Peltodytes caesus (Duftschmid, 1805)
The genus Peltodytes Régimbart, 1787 includes about 40 species and is distributed worldwide except for Australia and South America; 18 species occur in North America and 8 in the Palaearctic region but of these 6 occur in East Asia or China and only 2 are found in Europe. P. rotundatus (Aubé, 1836) is a mostly northern Mediterranean species occurring also in North Africa and the Near East, while P. caesus occurs throughout Europe except for the extreme north, and extends across North Africa and through Asia into China. In the UK P. caesus is locally common in southeast and central England north to The Wash and South Wales and southwest England below the Severn estuary; the distribution was formerly more extensive, extending sporadically north to Lancashire, but seems to have contracted over recent decades. Adults are present year-round, they usually occur among dense marginal vegetation or mats of algae in stagnant still or slow-moving water, often where there are bare patches of marginal soil (although in Watford we find them around the margins of dense reed beds), they generally occur in small numbers and often among populations of various species of Haliplus Latreille, 1802. Larvae are thought to feed exclusively on filamentous algae while adults are omnivorous; they predate aquatic worms, tiny crustaceans, hydrozoans and diptera eggs and also consume algae. Breeding occurs in the spring, eggs are laid on aquatic stems and foliage and larvae develop through the spring and summer and pupate out of the water among marginal soil and roots, they probably do not overwinter as fully-grown larvae as well as freshly-emerged adults occur during late summer. Adults are thought to overwinter among marginal substrate and, at least in central Europe, are known to fly.
Adults are small, 3.5-4.0mm, and resemble a broad and robust Haliplus but are readily identified by the terminal maxillary palpomere being slightly longer than the penultimate, the large convex eyes and the elytral suture which is raised beyond the basal quarter. Overall colour pale brown or yellowish-brown, pronotum with dark areas along the base which may be confluent, and elytra with rows of dark spots corresponding to punctured striae. Head smoothly convex and finely punctured, eyes separated by less than the width of an eye when viewed from above, antennae entirely pale; 11-segmented and filiform. Pronotum widest across the base and narrowed to slightly projecting anterior angles, basal margin produced backwards medially, surface with scattered fine punctures and a series of very strong and often elongate punctures across the middle in front of the basal margin. Elytra broad, almost straight in the basal half and rounded to a slightly mucronate apical margin, striae strongly punctured towards the base, becoming weaker apically; the basal punctures often distinctly larger, the cuticle otherwise shiny and only very finely punctured, sutural margin finely raised beyond the centre and often with tiny tubercles near the apex. Legs entirely pale or with the femora a little darker, posterior coxal plates bordered and with a small blunt tooth on the hind margin. Male with dilated basal pro-tarsomeres.