traditional view, these are still run as subfamilies. This broad demarcation is followed here. The family accordingly comprises 3335 species in 176 genera (as of 2010).
The majority of all hydrophilids develop in water, and most of them also live there as adults . The Georissinae live in humid habitats on the shore zone . The representatives of the subfamily Sphaeridiinae mainly live in dung and putrefactive substances without reference to water. In all groups, however, there are a number of exceptions with a different way of life.
Most hydrophilids have a more or less flattened, oval body. The head, pronotum and elytra usually form a closed contour, the pronotum is widest at the base (that is the side towards the elytra) and narrows in an even curve towards the head, it is almost always significantly wider than it is long. The rear edge usually has an additional inner edge that connects it firmly and immovably to the wing covers. Normally, the underside of the anterior breast segment (Prosternum) is also extended backwards into a strong process that engages in a recess in the mid-breast (exception: Georissinae). In connection with the mostly smooth, hairless or slightly hairy surface, the water resistance is reduced (streamlined shape). In some subfamilies such as the Helophorinae or the Georissinae, the pronotum is narrowed at the base and the body contour is thus constricted, these groups have no swimming ability. The head always sits close to the pronotum, mostly it is drawn in up to the eyes. Many species without buoyancy are short-oval and highly domed with a flat underside, but usually without the ability to ball. In contrast to the mostly slightly hairy top, the entire underside is usually covered with very fine and dense, water-repellent hair that supports breathing (see below).
The legs, which are usually strong, usually have five, rarely only four feet (tarsal) limbs. In the case of forms that live in water and are able to swim, a loose hem of elongated webbing hair is usually pronounced on the middle and rear legs. Most Hydrophilidae are relatively poor swimmers and only colonize stagnant or poorly flowing water. The legs of most species are flattened and have a recess on the inside into which the rails can be inserted. Some species also have antennae pits on the front chest. Almost all species have fully developed hind wings and are often good fliers.
The special feature of all members of this family are the seven- to nine-part bulb-like antennae, to which they owe their name. The last three links form a velvety, hairy club (in some groups one or two more links are attached to the club). The buttons on the lower jaws and lip are as long as the feelers or longer. This reversal of the usual length ratios goes hand in hand with a partial takeover of the sensor function (smell and taste) by the buttons, since the sensors are used for breathing. Breathing air is stored under the wing covers and in the fine, water-repellent hair on the abdomen; this also enables longer immersion through oxygen absorption in the air cover from the water ("physical gill" or plastron breathing ). To get air, the beetle touches the surface of the water with the left or right side of the head, sometimes changing sides in quick succession. The antenna of the respective side is placed on the surface of the water, along with the air is sent with antenna vibration and pumping movements of the abdomen over a fine hairline on the head side, covered by the folded down last antenna elements, to the fore chest and from there over the whole underside of the body and into the Tracheal trunks headed.
On the beetle's abdomen, five, rarely only four, belly plates (sternites) are visible; the tergites hidden under the wing covers are usually seven, i.e. H. two more to which no sternites correspond.
Development and larvae
Hydrophilids are separate sexes, and only a few species (e.g. Anacaena lutescens ) have been shown to have parthenogenic reproduction as an alternative . The females of almost all species have a spinning device at the end of the abdomen, with which a cocoon is spun, in which the fertilized eggs are deposited individually or in small clusters. The cocoon consists of a two-layer protective cover made of proteins (silk), it often has an artful structure that is typical for the species and can be used for identification. A chimney-like structure on the cocoon is used by many aquatic species or species that live in other moist substrates to renew the air they breathe for the brood. The cocoon is usually attached to stones or plants, with a few Hydrophilinae (e.g. Hydrophilus and Hydrochara ) it floats freely in the water. Other genera (e.g. Spercheus and Helochares ) carry it around with them attached to the abdomen of the abdomen until the larvae hatch.
Almost all species have three larval stages, a few species (in the genera Helophorus , Georissus and Sphaeridium ) exceptionally only two. The larval development is usually relatively rapid, usually one to two months. Almost all species in all subfamilies have predatory larvae, while the adults are partly herbivores (phytophag). Many groups that live in manure or wet detritus primarily hunt fly maggots. The larvae are usually elongated with the head stretched forward (prognathic) or slightly tilted downwards. They are usually only slightly sclerotized (especially the abdomen) and often predominantly whitish in color. On the side of the head there are usually five or six button-shaped larval eyes (stemmata or ocelli). The mandibles are redesigned to catch prey and have receded the basal chewing surface (mola) and the mobile, finger-like appendage (prostheca); they are often designed as crescent-shaped and often multi-toothed pincers protruding forward. The larvae also usually have long, four-membered maxillary palps that are as long as the antennae or even a little longer. Most species have relatively long, five-limbed legs, but there are species with partially or completely reduced legs. The abdomen is ten-segment and usually quite elongated, often it is curled secondary by skin folds. At the rear end of the tenth segment sit two usually short, one -parted appendages, the urogomphi (with Helophorus they are long and tripartite), these are sensory organs that warn of enemies approaching from behind. On the sides of the abdomen there are sometimes pointed projections, in the genus Berosus long, thread-like tracheal gills. Usually the first larval stage rises to the surface to take in air. Many hydrophilid larvae have a pair of open stigmas on the abdomen (on the eighth segment), which open into a small, air-filled vestibule (atrium). They usually sit in ambush in the tangle of plants in the area of the water surface, so that they can breathe air through the atrium, but otherwise also breathe submerged through the skin. The larvae of the soil-living Helophorinae, Epimetopinae and Georissinae are all air breathers with open stigmas.
Hydrophilids pupate outside of the water. For this, the last larval stage runs to the bank and digs a small pupal chamber in the leaves or soft soil, in which pupation takes place. The pupae of the hydrophilids are usually white in color with red eyes. They usually have numerous pin-shaped appendages with a bristle at the end, called styli, on the head, torso and abdomen. Their function is probably to prevent too close contact with the ground (with the risk of fungus growth). The number and position of the styli are genre-specific.
Most hydrophilids have one generation per year. In temperate latitudes such as Central Europe, the overwintering stage is usually the beetle (imaginal overwintering).
Way of life and distribution
The way of life of the hydrophilids is quite different depending on the subfamily and also variable within the subfamilies. The following list provides an overview.
- Helophorinae: The Helophorinae live in the northern hemisphere with the main distribution in the Palearctic , some species also in North America ( Nearctic ) and in northern Africa ( Ethiopian ) and in the northernmost section of the Oriental in East Asia. Of the approximately 200 species of the single genus Helophorus , 30 also live in Central Europe. Most species live aquatically in stagnant water (e.g. Helophorus aquaticus ), but a number of species are terrestrial and ground-living without any relationship to water (sub-genus Empleurus ), so they usually prefer habitats with little vegetation. The aquatic species place their egg cocoons in the water, with the extension extending like a straw to the surface of the water. The larvae are predators and little is known about the nutrition of the adults. Some of the terrestrial species are herbivores that have even been reported sporadically as agricultural pests. The aquatic beetles live on the ground without swimming ability.
- Epimetopinae: The approx. 25 species of this subfamily live in America, North Africa and East Asia. They live in different types of water.
- Georissinae : The 70 species of the only genus Georissus are distributed worldwide, three species also occur in Central Europe. Larvae and adults live in the damp bank mud on the edge of water. The beetles, which are only two millimeters long, walk on the surface, they are usually well camouflaged with a coating of dried mud. The larvae are predators, v. a. from fly maggots, which beetles eat decomposed plant material.
- Hydrochinae : The only genus Hydrochus is distributed worldwide, it comprises about 90 species, five of them also in Central Europe. The beetles and their larvae live in the bank zone of mostly small stagnant bodies of water between aquatic plants, on which the beetles feed.
- Spercheinae: The 20 species of the single genus Spercheus live almost worldwide, but are absent in North America. The only Central European species is Spercheus emarginatus . Both the beetles and their larvae live in plant-rich waters, where they often hang from below on the surface membrane of the water. As an exception with the hydrophilids, not only the beetles themselves, but also the larvae are herbivores. It has been reported how they can use specialized bristles to sweep up floating algae and filter them out of the water. This filtering way of life is unique among the beetles. In addition, there is also evidence of the diet of living and dead insects, including cannibalism.
- Horelophinae: This subfamily comprises a single genus with a single species: Horelophus walkeri . They only live in New Zealand (South Island), between moss on overcast rocks at the edge of flowing waters.
- Horelophopsinae. This subfamily was established by Michael Hansen after a species from New Guinea, of which only a single specimen existed. A second species has now been found in Japan. According to the morphology of the larvae, the classification in the rank of a subfamily is rather doubtful
- Hydrophilinae. This large subfamily with worldwide distribution mainly includes aquatic species, few genera live in humid land habitats such as riparian zones, e.g. B. Chaetarthrium and some Anacaena species. They live in stagnant as well as flowing waters, but clearly prefer areas with reduced currents, e.g. B. quiet bays and the bank areas. Only a few species, especially the larger forms of the genus Hydrophilus and their relatives (Tribus Hydrophilina), have quite good swimming ability as larvae and adults, most crawl over the bottom of the water or prefer the overgrown shore zone. The larvae partially digest their prey through digestive juices released to the outside ( extraintestinal ), so they bring them, if possible, above the water surface to avoid dilution. The larvae of the genus Berosus have a suction channel in the mandibles and usually do not come to the surface.
- Sphaeridiinae: Most species of this globally distributed subfamily live both as larvae and as adults in terrestrial habitats, only a few in water (e.g. some Cercyon species). They live on the ground, usually on rotting organic matter, e.g. B. plant litter, algae wadding u. and prefer moist habitats. Many species specialize in the excrement (dung) of herbivores. Their larvae are predatory, the faecal species mainly from fly maggots. Many species have numerous generations per year (multivoltin).
It is known from native water beetles that both sexes generate sounds for defense and courtship with stridulation organs by rubbing a stridulation surface on the underside of the wing covers on the sides of a back segment (tergite).
The family Hydrophilidae as defined here would be the only family within a narrowly defined superfamily Hydrophiloidea (Hydrophiloidea in the narrower sense). The predominantly rural (terrestrial) family Histeridae , together with the small families Synteliidae and Sphaeritidae, belong to the Hydrophiloidea (in the broader sense) as a sister group, some systematics alternatively see them as an independent superfamily Histeroidea. The Hydrophiloidea (including the "Histeroidea") are probably monophyletic according to all studies carried out so far, their sister group are the Staphylinoidea . The Hydrophiloidea i. e. S. and the "Histeroidea" are probably sister groups, some studies () also suggest that the Hydrophilidae family as defined here (corresponds to the subfamily in the narrower sense) could possibly be paraphyletic.
The relationships within the Hydrophilidae family have been extensively researched without any consensus among the various researchers. For this reason, some scientists completely reject an internal structure at this point in time. The most widely accepted hypothesis distinguishes a “helophorid line” from the subfamilies Helophorinae, Georissinae, Hydrochinae and Epimetopinae and a “hydrophilic line” with the subfamilies Hydrophilinae, Sphaeridiinae, Spercheinae, Horelophinae and Horelophopsinae. Both morphological and molecular studies provide clear indications that Sphaeridiinae and Hydrophilinae form a common family group. Of these subfamilies, however, only the Sphaeridiinae themselves are likely to be monophyletic and the Hydrophilinae, compared with them, are paraphyletic. In addition, a very isolated position of the genus Berosus suggests , which would probably not be related to the other Hydrophilinae.
- Large piston water beetle ( Hydrophilus piceus or Hydrous piceus )
- Small piston water beetle or spiked water beetle ( Hydrochara caraboides )
- Brown-footed water beetle ( Hydrobius fuscipes )
- Ringed water beetle ( Anacaena limbata )
- Matt colored pond beetle ( Helochares obscurus )
- Common dung ball beetle ( Sphaeridium scarabaeoides )
- Hairy dung ball beetle ( Cryptopleurum minutum )
- Helophorus aquaticus
- Helophorus grandis
- Helophorus nubilus
- Georissus crenulatus
- Bernhard Klausnitzer : Beetles in and around the water. Westarp Sciences, Magdeburg 1996, ISBN 3-89432-478-3
- Michael Hansen (1991): The Hydrophiloid Beetles: Phylogeny, Classification and a Revision of the Genera (Coleoptera, Hydrophiloidea). Biologiske Skrifter Vol. 40. (Kgl. Danske Videnskabernes Selskab) 367 pp.
- Miguel Archangelsky, Rolf G. Beutel, Albrecht Kornarek: 10.1: Hydrophilidae. In: Rolf G. Beutel, Richard Leschen (editor): Handbuch der Zoologie / Handbook of Zoology. Volume 1: Morphology and Systematics (Archostemata, Adephaga, Myxophaga, Polyphaga partim). (De Gruyter). pp.158 ff.
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- Speaking in the Tree of Life web project
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- Michael Hansen (1997): A new subfamily for a remarkable new genus and species of Hydrophilidae from New Guinea (Coleoptera: Hydrophilidae). Annales Zoologici 47 (1/2): 107-110.
- Horelophopsinae in the Tree of Life web project
- Yûsuke Minoshima, Masakazu Hayashi, Norio Kobayashi, Hiroyuki Yoshitomi (2010): Horelophopsis larvae? The unknown larvae collected with Horelophopsis hanseni Satô et Yoshitomi (Coleoptera, Hydrophilidae, Horelophopsinae). Acta Entomologica Musei Nationalis Pragae 50 (1): 8–9 (Abstracts of the Immature Beetles Meeting 2009 October 1–2, Prague, Czech Republic).
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- z. B. Short in the Tree of Life web project