Calyptomerus dubius (Marsham, 1802)
POLYPHAGA Emery, 1886
SCIRTOIDEA Fleming, 1821
CALYPTOMERINAE Crowson, 1955
Calyptomerus Redtenbacher, 1847
This is a mostly central and southern European species; it is generally common along the Mediterranean borders from Spain to Greece and extends sporadically north into the UK and the Baltic countries although it is known from only a few records in the southern provinces of Norway and Sweden, it is absent from North Africa but present on the Canary Islands and, following accidental introductions, has become established in various regions worldwide including South Africa, Australia, the United States and Canada (British Columbia). In the UK it is very local in southeast and central England and around the Welsh coast and is also known from Anglesey. Adults occur year-round and are active over a long season from early in the year although other than in the warmest weather, when they may be swept from dry grass, they are seldom recorded. In northern Europe they are mostly associated with decaying grass and may be abundant among mouldy hay, straw and even garden compost while in warmer southern regions, where they may occur in abundance in Malaise traps in urban areas, they are more eurytopic, occurring among woodland litter and decaying fungus-infested logs etc. Little is known of the biology in the wild but under artificial conditions oviposition has been observed in the spring when females laid pairs of eggs on mouldy bread each day for about two weeks, the resulting larvae developed among the medium, pupated after a month nor so and adults eclosed about two weeks later. Adults collected through the summer did not breed, suggesting a univoltine cycle with adults overwintering and breeding through the spring. Adults may occasionally occur by sweeping established dry grassland in the summer but otherwise they will need to be looked for by sieving decaying grass etc, an old bale of straw or hay left out through the winter is ideal, especially the decayed layer in contact with the ground, and taking samples for extraction may also be productive. Although very local and infrequently recorded, adults may sometimes occur in large numbers.
Calyptomerus dubius 1
© Lech Borowiec http://www.cassidae.uni.wroc.pl/Colpolon/index.htm
Calyptomerus dubius 2
© U.Schmidt https://www.kaefer-der-welt.de/index.htm
1.1-1.6 mm. Convex and broadly-oval, in life capable of retracting into a tight ball, entirely pale brown or darker brown with paler margins, dorsal surface extremely finely punctured and with rather dense pale pubescence, most easily distinguished from species of Clambus Fischer, 1820 by the form of the head and pronotum. Head proportionally very large; longer than, and almost as wide as, the pronotum, with weakly convex eyes situated at the extreme lateral margin, strongly diverging temples and long, weakly-diverging cheeks, dorsal surface evenly though rather weakly convex. Antennal grooves form an even and continuous curve with the frons in front of the eyes, antennae 10-segmented, the two basal segments much wider than those following and 9 & 10 forming a compact club. Pronotum widely transverse and evenly convex, basal and lateral margin form a continuous curve without distinct posterior angles, anterior margin curved; from above forming a rounded angle with the lateral margin. Elytra weakly curved across the base, broadest in the basal third and narrowed to a rather truncate or separately-rounded apical margin, surface evenly and rather strongly convex. Legs long and very slender, all tarsi with four simple segments.