Ilybius quadriguttatus (Lacordaire, 1835)

Native to Europe and Asia Minor and occasionally found further east into western Russia, this species is generally common throughout this range although it seems to have declined over recent decades due to habitat abuse; it occurs from Spain to the Caspian Sea in the south, although it is absent from some of the Balkan countries, and to the UK and Central Finland in the north. It is common throughout England and Wales though less so in the north and absent from Scotland, it is common in Northern Ireland, less so in central Ireland and absent from much of the south. The typical habitats are well-vegetated permanent ponds and ditches of near-neutral pH, usually with plenty of decaying organic material on heavy soils although in northern areas of Europe it is common in lowland peat bogs and marshes, and in central Europe it occurs to boreal and alpine altitudes. Adults are present year-round; they overwinter in soil or under debris away from the water margin although we have swept them from local ponds in December, and are active over a long season from early in the year, peaking in abundance during May and June. Mating occurs mainly in spring and early summer although we have found mating pairs through the summer as well, larvae develop through the summer and pupate in soil out of water, and they probably all produce adults in the autumn. Adults are easily sampled by sweeping among marginal vegetation but this is among the few species that may be observed swimming in open water near pond margins in warm sunny weather.

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10.5-12.2 mm. Body unmetallic dull black with the clypeus, two spots on the vertex and sometimes the anterior pronotal margin red, there are also two red spots towards the lateral margin of each elytron; one near the middle and one towards the apex, which may not be visible in set specimens but lifting an elytron will reveal them, legs dark brown, antennae paler brown. Ventral surface dark reddish-brown. Metasternal extremities wide to the subapical angle. Elongate-oval and broadest behind the middle, elytra with irregular mesh-like reticulation which is elongate in places and may be weaker towards the base. Anterior margin of eyes strongly incised. Posterior margin of the hind femora with a short and discrete line of setae towards the apex. Hind tarsi with claws unequal in length; the outer claw shorter and more strongly curved in both sexes. In males the last sternite has a short median keel and a continuously curved apex and the front and middle tarsi are broad and furnished with ventral sucker hairs, females have a small notch in the apical margin of the last sternite and the tarsi are narrower and lack sucker hairs. Very similar to Ilybius guttiger (Gyllenhal, 1808) which is consistently a little smaller, 8.5-10.0 mm, males have a smooth outer claw on the front tarsi whereas in quadriguttatus it has a ventral tooth, in female guttiger the hind tibiae are strongly punctured ventrally while in quadriguttatus they are very weakly punctured.