SPHAERIUSIDAE Erichson, 1845

Minute Bog Beetles

MYXOPHAGA Crowson, 1955

SPHAERIUSOIDEA Erichson, 1845

Sphaerius Waltl, 1838

S. acaroides Waltl, 1838

0.6-0.8mm

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A single species, Sphaerius acaroides is found in the UK. Tiny and easily overlooked, it is known from only two sites in Britain and, once examined very closely, should be easily identified.

Around the World

This monotypic family contains a single genus, Sphaerius Waltl, 1838, in which there are around 23 species. They are all tiny, 0.5-1.2mm. and hemispherical in form. They are distributed throughout all the continents except Antarctica. Three species occur in the U.S.A and of the eight Palearctic species three are known from Europe. All species are aquatic or semi-aquatic occurring in a variety of damp habitats; mud, wet loamy soil and sandy or stony substrates. Some species store air beneath the elytra. In all species so far observed only a single egg is laid at a time. The larvae are broad and trilobite-like in form and live among detritus underwater. Only a single species, Sphaerius acaroides Waltl, 1838, is known from the U.K. It is widely distributed throughout Europe except for the Iberian peninsula and has recently been recorded from the Caucasus, Hungary and Romania.

Sphaerius acaroides Waltl, 1838

There are only two modern records of Sphaerius acaroides from the U.K.; from south Dorset and Huntingdonshire. It occurs in damp situations near the margins of water. Crowson states ‘under pieces of wood etc. in wet places on areas of alkaline peat in Central and Eastern England, but I have been able to find no records of its occurrence during the last thirty years.’ It may well be overlooked; living in such situations, slow moving and with a secretive lifestyle this tiny beetle can hardly be easy to record. During recent studies from Serbia (Mesaros, 2013) samples from likely situations were extracted using a Berlese Funnel and the beetles were found in abundance, all examined habitats were located on the shore of stagnant water, on sandy substrate not fully overgrown with emergent vegetation, but with high humidity and a sufficient quantity of decomposing dead leaves of  Phragmites  and Carex.  Adults  move  slowly,  feeding on  algae 

among litter or under stones etc. The larvae are aquatic while the adults are marginal. Adults have been recorded in April, May, July and October. It seems likely that similar extractions might be the best way of sampling for the species in the U.K.

Very small, 0.6-0.8mm. Convex and with a flat undersurface this should not be mistaken for any other U.K. species; the smallest of our hydrophilids, histerids, cryptophagids or ptilids are either too large or sufficiently distinct under a microscope to be discounted. The most likely confusion will be with oribatid mites that are often common in such samples. Any suspicion that this species is present should be easy enough to confirm with its unusual combination of characters.

Short oval and very convex in form, entirely dark brown or black, and glabrous. The head, when not withdrawn, is prominent with relatively large eyes which are widely separated. Antennae are 11-segmented with the two basal segments enlarged and an elongate 3-segmented, pubescent club.The notopleural sutures situated laterally on the prosternum are distinct. Only three abdominal sternites are visible. Scutellum triangular. The hind coxae are expanded into very large plates which cover the first visible sternite and the meta femora. The legs are short with broad and flat tibiae which have a prominent subapical tooth behind an excavated outer margin. The inner margin of the front femur has a strong tooth about half way along. Tarsal formula 3-3-3. Claws unequal.

MYXOPHAGA

This small sub order contains four families and around 65spp. They are aquatic or semi-aquatic and feed on algae. Only one species of the Sphaeriusidae occurs in the U.K. The families are:

  • HYDROSCAPHIDAE LeConte, 1874 with 3 genera and 23 species. Small algae feeders, around 2mm in length, that are very widely distributed from the icy snow melts of the north to the hot springs. They are brown and look superficially like small a Tachyporus Sp.; the wings are fringed with long setae. All species lay one egg at a time.

    • Hydroscapha LeConte, 1874 with 18spp. Endemic to Europe and the U.S.A.

    • Scaphydra Reichardt, 1973 with 3 spp.

    • Yara Reichardt & Hinton, 1976 with 2 spp.

  • LEPICERIDAE Hinton, 1936. With 3 species of Lepicerus

  • TORRIDINCOLIDAE Steffan, 1964 With 7 genera, all described since 1964.

  • SPHAERIUSIDAE Erichson, 1845 with a single genus containing around 23 species.

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