Stenus guttula Müller, P.W.J., 1821
This species is widespread across the western and central Palaearctic regions, extending from Europe and Asia Minor into eastern Siberia; it is generally common across Southern Europe and North Africa, including the Mediterranean and Atlantic islands, but less so moving north where it becomes much more local, reaching as far as the UK, Denmark and the Baltic countries although only in southern parts of Fennoscandia. In the UK it is locally common throughout England and Wales, including all the islands, much more local and scarce in the Scottish Highlands and rare and scarce and mostly coastal in Northern Ireland. The usual habitat is wetland margins, lake shores, banks of slow-moving streams and rivers, seepages and coastal estuaries and dune pools, usually on clay or fine sandy or gravelly substrates and usually with patchy vegetation and areas of moss and layers of decayed vegetation. Adults are diurnal and predatory, hunting on the surface during warm weather but otherwise remaining concealed under debris or in moss or soil etc, they do not climb vegetation but are capable of moving rapidly on the water surface when necessary. Little is known of the biology but, typical of the group, reproduction probably occurs in spring and early summer and the larvae predate other small invertebrates on or within the soil. Adults are present year-round; the overwintering sites are not known but are unlikely to be far from water, they are rarely recorded during the winter but are active over a long season from March until October, they are most common in the spring and peak in abundance during June. They may be sampled by searching under debris or matted vegetation or by catching then as they run during sunny spells, flooding areas of likely substrate can often make them active and pitfall-trapping may produce numbers of adults, usually along with plenty of other marginal carabids and staphs and so traps will need to be examined frequently.
Stenus guttula 1
From Tottenham, 1954
4-5 mm. A slender species that may be identified in the field with a lens; body shiny black with a large oval orange mark on each elytron, antennae dark brown or black at the base, palps pale or slightly darkened towards the tips, legs bicoloured; pale with the femora dark from near the base, tibiae darkened apically and tarsi at least partly darkened, usually about the apex of the basal segments. Forebody and elytra strongly and densely punctured and with sparse pale pubescence throughout. Head bluntly keeled between massive eyes that occupy most of the lateral margin. Pronotum elongate, curved laterally and weakly constricted in the basal third, surface uneven; without a median furrow but with a small, impunctate and shiny raised area either side of the disc. Elytra quadrate or nearly so and slightly longer than the pronotum, curved laterally from rounded shoulders to recurved apical margins, surface uneven, densely and in placed confluently punctured. Abdomen strongly bordered, all tergites densely punctured; the apical tergites more finely so, basal tergites only weakly depressed across the base and without median eels. Legs slender and very long, tarsi without bilobed segments. Similar to S. bimaculatus Gyllenhal, 1810, but smaller, more slender and lacking a keel on the basal abdominal tergites. Aedeagus characteristic.