|Includes two British species; Noterus clavicornis (DG.) and
N.crassicornis (Muller, O.F.). They are small (3.5-5.0mm) and somewhat shiny brown water beetles, strongly
convex above and flattened below, in the dytiscid water beetles the dorsal and ventral surfaces are of about equal
convexity. Characteristically boat shaped with elytra broadest before middle. Head relatively small with weakly
protruding eyes and short, transverse labrum. Apical segment of labial palps much longer than others. Mandibles bifid
at apex. Antennae eleven segmented with middle segments expanded, greatly so in males. Pronotum broadest at base,
anterior margin bordered by a series of punctures which may be joined by a furrow, hind margin produced at middle so covering
scutellum. Elytra bordered laterally, surface with scattered large punctures which tend to form irregular rows and are
more dense towards the apex. Prosternal process diagnostic for our species; with a ridge from front margin to median
constriction in clavicornis, smoothly arched in crassicornis. Hind coxae produced into plates posteriorly
which overlap the second and third abdominal sternites. Front and midlegs short with broad tibiae, modified for burrowing or crawling.
Hind legs modified for swimming with long hairs to tibiae and tarsi. All tarsi 5 segmented.
There has in the past been some confusion as to the nomenclature; N.clavicornis having been known as N.capricornis while N.crassicornis has been known as N.clavicornis. Specimens may generally be separated by size and this character, being obvious, has often been quoted after the name so as to avoid confusion ie N.clavicornis (DG) 'the larger species' is between 4.0 and 5.0mm in length while N.crassicornis (Mull.) 'the smaller species' is between 3.5 and 4.0mm (Friday) in length. In Balfour-Browne the larger species is given as capricornis Hbst. (sparsus Mars.) and the smaller as clavicornis DeGeer. In Joy, who separates the species on the purely comparitive elytral puncturation and gives a more optimistic size difference, they are given as:
Elytral puncturation much smaller; L3.5-4.0mm............. crassicornis Mull (clavicornis Brit.Cat)
Elytral puncturation large and deep L4.2-4.5mm............ clavicornis DG. (sparsus Mars)
Holmen (1987) quotes some overlap, 4.0-5.0mm for N.clavicornis and 3.5-4.2mm for N.crassicornis.
In any case with the majority of specimens size will be diagnostic but there is never a need to rely on this; the prosternal process is distinctly ridged in N.clavicornis, in N.crassicornis males this flat in females it is convex but in either case without a ridge. In males the fifth antennal segment of N.clavicornis is about the same length as the sixth, in N.crassicornis the fifth is about twice as long as the sixth. With experience the elytral puncturation is distinctive, in N.crassicornis they form much more distinct longitudinal rows on the anterior surface.
Noterus clavicornis is by far the more frequently recorded species, N.crassicornis is a very local, mostly fenland, species. Both have a widespread distribution in the southeast and should be expected from our Watford area.
Adults are probably carniverous. Typical habitat is stagnant water rich in decaying vegetation, they are often found swimming or crawling among dense vegetation. They carry an air supply beneath the elytra which they come to the surface to replenish.
Holmen 1987 The Aquatic Adephaga of Fennoscandia and Denmark. Fauna Entomologica Scandinavacia Vol. 20 E.J.Brill, Leiden