Chaetocnema aerosa (Letzner, 1846)
Widespread in Central, Eastern and Northern Europe but generally very local and rare, especially in the north where it reaches southern provinces of Sweden and Norway and a few coastal sites in Finland. To the south it extends from Spain to Northern Italy and Ukraine, and is recorded from Turkey, Egypt, Israel, Siberia and parts or European Russia. In the UK it has always been very rare; there are older records from Hampshire, Cambridgeshire and Surrey but it was last recorded in the early 1960s and is now considered to be extinct here. Typical habitats are wetland margins, marshes, fens, ditches and floodplains where the host plants are common. Adults are present year-round, they overwinter among tussocks or litter etc., and are active over a long season (in southern Europe almost throughout the year), peaking in abundance during June and July. Host plants include various Cyperaceae; in the UK on Common Spike-rush (Eleocharis palustris (L.)) and Many-stalked Spike-rush (E. multicaulis (Sm.)), but more generally also on Ovate Spike-rush (E. ovata (Roth)). Little is known of the biology but mating occurs in the spring and oviposition continues into July. Larvae develop inside low stems and roots of host plants during the summer, and adults probably appear during August and September. Adults may be sampled by sweeping host plants; they are fully-winged and also capable of moving across water, using their front and middle legs to skate and maintaining a bubble of air beneath the head and thorax.
© Lech Borowiec
1.6-2.2mm. Broadly-oval (with the exception of arida, broader and less elongate than our other members of the genus) and discontinuous in outline, glabrous, body metallic bronze or greenish to almost black, antennae dark with two or three basal segments pale, at least below, legs brown with femora and sometimes tibial apices darkened. Head hypognathous with large convex eyes, finely punctured throughout, vertex flat, space between antennal sockets wide and flat, without a vertical ridge, frontal groove arcuate and not reaching the upper margin of the eyes. Pronotum transverse, broadest behind the middle and narrowed to weakly protruding anterior tubercles and obtuse posterior angles, apical margin evenly curved, basal margin gently sinuate, surface closely and moderately-strongly punctured throughout or with a short impunctate medial area towards the base, without basal fovea or impressions. Elytra broadest about the middle and curved from sloping shoulders to a continuous apical margin, striae confused around the scutellum but otherwise mostly seriate, humeral calli usually well developed. Hind femora greatly enlarged, front and middle femora normal. Front tibiae smooth externally, middle and hind tibiae with a single external tooth towards the apex, hind tibia with a singly apical spur. Basal segment of front and middle tarsi expanded in males.
Very similar to C. sahlbergii (Gyllenhal, 1827) which is narrower and more elongate, and here the frontal grooves extend laterally to meet the inner margin of the eyes.