COLYMBETINAE Erichson, 1837
Includes a few medium-sized water beetles, several are common and widespread and should soon be found when sweeping among marginal vegetation.
This small subfamily includes about 130 species in 11 genera although this is likely to change as the largest genus, Rhantus Dejean, 1833 with about 100 species, is under constant revision and will split in due course. Historically the group has included a diverse range of medium sized dytiscid groups that are now treated as distinct subfamilies e.g. Copelatinae, Agabinae and Hydrotrupinae, but even in the modern narrow sense it is unlikely remain intact. Certain other groups e.g. Matini and Coptotomini are still variously accepted as tribes but for this discussion, and so far as the European fauna is concerned, members of the subfamily are included in the Colymbetini Erichson, 1837. The group has a cosmopolitan distribution, mostly due to the very widespread genus Rhantus which is notable for its ability to colonize islands throughout the world, with the greatest diversity in the Holarctic region. The only other widespread genus is Colymbetes Clairville, 1806 which is Holarctic in distribution and includes 21 species. The Nearctic fauna includes 22 species of 4 genera; 10 species of Rhantus and 9 species of Colymbetes Clairville, 1806, as well as two genera restricted to the region, the monotypic Hoperius Fall, 1927 and Neoscutopterus Balfour-Brown, J., 1943 with 2 species. The European fauna is roughly equal in diversity with 22 species of four genera; 5 species of Colymbetes, 11 species of Rhantus, the 4 species of the Mediterranean genus Meladema Laporte, 1835 and the monotypic Melanodytes Seidlitz, 1887 from Italy and the Balkans.
The remaining genera, at least according to the present narrow perception of the group, are much more restricted in distribution. The monotypic Anisomeria Brinck, 1943 is endemic to Juan Fernandez Islands, about 660 km west of the coast of Chile. The monotypic Carabdytes Balke, Hendrich & Wewalka, 1992 is endemic to New Guinea. The monotypic Bunites Spangler, 1972 is Neotropical. Rugosus Garcia, 2001 includes 2 species from Venezuela. The monotypic Senilites Brinck, 1948 is endemic to Tristan da Cunha in the South Atlantic. These are variously included within distinct tribes e.g. Anisomeriini Brinck, 1948 and Carabdytini Pederzani, 1995 and the final classification of the ‘subfamily’ is likely to change, but so far as our European fauna is concerned things seem to be stable. Nartus Zaitzev, 1907, formerly a subgenus of Rhantus but now variously considered sufficiently distinct to form a separate genus, includes one widespread Nearctic species and the familiar European N. grapii (Gyllenhal, 1808) which extends to the UK. Species of Nartus differ from those of Rhantus in being entirely dark, having sharply produced posterior pronotal angles and the labrum weakly emarginate medially. Our UK list also includes a single species of Colymbetes and 5 species of Rhantus although R. bistriatus (Bergsträsser, 1777) was last recorded in 1904 and is now considered extinct here.
Members of this subfamily are medium sized water beetles, 8-20mm in length, and typical of the family; oval to rather elongate and usually rather depressed, most are pale to dark brown with various patterns or maculation and microsculpture. The group is broadly defined as follows. Head transverse and smoothly convex, sometimes with a transverse impression across the base, eyes relatively large, continuous in outline with the lateral margin of the head and incised anteriorly, clypeus with distinct oval to transverse and widely separated fovea, antennae and palps simply filiform. Pronotum transverse, widest at the base and narrowed to projecting anterior angles, posterior angles acute and basal margin curved or weakly bisinuate, surface finely punctured and microsculptured and the lateral margins with or without a raised border. Colour very variable but often pale with darker bands to the anterior or posterior margins and/or dark maculae to the disc. Prosternal process moderate to very long, more or less level with the apex of the prosternum and rounded to sharply pointed apically, the surface flat or convex and without a median groove. Scutellum always relatively large. Metasternum either flattened or impressed anteriorly to accommodate the prosternal process. Metasternal wing variable but usually relatively broad, metacoxal lines always present and diverging anteriorly but becoming obsolete and not reaching the metasternum, the space between relatively broad throughout. Elytra usually with 2 or 3 rows of larger puncture which may be emphasized by rows of dark spots, colour very variable but often pale with dark mottling that may form intricate patterns, epipleura broad under the shoulders but narrowing and usually only narrowly visible beyond the middle. Legs slender and moderately long, the femora hidden from above and the pro- and mesotibiae short and only moderately expanded apically. Hind femora may have scattered setae but they lack the small row or ‘comb’ of setae on the posteroapical angle seen in members of the Agabinae. Dorsal surface of metatibiae with a row of bifid setae, in the male also with swimming hairs. Basal metatarsomeres lobed apically, sometimes only weakly so, and in the male with swimming hairs along the hind margins. Claws unequal in length. Males may be distinguished by the basal pro- and mesotarsomeres which are dilated, at least to some extent, and clothed ventrally with adhesive setae.
Among our UK fauna the group is easily distinguished; from Copelatus by the widely separated metacoxal lines (although Copelatus is safely separated using habitus pictures), from Dytiscinae by the incised inner margin of the eyes and from Agabinae by the absence of a metafemoral comb. The only other water beetle of comparable size, Hygrobia, is abundantly different. Colymbetes fuscus (Linnaeus, 1758) is distinguished from our other water beetles by the combination of its large size and very strong transverse elytral microsculpture. Nartus grapii (Gyllenhal, 1808) is our only uniformly black member of the subfamily and as such is likely to be confused with various Agabinae in the field. All our species of Rhantus have a bicoloured pronotum and mottled elytra, and with a little experience they will be immediately recognized in the field although Agabus nebulosus (Forster, 1771) might confuse the inexperienced. In R. exsoletus (Forster, 1771) and R. suturellus (Harris, 1828) the dark markings are confined to the basal or apical pronotal margins while in R. frontalis (Marsham, 1802) and R. suturalis (Macleay, 1825) the disc has a central dark macula and sometimes less pronounced lateral macula.
Colymbetes fuscus is generally common throughout the UK including all the islands. Nartus grapii is locally common in Ireland, South Wales and south and central England although it is more prevalent in the east and almost absent from the West Country. Three of our Rhantus are widespread and locally common while R. frontalis occurs across Ireland but has a disjunct mainland distribution, occurring in southern and eastern England and southern Scotland with only very occasional records elsewhere. Adults of all species occur year-round and peak in abundance during the summer. Typical habitats are small to medium sized stagnant water bodies, often with plenty of vegetation and at least partly shaded, although R. suturellus is also typical of thinly vegetated upland ponds and channels on peat. Without a great deal of experience, identification in the field is very difficult and specimens should be examined critically under the microscope, fortunately there are several very good keys that work with external morphology and colour and so dissection is not necessary.