Gyrinidae
   2 genera, 12 species. Very characteristic beetles highly adapted for life on the water's surface; the eyes are divided into an upper pair for overhead and a lower pair for aquatic use, all being large and prominent. The front legs are long with stout tibiae and well developed claws, mid and hind legs are adapted for swimming; short with greatly dilated tibiae and tarsal segments, especially in the hind pair. Tarsi 5-5-5. Antennae short and compact with close fitting and strongly transverse segments. Species are oval or subparallel-oval with bordered pronotum and elytra.

   The hairy whirligig, Orectochilus villosus (Muller), is the only British member of the genus, it is of typical gyrinid form but covered in thick set yellowish grey pubescence and has the lateral edges of the pronotum and elytra lighter. Widely distributed but local, it is active on the water surface at night, by day hides under stones or among grass roots at the waters edge.
   Gyrinus species are diurnal and swim rapidly in circles especially during hot weather, when disturbed they dive rapidly so speed is needed to catch them. They are carniverous, catching insects etc. on the surface, but all feed vigourously on carrion often in large numbers. Open water may host large 'schools' often comprising more than one species, but they seem equally at home among reeds or flooded waterside vegetation. Adults have been recorded from March onwards and large numbers are common late in the summer through to October or November depending on season. Mating has been observed during March and May but full grown larvae have been found from March to September which suggests hibernation in both stages. Eggs are laid either singly or in rows or clusters on near-surface vegetation. Upon hatching the larvae drop down into the bottom debris or ooze where they remain until nearly fully grown, at this stage they live among vegetation. Pupation takes place out of the water within a cocoon constructed from mud or debris among the roots of waterside vegetation. This stage is rapid with adults hatching within a week or two. Adults hibernate among mud and around vegetation and may be active during mild spells in winter.
   The genus Aulonogyrus has been claimed to occur in Britain but has now been deleted from the list.
   Much name changing and confusion has been associated with the British list over the years and from a practical viewpoint it is best ignored; the Coleopterist website checklist should be consulted for nomenclature, and Laurie Friday's book should be used for identification. For a more comprehensive key that given in the B-B C newsletter 20:3-7 (1981) by Garth Foster is very good indeed. Other keys are best ignored, and ecological accounts treated with great caution because of the nomenclature history of the group. Balfour-Browne's account gives much interesting information and, being now available on CD ROM is well worth a read.
   Friday's key is a pleasure to use, admittedly it relies on underside characters but to any serious student this should be happily accepted as educational and precise, the descriptions are lucid and the figures easy to appreciate so several species are readily identifiable, for some one must be prepared to dissect out the genitalia. The Gyrinidae are a good group to start dissecting as the structures are large and robust and, as often as not, partly protuding anyway. Genitalia of both sexes can be mounted dry with the specimens and will remain fully diagnostic without any distortion.
   According to Friday several species can be expected in our area: G.aeratus Stephens, G.caspius Menetries, G.minutus Fabricius, G.substriatus Stephens.

Heslop-Harrison, J.W.. Ent. Mon. Mag. 1936 72:97-108
 
Gyrinus marinus F Gyrinus marinus M Gyrinus substriatus F Gyrinus substriatus M