Gyrinus minutus Fabricius, 1798
ADEPHAGA Clairville, 1806
GYRININAE Latreille, 1810
GYRININI Latreille, 1810
Gyrinus Geoffroy, 1762
Gyrinulus Zaitev, 1908
This is the only European member of the subgenus Gyrinulus Zaitzev, 1908, it is among the most widespread members of the family with an Holarctic distribution; it occurs across Canada and North America and is locally common throughout the northern Palaearctic region from Spain to north-eastern China, Sakhalin and Hokkaido; in Europe it is mostly northern species, extending into the UK and above the Arctic Circle in Fennoscandia, and is absent from the most southern parts and North Africa. The UK distribution is mostly northern and it has declined over much of its southern range in recent decades; it is now more-or-less confined to a few widely scattered heathland locations in England and to Snowdonia in Wales, but it remains locally common across Ireland and Scotland including the Western Isles and Shetland. Adults occur year-round, they overwinter on submerged vegetation and are active over a long season, from February until late in the autumn and peaking in abundance in mid- and late-summer, they generally occur around open margins of larger water bodies such as lakes and reservoirs but also inhabit poorly-vegetated moorland ponds and slow-moving sections of rivers. Compared with other gyrinids they are much less vigorous swimmers and their weaker gyrations soon become obvious in the field, they may dive when disturbed but a group will often disperse by swimming away from any disturbance. Reproduction begins in early spring when mating occurs on the water surface and oviposition occurs in late spring, eggs are laid on aquatic vegetation and larvae emerge after a week or so. Larvae are completely aquatic and do not need to visit the surface to exchange gasses, they are predatory and move among submerged vegetation in search of prey, they develop rapidly and are fully-grown within three or four weeks at which time they leave the water to construct a cocoon on marginal or emergent vegetation. The pupal stage is brief; adults eclose within a week or so and the entire cycle from egg to adult has been completed in about seven weeks under laboratory conditions. This species is known to fly and adults may suddenly appear at new sites through the summer.
This is the smallest of our whirligig beetles with adults not exceeding 4.5mm, and this small size coupled with the narrow form and entirely pale ventral surface makes it distinctive in the field, it is also unique among our species in having a short longitudinal ridge on the scutellum and so should not be confused with any other species. Body entirely black and somewhat dull due to reticulate microsculpture, and in most specimens there is a distinct metallic bronze reflection to the lateral margins of the elytra. Underside and legs pale brown or orange. Pronotum with a narrow and smooth longitudinal central line contrasting with the reticulate disc, and the mesosternum with an entire longitudinal median groove. Elytral striae finely punctured and complete to the apex, the subapical transverse series’ of punctures weakly impressed but obvious.