Agabus sturmii (Gyllenhal in Schönherr, 1808)
This species is generally common from lowland to low mountain altitudes throughout central and northern Europe, from Spain and France through northern Italy to Asia Minor and north to the UK and central Fennoscandia, further east it extends through northern Russia to Siberia. Here it is common and often abundant throughout the UK including all the islands with the exception of Shetland and is, along with A. bipustulatus (Linnaeus, 1767), our most common member of the genus. Adults occur year-round, generally in small and well-vegetated water bodies but also occasionally in slow-flowing drains in marshy areas, they have been recorded from a wide range of habitats including peaty and brackish-water ponds but may be absent in limestone and chalk districts in southwest and central of England, in general they might be expected from any small and permanent body of water e.g. we have recorded them in numbers from cattle troughs and weed-choked garden ponds. They overwinter among marginal substrate or under debris and become active in April or May, at which time they may be abundant alongside other common dytiscids such as Colymbetes, Ilybius ater, I. fuliginosus or Agabus bipustulatus. Adults are fully-winged and capable of flight but are rarely recorded at light or in flight-interception traps; they breed in the spring and oviposit in stems or among roots of submerged vegetation. Larvae develop through the summer and are fully-grown by July or August, they pupate among marginal substrate and new-generation adults appear from July to September, these are active until late in the autumn when they will leave the water to overwinter; so far as is known this species does not overwinter in the larval stage. Both adults and larvae are predatory, feeding on small invertebrates and the early stages of other insects, and adults will readily consume carrion.
Adults may be recognized in the field by broadly-oval shape and contrastingly coloured pronotum and elytra. 7.7-9.0mm. Head rather coarsely reticulate, pronotum and elytra with double reticulation, the finer mesh visible at X30 but often very weak or even missing from the pronotal disc. Head, shiny and distinctly metallic; black with two spots on the vertex and the anterior margin red, antennae and palps pale but variously darkened towards the apex. Pronotum metallic black with broadly pale lateral margins, scutellum finely microsculptured but without the larger mesh. Elytra distinctly paler than the pronotum; pale to dark testaceous with the lateral and basal margin, or at least the shoulders, pale, reticulation on the disc more uniformly quadrate compared with the pronotum, larger punctures weak or obscure and forming rather random longitudinal series. Epipleura narrowed from the level of the first abdominal sternite. Legs pale or with the femora darker. Male with the basal pro- and meso-tarsomeres broadly dilated, aedeagus distinctive; robust, strongly curved and with a subapical spine. Claws on hind legs equal in length in both sexes. A rare melanic form has been described from the Stoer peninsula in North Sutherland, see: Foster, G.N. 1980 Ent. Mon. Mag. 116: 213-214