Hyphydrus ovatus (Linnaeus, 1761)
This is a widespread and generally common species throughout Europe, The Middle East and Western Asia; it occurs to the far north and in Sweden is found in boreal bogs among sphagnum and Carex. It is generally common through England and Wales becoming less so further north and very local and scarce in southern Scotland. The typical habitat is permanent and well vegetated still water bodies; stagnant ponds, marshes and gravel pit margins etc. where the adults live among aquatic vegetation or bottom substrate, they also occur in brackish ponds by the coast and in the spring may occur in numbers in temporary pools. Adults occur year round, being commonest in the spring and autumn, and soft, newly eclosed specimens have been found in September. Eggs are laid on the leaves and stems of aquatic vegetation during the spring and possibly into the summer, the larvae emerge soon afterwards and all three instars live among vegetation; the first instar among the upper stems and foliage where they feed upon tiny insect larvae and the early stages of planktonic crustaceans, the second instar feeds upon larger prey lower down among the plants , and the final stage inhabits the substrate and roots where they consume diptera larvae and tubifex worms etc. Pupation occurs in late summer and new generation adults appear soon after. Adults often occur among swept samples from marginal vegetation or bottom substrate, and during warmer conditions, especially in the spring, are attracted to light in suitable situations.
4-5mm. The small size and globular form is very distinctive among our UK fauna. Most specimens are dark reddish brown but very dark individuals may occur in populations and some have indistinct elytral patterns or markings, especially around the elytral margins in teneral specimens. The head is densely punctured and finely microsculptured with large eyes and pale antennae and palps, generally with 2 depressions on the vertex. Pronotum densely and often confluently punctured in the male, sparsely so in the female where the disc is often impunctate. Scutellum not visible. Pronotum and elytra discontinuous in outline; elytra finely pubescent, in the male densely punctured with punctures of various sizes which are usually longitudinally confluent along the base, in the female fine and sparse. The female dorsal surface is strongly microsculptured and so appears dull although shiny females occasionally occur; males are always very finely microsculptured and so appear shiny. Underside and legs pale. Front and middle tarsi appear 4-segmented; the third is strongly bilobed and the fourth very small. Segments 1-3 with a fine median furrow; dilated in the male. Middle and hind tibiae and tarsi with long swimming hairs. Hind tibiae with 2 strong apical spurs, the inner spur longer. Hind tarsi obviously 5-segmented.
Hyphydrus Illiger, 1802
This large genus includes more than 140 species of small and very convex water beetles, originally divided among 4 subgenera, the species are now included in 20 ‘species groups’. With the exception of northeast Asia and the New World the distribution is worldwide with the greatest diversity in tropical regions. Some species are very widely distributed e.g. H. lyratus Swartz, 1808, as various subspecies, occurs throughout the Palaearctic, Oriental and Australasian regions; many are endemic to Africa or Australia and some occur on remote pacific islands. They are small beetles, 2.5-6.5mm, of a characteristic broad-oval and very convex form with large eyes and long appendages, and as such are distinctive among the European fauna. Among the world Hyphydrini Sharp, 1882, which is a large tribe containing about 15 genera; they are distinguished by the combination of a well-sclerotized spermatheca and the possession of a distinct and usually broad bead along the anterior margin of the clypeus. The pronotum is transverse with obtuse hind angles which are never produced backwards, and the elytra lack carinae or distinct impressions, being randomly punctured, the epipleura have a distinct and sharply-defined triangular impression towards the base to accommodate the ends of the femora when the beetle is at rest. The apex of the Prosternal process is usually lanceolate, never truncate, and extends back to the base of the metasternum. The metacoxae are large but lack lobes and so the bases of the metatrocanters are exposed. The terminal segment of the pro- and mesotarsi is very short and the metatarsal claws are unequal, the outer claw being very short and often difficult to see. Typical colouration is drab brown to yellow with darker or paler markings but some species are completely brown or black and some e.g. H. aubei Ganglbauer, 1891 have striking colour patterns. Males tend to be shiny while females are generally dull although shiny specimens are known in some species. The Palaearctic fauna includes 8 species of which 4 are known from Europe but only 2 of these are widespread, see below. H. maculatus Babington, 1841 is an Afrotropical species which occurs in North Africa and the Canary Islands, and H. anatolicus Guignot, 1957 is a local southern species distributed from Italy to Greece.
H. aubei Ganglbauer, 1892
This species has a more restricted southern and western distribution in Europe and is generally rare in central regions; it reaches the channel coast in France and this is probably the northern extent of its range, it is included on some recent UK lists on the strength of specimens from the Channel Islands during the early twentieth century but it has not been found there since. The biology is much the same as with H. ovatus; larvae develop in spring and summer and new-generation adults occur late in the year, they have been recorded from all kinds of still and well-vegetated water, more rarely from slow-moving parts of rivers and drainage ditches, and also from brackish water by the coast. Adults are on average a little smaller than those of the previous species but are otherwise very distinctive due to the strongly punctured (at least in the basal half) elytral sutural margin and the elytral pattern which consists of several series of sharply-defined longitudinal dark maculae variously connected by transverse bands.