AGABINAE Thomson, C.G., 1867
Medium sized beetles, including some of the most common water beetles in Britain. Species of Agabus and Ilybius can be found all year round and in almost any water source.
Formerly included as a tribe of the Colymbetinae Erichson, 1837 this large group includes more than 400 species in about a dozen genera and has a worldwide distribution. The greatest diversity is found across the Holarctic region and species of this subfamily form the largest proportion of dytiscid faunas in many areas across the world. Species occur in all fresh and brackish water environments but typical habitats are vegetation-rich margins of ponds and lakes, temporary pools and slow-moving water but many are adapted to particular situations and any fresh water e.g. upland tarns, cattle troughs, woodland pools, garden ponds or flooded grassland is likely to host some species. Adults of many species disperse by flight and so new habitats may quickly become colonized, especially during the warmer months, and at this time many will be attracted to light or fall onto polarized surfaces, many occur throughout the year and are readily sampled by sweeping marginal environments. Larvae will also occur regularly in the sweep-net and with a little care may be reared in aquaria. Life-cycles are very varied and generally adapted to habitat type and latitude but in temperate climates there are two basic cycles; many oviposit among vegetation during the spring, the aquatic and predacious larvae develop through the summer and pupate among marginal substrate later in the year and produce adults that will overwinter either in the water or among marginal substrate, or these adults will oviposit and produce overwintering eggs that develop in the spring. A two year cycle involves mating in the spring or summer and producing eggs or larvae that will overwinter and develop the following year to produce overwintering adults that breed the following year. Some species are very adaptable and may be found ad adults and larvae at any time of year. Both adults and larvae are predatory and a very wide range of prey have been recorded including insect larvae, small fish, worms and molluscs and most will readily devour carrion. During the summer adults many occur in large or very large numbers, especially during the warmest months when smaller water bodies shrink or dry up completely, and at such times they may be found in abundance under debris or in damp soil crevices on dry pond beds. Even with relatively small faunas such as that of the UK many species are common and widespread, and for all these reasons the subfamily is a very rewarding group to study. Beyond these practical considerations is the fact that water beetles hold a particular (and understandable) fascination for
Platambus maculatus larva
many coleopterists and so the distribution and ecology of many species are relatively well understood and there is a wealth of literature, including several good keys and many excellent photographs, available both in the literature and on line, and for those interested there is a large body of work concerning the history of the subject in the UK; the fascinating and very informative volumes by Balfour-Browne (British Water Beetles, 1940, 1950) are essential reading for any serious water beetler.
Adults are of a characteristic appearance but superficially similar to those of the Colymbetinae, they differ essentially in having a small ‘comb’ of setae near the apical angle of the inner margin of the posterior femora, in Colymbetinae these are absent or there may be one or a few scattered setae. Members of the Dytiscinae have the eyes entire while those of the present group are emarginate anteriorly, members of the Copelatinae are distinguished by the form of the posterior coxal lines. Confusion with members of the Laccophilinae Gistel, 1848 or Hydroporinae Aubé, 1836 is unlikely due to the size and habitus but in both groups the scutellum is hidden beneath the pronotum and so not visible from above, in Agabinae it is exposed and obvious. A general description of the group is as follows. Small to medium sized beetles, 5-15mm; continuous in outline but rather variable in shape from broadly oval and rounded to elongate and parallel-sided. Most are dull, black to dark or pale brown but many have pale spots to the head, pronotum and/or elytra and many have pale margins or stripes to the elytra, some e.g. Agabus nebulosus (Forster, 1771) are entirely pale with darker patterns. Body glabrous or with very fine recumbent pubescence to the elytra and a few fixed trichobothria. Punctation, including any partial elytral striae, very variable. Microsculpture usually present and often diagnostic for a group or a species. Head mostly concealed within the thorax, smooth or with only weak impressions to the vertex or frons, eyes weakly convex and usually continuous with the outline, emarginate anteriorly and widely visible from above, antennae simply filiform and palps with the apical segment entire. Pronotum transverse, broadest at the base and narrowed to the anterior margin, all angles distinct, lateral bead well-developed and surface usually simply convex. Prosternal process variable, from short and only just touching the metasternal lobe to elongate, acute and recessed into a well-developed anteromedial notch on the metasternum, metasternal ‘wings’ broad to very narrow, metasternal lines usually present. Metacoxal lobes large and rounded with lines converging at the centre then diverging around the base. Elytra continuous in outline with the pronotum, smooth in outline and continuously rounded or variously acuminate apically, epipleurs variable but usually well-developed from the base to at least the middle, without distinct striae but often with a few variously developed rows of punctures, punctation, when present, extremely fine and random, microsculpture variable from very fine and dense to large and cellular, sometimes doubled; each cell with an internal fine mesh. Hind wings variable between species, from fully developed to short-winged or apterous. Legs robust and moderately long. Pro- and meso-coxae variously convex and round or weakly transverse, trocanters large and obliquely joined to broad and flattened femora, femora often extensively punctured and with sensory setae, often constricted before the apex, tibiae broad and flattened, usually with swimming hairs and well-developed terminal spines. Hind legs long and slender with large coxae and well-developed trocanters that are often accommodated within a basal narrowing of the femora. Hind femora in most species with a short row of setae near the posterior apico-ventral angle, meta-tibiae without bifid setae on the upper surface, in most with swimming hairs on the outer margin and also on the meta-tarsomeres. Tarsi 5-segmented, the segments variously flattened and smoothly continuous in outline, in many the pro-tarsomeres are sexually dimorphic being to varying extents enlarged and equipped with ventral sucker hairs in the males. Claws usually long and only weakly curved or toothed at the base, the relative lengths and shapes of each pair of claws sometimes provides important diagnostic features.
Around the World
Because our UK fauna represents large and widespread genera, and because the group is morphologically rather narrow (in a superficial sense), a familiarity with our species offers a good insight into the group as a whole. Other genera within the subfamily are as follows but it is very likely that there will be changes in the light of modern molecular and other studies and, as the group is a popular one, more species are regularly being added. Agametrus Sharp, 1882 includes six Neotropical species and one from Central America. Andonectes Guéorguiev, 1971 includes 14 Neotropical species. Agabinus Crotch, 1873 includes two Nearctic species. Hydronebrius Jakoviev, 1897 includes 3 species from southern Asia and a single widespread species, H. cordaticollis (Reitter, 1896) from Europe and northern Asia. Hydrotrupes Sharp, 1882 is a monotypic Nearctic genus formerly classified as a distinct subfamily. Ilybiosoma Crotch, 1873 is mostly Nearctic, extending south to Central America and there are single species recorded from Asia and Africa, it includes about 16 species some of which were formerly included as a distinct group within Agabus. Leuronectes Sharp, 1882 includes 5 Neotropical species. Platynectes Régimbart, 1879 is divided into 3 subgenera; the monotypic Australonectes Guéorguiev, 1971 from Australia, 33 species of the widespread Asian and Australian subgenus Gueorguievtes Vaxirani, 1976 and 11 species of the Australian and Neotropical Platynectes s.str.
Our three UK genera represent large and widespread groups. Platambus Thomson C.G., 1859 now includes more than 60 species and is Holarctic in distribution, it was formerly more narrowly defined to include only 25 or so Palaearctic, Oriental and African species but various other groups, including some Nearctic species are now included. Adults are distinguished by the wide elytral epipleura which taper gradually to the apex but our single UK species, which is also the only species extending into Northern Europe, is distinctive by the size and elytral pattern. Agabus Leach, 1817 is a large Holarctic genus with a few species extending in range to North Africa and Central America, presently the group includes more than 200 species, about half of which occur in the Nearctic region. In this genus the epipleura are narrowed after the middle and are absent at the elytral apex. Formerly the distinction between this and the following genus was based on the relative lengths of the meta-tarsal claws; in Agabus they are equal or nearly so but in Ilybius Erichson, 1832 they are unequal with the anterior (outer) claw being shorter and often more curved than the inner claw. This distinction still holds for most of our species but some Ilybius e.g. I. wasastjernae (Sahlberg, C.R., 1824), I. montanus (Stephens, 1828) and I. chalconatus (Panzer, 1796) have equal claws and need to be separated using a combination of characters. Ilybius was similarly more narrowly defined and based primarily on the unequal meta-tarsal claws, it included about 70 species from the Holarctic region and North Africa but this distinction no longer holds as individual species are assigned according to chromosomal, molecular genetic and genitalia characters. Our UK species of these genera are readily identified using morphology although some e.g. I. chalconatus and I. montanus will need to be dissected; in modern keys both genera are treated as a single group. According to the latest checklist our fauna includes 10 species of Ilybius and 18 species of Agabus.