Esolus parallelepipedus (P.W.J. Müller, 1806)

This is a mostly central and southern European species, it occurs sporadically from Portugal to the Black Sea, north to the UK, Germany and Poland and there are isolated records from southern Fennoscandia, it is generally common across Wales, northern England and Scotland north to Orkney and across Ireland but otherwise very local and scarce and absent from much of the south east. Typical habitats are shallow stretches of streams and rivers in foothills and low mountain areas; they are absent from higher mountain altitudes and only rarely occurs in lowland regions, they seem to prefer moderately-fast water with little or no vegetation on stony substrates with abundant filamentous algae. Both adults and larvae have been recorded throughout the year, adults are long-lived and active over a long season from early spring but due to their cryptic lifestyle only rarely found during the winter. Adults use plastron respiration and spend most of their time under stones and debris, they usually occur in numbers but seldom out of water and then among wet marginal moss etc. they are known to fly but dispersion is thought to be mainly within the current and generally over short distances. Females lay small numbers of eggs among algae or on stems of aquatic plants from July and larvae develop through the summer, it is usually second instars that overwinter and development is completed during the following spring and summer, they pass through six instars and leave the water to pupate among marginal substrate from June or July, this stage lasts two or three weeks and the resulting adults soon return to the water. Adults persist after oviposition and final-instar larvae have been observed later in the year suggesting that adults may breed again and that some larvae may pass a second winter but this may depend on conditions and the details are not known. Adults and larvae occur under stones and debris in well-aerated shallow water, often together and usually in numbers, both stages rarely occur away from this habitat and dispersal occurs at night, adults crawl upstream or enter the current to be carried downstream for short distances before returning to the bottom, movement over longer distances may occur in spring and autumn during spates but here survival rates are low and this is probably not an important means of dispersal. June and July are the best months to find adults as they occasionally appear on wet riparian substrate or moss etc. but they are otherwise rarely found by the usual sampling techniques, disturbing submerged stones etc. so that the resulting debris passes into a net is generally the best way to find both adults and larvae.