GEORISSIDAE Laporte, 1840
Minute Mud Beetles
This widespread but local species is mostly coastal and will need to be looked for very carefully among marginal silt and debris.
POLYPHAGA Emery, 1886
HYDROPHILOIDEA Latreille, 1802
Georissus Latreille, 1809
G. crenulatus (Rossi, 1794)
This family has variously been included as a subfamily of the Hydrophilidae. The 80 or so species are all included within the cosmopolitan genus Georissus Latreille, 1809 with the widest diversity occurring in the Old World tropics. They are all small, 1.5-2mm, and highly distinctive; broad, convex and constricted between the pronotum and the elytra. They are typified by our U.K. species, G. crenulatus (Rossi, 1794) but there is great variation in the surface structure and the form of the pronotum; in some tropical species it is strongly transverse, parallel-sided and heavily sculptured. Most species are drab; black or brown but pale brown with darker markings is a feature of some tropical species, and many are to some extent metallic. The head and pronotum are usually granulate and the elytra have strongly punctured striae and, often, raised longitudinal carinae. The scutellum is very small, in some cases hardly visible. Although the palps are as long as the antennae they are often not visible as the head is vertically inclined, or nearly so. The antennae are 7-9 segmented with a 3 or 4 segmented club. In most cases the eyes are convex and protruding. Among the Hydrophiloidea the form of the prosternum is unique; weakly sclerotized and lacking an intercoxal process while the fore-coxae are fused with the trochanters so forming large and flat plates. The mesosternum is transverse, pentagonal and steeply declined anteriorly. The first abdominal sternite, consisting of fused sternites 1 and 2, is very large, convex and depressed apically. Tarsi 5-5-5, or (rarely) 4-4-4, the terminal segment being shorter than the penultimate. Wings variable.
Adults are often considered to be terrestrial insects that live in marginal or wet habitats; they move in the water with alternate leg movements and live among fine sediments or mud, especially in sparsely vegetated areas. Some species are attracted to light. In general the adults are vegetarian, feeding on vegetation or decaying organic matter, while the larvae are predatory, feeding on nematodes and Diptera larvae etc. within the first centimetre or so of sediment. Usually the adults are covered in a layer of mud or sand grains and so are cryptic, they also remain motionless when disturbed and so are difficult to detect. Adults of species that fly have been found in wet habitats far from permanent water or rivers.
Georissus crenulatus (Rossi, 1794)
This is the only species to occur in northern Europe; there are two others, found rarely, in central Europe from which crenulatus is distinguished on the form of the sculpture on the anterior part of the pronotum. In the U.K. it occurs north to Angus in Scotland, in the south it is mostly coastal, including the Isle Of Wight and Anglesey, and it is widespread in East Anglia, South Wales and Cumbria. There are several inland records from Ireland. Although a very local species they may be abundant where they occur. Adults occur from April to September, and have also been recorded in the winter. They occur in open marginal habitats, often by running water, among sediments with algae, moss and plant debris. They are usually covered in substrate and move slowly, remaining still when disturbed and so are difficult to find although may be seen by flooding likely habitats, then they may be detected briefly before sinking under the weight of the attached sediment. Pitfall trapping in likely habitats has also produced adults.
While there are many morphological peculiarities in Georissus, the tiny size and very distinctive appearance is sufficient to identify the species among the British fauna. The upperside is dark, glabrous and shining, and often distinctly metallic. Head deflexed, with a Y-shaped impression on the frons. Antennae 9-segmented with a pubescent-3 segmented club. Pronotum quadrate, broadest at the base and narrowed to the rounded front angles, hind angles rounded. The apical third is longitudinally sculptured and flattened and the lateral and basal margins are lined with large punctures, otherwise the pronotum is smooth. Elytra strongly convex, almost parallel, and weakly produced apically. Each with 10 strongly punctured striae. Tarsi 4-4-4. Wings variable.