Rhantus frontalis (Marsham, 1802)

Suborder: 

Family:      

Subfamily: 

Genus:

ADEPHAGA Clairville, 1806

DYTISCIDAE Leach, 1815

COLYMBETINAE Erichson, 1837

Rhantus Dejean, 1833

This species is Holarctic though rather patchy in distribution. In North America it occurs sporadically across the northern United States, in Canada it is known from several sites in southern Manitoba and Saskatchewan and is widespread and common across Alberta. In Europe it is locally common across central and northern Europe from the Pyrenees to Italy and Ukraine, although absent from most of the Balkan Peninsula and the Mediterranean islands, and north to the UK, Denmark and Fennoscandia where it is common in the south but individual specimens have been recorded north to the Arctic Circle. To the east it extends across the Northern Palaearctic region into Siberia and Mongolia. The UK distribution is disjunct; it is locally common though mostly coastal or near-coastal across East Anglia and the southeast and North Somerset , beyond this it occurs across Southern Scotland, where it may be expanding, and throughout much of Ireland, but there are also records from Dorset, IOW, Anglesey and Man. The usual habitats are stagnant ponds and ditches, usually with abundant aquatic vegetation and areas of exposed substrate, usually in exposed lowland regions although in Northern Europe they also occur in woodland ponds and are sometimes abundant in peat bogs. Adults occur year-round although they are seldom recorded during the colder months as they (probably) overwinter away from water, they are active between March and October or November and peak in abundance during June. Reproduction occurs in spring and early summer, according to some sources in more temporary water bodies, females oviposit in aquatic plant stems and larvae develop during late spring and summer, pupation occurs among marginal substrate etc and new-generation adults appear during the summer. Both adults and larvae are predatory on other insects, mostly diptera larvae, though adults may be more general predators as they have been observed taking small leaches in the laboratory (Cywinska A. and Davies, R.W., 1989). Adults may be sampled by sweeping among vegetation in ponds and other still water bodies, they are known to fly but they seem not to come to light and flight may be restricted to autumn and spring movements between ponds and overwintering sites.

Rhantus frontalis 1

Rhantus frontalis 1

Rhantus frontalis 2

Rhantus frontalis 2

9.4-11.4mm. Body glabrous, elongate, broadly-oval and weakly convex. Colour distinctive; head black, usually with the anterior margin and two spots on the vertex pale brown but these are variable, pronotum pale with the apical and basal margins and a transverse bar and sometimes a lateral spot across the middle black. Ventral surface bicoloured (a good field character); prosternum, epipleura yellow, meso-and metasternum black, in females the sternites are black with apical margins and sometimes a median spot yellow, in males this is reversed. Palps pale with darkened apices, antennae pale with segments 4-11 darkened apically. Pronotum broadest near perpendicular posterior angles and smoothly curved and narrowed to projecting anterior angles, apical margin curved forward, basal margin straight across the middle then sinuate towards the angles, lateral margin finely bordered throughout, surface very finely granulate and punctured, without sculpture but usually with larger punctures behind the basal and apical margins. Elytra substantially yellow with numerous small dark spots; the lateral and sutural margins and two narrow longitudinal lines in the basal third yellow. Legs entirely pale, surface very finely punctured throughout and with two longitudinal lines of much stronger punctures. Metafemora without a small ‘comb’ of stiff setae at the outer apical angle (a feature present in Agabinae). Males may be distinguished by the broadly dilated basal front and middle tarsal segments and the unequal front tarsal claws; the inner claw about 1.5X longer than the outer.