Rhantus exsoletus (Forster, 1771)

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Genus:

ADEPHAGA Clairville, 1806

DYTISCIDAE Leach, 1815

COLYMBETINAE Erichson, 1837

Rhantus Dejean, 1833

This species occurs across much of the Palaearctic region from Europe to eastern Siberia; it tends to be sporadic and scarce in warmer southern regions but is generally common across central and northern latitudes. In Europe it is absent or scarce in many southern countries e.g. Spain, Bulgaria, Greece and the Mediterranean islands, and it does not occur in North Africa.  Although sporadic it is generally common across central Europe and extends north to the UK and beyond the Arctic Circle in Scandinavia. The species is locally common across Britain and Ireland with the exceptions of the West Country, the far north of Scotland and the south of Ireland, it is also known from most of the islands, including the Western Isles, but seems to be absent from Man, Orkney and Shetland. This is essentially a species of well-vegetated lowland ponds, reservoirs, marshes and lake margins, it occasionally occurs in shallow and  slow-moving stretches of rivers and we have also swept adults from sandy pools devoid of vegetation on heathland in Surrey, and it is usually absent from coastal pools. Adults occur year-round; they overwinter away from water and are active from March until October, peaking in abundance during May and June. Mating occurs underwater soon after adults have arrived from their overwintering sites and females oviposit over a relatively long season starting in March or April and extending into June. The biology is typical of the group; eggs are laid singly into aquatic stems and larvae emerge after two or three weeks. The predaceous larvae are unusual among dytiscids in being golden yellowish in colour; they develop in water but leave in order to pupate among marginal soil or under debris etc. It is thought that pupation always occurs during the summer and that only adults will overwinter, although it has been suggested that eggs produced late in the year may diapause and produce larvae during the following spring. Adults and larvae predate a wide range of aquatic insects and other small animals and, very unusually, both stages were found (in acidic lakes in Norway) to consume frog eggs (despite the spawn mucus which generally defends them) and tadpoles. Adults may be sampled by sweeping among marginal vegetation, they often occur in numbers and may persist into November during mild years, they are otherwise thought to disperse by flight to overwintering sites (so far unknown) during October. Migration back to the water in the spring is also by flight but it may be that after finding a suitable habitat the flight muscles degenerate.

Rhantus exsoletus 1

Rhantus exsoletus 1

Rhantus exsoletus 2

Rhantus exsoletus 2

Rhantus exsoletus 3

Rhantus exsoletus 3

9.0-10.2mm. Readily identified by the absence of a short ‘comb’ of bristles (which is present in species of Agabinae) on the apical margin of the hind femora, lack of transverse elytral microsculpture, and the general shape and colour. Broadly elongate-oval and continuous in outline, glabrous and moderately shiny above, elytra with weak irregular microsculpture which tends to be elongate and is often faint towards the base. Head black with pale spots on the vertex and in front of the eyes but this varies and extensively dark or pale specimens occur. Eyes excised anteriorly. Palps pale but always darkened apically. Antennae pale with segments 5-11 darkened apically. Pronotum pale with a dark band across the centre of the base and sometimes also behind the apical margin, these tend to be diffuse or indistinct. Scutellum dark, as the surrounding cuticle. Elytra pale with numerous small dark marks which extend almost to the lateral margin, these vary so that specimens may appear extensively dark, surface smooth and without striae. Ventral surface pale brown or orange-brown with at most the mes- and metasternal margins and a diffuse spot towards the lateral margins of the sternites diffusely black. Legs pale brown. Males may be distinguished by the front tarsal claws, both of which are much longer than the terminal tarsomere.