Platambus maculatus (Linnaeus, 1758)
This very distinctive water beetle is generally common from lowland to low mountain altitudes throughout Europe, extending north above the Arctic Circle in Fennoscandia and east through Asia Minor and Russia to Siberia and Mongolia. The UK distribution includes the entire mainland and a few of the Western Isles but it is otherwise absent from the islands and there is only a single record from southeast Ireland. The species prefers clear water with marginal vegetation and in some continental areas it occurs predominantly in mountain and highland streams and water courses, but more generally typical habitats are vegetated rivers and lake margins where water movement provides the correct conditions although there are increasing numbers of continental records from smaller pools, ponds, lakes and reservoirs including brackish waters in the Gulf of Bothnia. They may also occur in urban areas e.g. they were common in a small stream in Uxbridge town centre for many years during the 1970s and 1980s, but they are sensitive marginal degradation and the widely fluctuating water levels that are a feature of many urban rivers nowadays. Adults are flightless and occur year-round, they mate during the summer and oviposit among mats of algae or on submerged plants in late summer and autumn, these eggs will produce overwintering larvae that complete their development and produce adults the following year. In southern European latitudes larvae occur from autumn to spring but further north they occur year-round. Larvae leave the water to construct a pupal cell among marginal soil or debris and the resulting adults return to the water after a period of maturation. Both adults and larvae are predatory and feed on the early stages of other insects etc.
Among the UK fauna the size and distinctive colouration should be sufficient to identify this species. 7.4-8.5mm. Broadly elongate-oval and continuous in outline. Dorsal surface usually shiny, with weak and large-meshed microsculpture and very fine punctures but these can be very strong in some females which appear correspondingly dull. Head black, typically with two pale spots between the eyes and variously pale anteriorly but extensively dark specimens occur, antennae and palps pale. Pronotum pale, typically with the anterior margin narrowly and the base variously dark brown. Elytra pale with a pattern of longitudinal dark markings, these may be extensive and confluent so that the elytra appear almost entirely dark but the lateral margin is always pale; typical specimens have the base narrowly dark and the suture extensively darkened to an interrupted lateral area or discrete dark maculae but this varies; specimens with almost entirely dark elytra can usually be distinguished by the pronotal pattern but in any case the distinguishing feature of the genus i.e. the elytral epipleura which are wide almost to the elytral apex, are easily observed even in the field. Legs and ventral surface of the body dark red. Basal segments of pro- and meta-tarsi dilated in the male.