Dianous coerulescens (Gyllenhal, 1810)
This is the only western Palaearctic member of the genus; it occurs in temperate regions throughout Europe extending north to 65o in Scandinavia and south to 38o in Iran, the eastern distribution extends to Kazakhstan and western Siberia, and there are 5 subspecies. The nominotypical subspecies occurs throughout the range and is sympatric with the others; ssp. hispanus Puthz, 2002 is Spanish, ssp. italicus Puthz, 2002 is Italian, ssp. elegans Iablokoff-Khnxorian, 1957 occurs in Armenia and Turkey, and ssp. korgei Puthz, 2002 is restricted to Turkey. The U.K. distribution is sporadic; it has been recorded throughout Wales and in England from the Severn estuary to the Scottish borders and there are several groups of records further south, from Kent and the northern West Country coast as well as Lundy, and in Scotland there are a few records north to Inverness. The typical habitat is among moss and vegetation in the splash zone of weirs, torrents and waterfalls etc. but they also occur around cliff seepages and beside fast fresh i.e. unpolluted running water generally. The adults are diurnal predators, moving rapidly among vegetation and stones in search of prey, the shiny blue metallic colouration and red elytral spots being very distinctive, especially in bright sunshine, they are able to walk on the water surface due to fine hydrophobic hairs beneath the tarsi, and can propel themselves rapidly forward, up to 40cms-1, using abdominal secretions. They generally occur from mid-spring and into the summer. The larvae develop in similar habitats to the adults and pupate in a silken cocoon among vegetation. Here the species is very local and sporadic but may be abundant where found, on the continent it is similarly sporadic but especially abundant around the foothills of low mountain ranges.
6-8mm. Body entirely shiny metallic blue to greenish-blue, upper surface moderately strongly and densely punctured. The head is transverse, with prominent convex eyes and long tapering temples. Antennae and palps dark, antennae long and weakly clubbed. Pronotum elongate, broadest in front of the middle and constricted in front of the base. Elytra quadrate to slightly elongate, much broader than the pronotum and with prominent shoulders, lateral margins weakly rounded and each with an orange spot behind the middle, the punctation is generally a little stronger than that on the pronotum. Abdominal tergites 1-4 strongly bordered, without basal ridges and finely punctured tergites 5 and 6 without borders. Last segment with a pair of long stout setae (absent in Stenus). The legs are long and slender, generally entirely dark although the tibiae may be lighter towards the apex. Hind tarsi long with the first and fifth segments longer than the others and the fourth weakly bilobed. Distinguished from all U.K. Stenus by the paired abdominal setae, the metallic colouration and the long, tapering temples.