Nebrioporus elegans (Panzer, 1794)

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ADEPHAGA Clairville, 1806

DYTISCIDAE Leach, 1815

HYDROPORINAE Aubé, 1836

HYDROPORINI Aubé, 1836

Nebrioporus Régimbart, 1906

This species has a rather narrow western Palaearctic distribution, it is more-or-less restricted to Europe from Portugal to Serbia in the south and to Germany and the UK in the north, the eastern limit is Hungary and Slovakia and it has sometimes been referred to as occurring in North America but these records are for another species. In the UK it is generally common throughout lowland England, Wales and southern Scotland, including most of the islands, much more local and rare further north to Orkney and there are scattered records from Ireland. Adults occur-year-round, they are active over a long season and peak in abundance in the middle of summer. They may overwinter out of water, as is typical of the family, but we once found a large assemblage of adults beneath a partly-submerged rock in a shallow and fast-flowing stretch of the River Colne in West Middlesex during December. Little is known of the biology but adults are widely eurytopic and occur in both still and running water in a wide range of habitats, often in shallow stretches of rivers with little vegetation but also much more generally in ponds, reed beds, drainage ditches and often in artificial or temporary water bodies. They are known to fly and during the summer may either move on from drying ponds etc. or enter the soil or remain among damp soil under logs or debris and at this time they may also occur in numbers.

4.4-5.0 mm. Broadly-oval and discontinuous in outline, with a large head and robust legs, entirely pale brown to yellow with the apical and basal pronotal margins and elytra variously darker. Head transverse and smoothly curved anteriorly, surface evenly convex and finely punctured, eyes large and only slightly protruding from the outline, antennae with at least the apical segment partially darkened. Pronotum transverse, broadest about the middle and smoothly curved from obtuse posterior angles to protruding anterior angles, basal margin produced medially, surface often uneven but without definite structure, closely punctured throughout. In most specimens the anterior pronotal margin is narrowly darkened and the basal margin has a broad transverse marking either side of the middle but this is variable. Scutellum not visible. Elytra smoothly curved from rounded shoulders to a finely acuminate apical margin, each with a small subapical tooth, surface closely punctured throughout and lacking striae, the typical colour is pale with longitudinal dark marks that may be confluent or expanded into blotches in places, but extensively pale and dark specimens are common.  Tarsi 5-segmented but the front and middle tarsi appear to be four-segmented due to the diminutive fourth segment.

Nebrioporus elegans 1

Nebrioporus elegans 1

Nebrioporus elegans 2

Nebrioporus elegans 2

Nebrioporus elegans 3

Nebrioporus elegans 3

Distinguished from N. canaliculatus (Lacordaire, 1835) by the subapical elytral tooth and from N. assimilis (Paykull, 1798) by the pronotum being broadest about the middle, but very similar to N. depressus (Fabricius, 1775). At present only males of the depressus/elegans pair can be safely separated; in the present species the front tarsal claws are smoothly curved while in depressus the inner claw unevenly and more strongly curved than the outer claw. The aedeagus of depressus is broadly rounded and the apical third often appears slightly dilated while in elegans it is smoothly and continuously narrowed to a finely-rounded apex. N. depressus is a mostly northern and western species in the UK, including Ireland, with scattered records from the south of England, and sterile hybrids between the species sometimes occur in which these characters are intermediate.

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