Noterus clavicornis (De Geer, 1774)
The 'Larger Noterus'
This is the most common and widespread European member of the family; it is native to the western Palaearctic and the Near East and has a very wide distribution, from Portugal and Spain east through southern and central Europe to Mongolia and China, and from Italy north to southern Scandinavia and the U.K. It is also recorded from Turkey, Syria, Lebanon, Iran and Iraq. Here it is common throughout England and Wales including the Isle of Wight, Anglesey and Man although in the West Country records tend to be mostly near-coastal. Most Scottish records are from the southwest; Dumfries, Galloway and Ayre north to Glasgow, and there is an older record from Fife. The species is well-known from brackish water environments and there are coastal records throughout its range. Adults occur year-round; they overwinter among submerged vegetation and detritus etc. and become active in March or April, they generally inhabit permanent water bodies rich in submerged vegetation and organic detritus, and while they are good swimmers they spend much of their time crawling among vegetation or burrowing through substrate in search of prey. In our south Hertfordshire area they are common in ponds and reservoirs etc throughout but probably more especially so in still water beside reed beds where we have recorded adults in every month of the year. In south Hampshire we have recorded them as abundant in early June around the margins of permanent shallow ponds on heather moorland. Eggs have been found in late spring under decaying Scirpus (Bulrush) leaves, and young larvae have been found in late June. Mature larvae have been recorded in August; they mine among the roots of grasses, sedges and rushes etc. in search of prey, in captivity they have been observed feeding upon dead insects and carrion and these are likely to form a part of their diet in the wild. They obtain air by penetrating roots and stems with the pointed tips of the abdomen and will not swim to the surface to breath unless forced to do so. When fully grown they construct a cocoon low down on a stem or root from secretions and particles of debris etc. and inflate it with air obtained from the plant, and both cocoons and freshly eclosed adults have been recorded in August. Adults remain active at least until November but in mild conditions may found through the winter.
4.0-5.0mm. the characteristic oval, convex form and pale to dark olive-brown colouration soon become obvious in the field so that with a little experience Noterus will not be confused with the dytiscids with which it is usually found. The head is transverse and discontinuous with the outline of the pronotum and elytra, with round and weakly convex eyes that are not notched but gently sinuate behind the antennal insertions, the surface is shiny and weakly microsculptured with sparse and weak punctures anteriorly, the clypeus is emarginate and the anterior margin of the labrum is straight. Antennae 11-segmented, in the male the middle segments are dilated with the fifth about the same length as the sixth. Pronotum strongly convex with protruding anterior angles and perpendicular hind angles, the basal margin is produced medially over the scutellum and the surface is weakly microsculptured, this just visible at X10. Elytra convex, becoming flattened towards the apex, with large punctures forming very indistinct rows in the anterior half and more dense and random towards the apex. Legs pale to dark brown. Pro-tibiae short, broad and sinuate along the inner margin; meso-tibiae longer and triangular, meta-tibiae long and parallel-sided; meso- and meta-tibiae with long swimming hairs and two long spines on the inner apical angle. Tarsi 5-5-5; each with two weakly curved and equal claws. Pro- and meso-tarsi dilated in the male.
Distinct from our other species, N. crassicornis, in the form of the prosternal process; in clavicornis it is raised into a ridge which extends from the front margin to the intercoxal constriction, and the anterior margin of the prosternum is produced into a small and sharp point in front of this ridge.