GYRINIDAE | uk beetles

ADEPHAGA Clairville, 1806

3.5-7.8mm

GYRINIDAE Latreille, 1810

Whirligigs

Found throughout the UK, these unusual beetles are instantly recognizable from their rapid skating movements on the water surface.

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Around the World

Worldwide this family contains around 1000 species in 15 genera arranged into 3 subfamilies. All but 2 species are in the Gyrininae Latreille, 1810. The Heterogyrinae Brink stat. N. contains a single species which is endemic to Madagascar; Heterogyrus milloti Legros, 1953. Two tribes of the Gyrininae Latreille, 1810 are represented in the U.K. The Orectochilini Regimbart, 1882 includes 3 genera; the Neotropical and southern Nearctic Gyretes Brullé, 1835, the mainly Palaearctic and Oriental Orectochilus Dejean, 1833 (which also contains a single Afrotropical species) and the Afrotropical Orectogyrus Régimbart, 1884. The Gyrinini Latreille, 1810 includes the cosmopolitan Gyrinus Muller,O.F., 1764, Aulonogyrus Motschulsky, 1853 (absent only from the New World) and Metagyrinus Brinck, 1955, a poorly understood eastern Palaearctic genus containing 3 species. A third tribe, the Enhydrini Régimbart, 1882, includes 5 genera, only one of which, Dineutus McLeay, 1825, occurs in the Palaearctic but not in the U.K. Around 60 species of Gyrinidae occur in the U.S.A. and Canada, mostly among Dineutus (14spp.) and Gyrinus (41spp.) but also including the only species of Spanglerogyrus Folkerts, 1979.

Ecology

Commonly known as whirligig beetles, these are aquatic insects that can dive and fly but spend most of their time on the water’s surface, semi-submerged. These are the only beetles to spend their adult lives in this environment. Most species inhabit fresh water but a few occupy mixohaline habitats. A peculiar feature is the divided structure of the compound eyes; these form ariel and aquatic parts, actually consisting of two independent pairs of eyes, and are an adaptation to the beetle’s way of life, allowing simultaneous vision above and below the water. In the U.K. Gyrinids are a familiar sight on ponds, lakes and slow moving water, although not nearly so familiar as they were only a few decades ago. They often occur in large aggregations or ‘schools’ in late summer and autumn which may consist of several species. Social interactions within the school are complex are just becoming understood. Whirligigs are predators,

feeding on insects etc. on the water’s surface but they are also scavangers of dead insects etc. The specially adapted antennae contain a sense organ called Johnston’s organ in the pedicel which detects disturbances on the water’s surface and allows the beetle to sense the location of prey or to signal the presence of a threat to others in the group. Adults use a distasteful chemical defence called Gyrinidal to deter predatory fish etc., after sensing this the predatory organisms usually ignore them. Mating takes place on the surface. Eggs are attaches to aquatic vegetation, generally in small groups along stems. There are usually 3 larval instars. Larvae are generally bottom feeding predators that feed on a variety of soft bodied organisms e.g. worms, chironomid larvae or odonatid nymphs. They can remain submerged through all instars by obtaining dissolved oxygen through abdominal gills. Pupation occurs in the marginal substrate or on plant stems in a cocoon constructed of plant material or debris. Adults eclose after a few weeks. In temperate regions the adults usually overwinter. Gyrinids are exceptionally fast swimmers and their characteristic circular motions on the surface give them the name whirligigs. They dive very rapidly when alarmed as anybody who has tried netting them will know. When swimming they generally use the middle legs only but when alarmed both the middle and hind legs are used to achieve rapid escape. They are able to skim the surface with the aid of surfactants secreted by pygidial glands. Most species are diurnal but in unfavourable conditions at least some species become nocturnal. Naturally nocturnal species e.g. Orectochilus, are less gregarious than diurnal ones.

Description

Gyrinids are very distinctive and should never be confused with any other group. They range from 3-25mm, although European species do not exceed 9mm, most are flattened or weakly arched in cross-section and the overall shape is near ellipsoid with the appendages fitting closely into the streamlined body. Most species are black or black with a metallic reflection although in Aulonogyrus Motschulsky (with 2 European species, neither of which extends into the U.K.) the pronotum and elytra have wide yellow margins. The dorsal surface may be variously microsculptured and/or punctured, and in some groups is pubescent. The front legs are raptorial and used to grab prey while the middle and hind legs are short and broad, adapted for paddling. Males have the pro-tarsi dilated and are generally shorter and narrower than the females. The head is prognathous and proportionally large, with large and weakly convex eyes divided into dorsal and ventral parts, the dorsal parts being obvious from above, and unique 11-segmented antennae; very short with the two basal segments large, the second flattened and fringed with long setae, and the terminal 3 or 4 segments often fused, the terminal segment generally the largest. The clypeus is transverse and narrow, delimited by a distinct frontoclypeal suture, and covers the labrum, the mandibles robust, strongly curved and often bifid apically. The pronotum is broadest at the base and narrowed to protruding anterior angles, the lateral margins distinctly bordered and narrowly explanate, and there is generally an interrupted series of punctures behind the anterior margin. The prosternum is transverse and short with a median ridge extending back to between the pro-coxae where it meets the produced anterior margin of the mesosternum; the mesosternum is large and rhomboid, lacking a transverse suture and with the posterior margins attached to the large and triangular meso-coxae. The meta-coxae are very large and extend laterally to the elytral epipleura. Abdomen with sternites 2-7 visible; 2-4 fused although with visible sutures and divided by, and fixed to, the large meta-coxae, sternites 4-7 are distinct. All species have well-developed wings and are good fliers. The scutellum is visible between the elytral bases and is sometimes sculptured. The elytra are broadest about the middle, variously convex and rounded laterally, narrowly explanate and distinctly bordered, and at least to some extent truncate apically. The striae are generally strongly punctured although they lack a scutellary row, and there is a transverse series of punctures towards the apex. The front legs are normal with long femora and tibiae, flattened 5-segmented tarsi and smooth, curved claws, the coxae are moveable and the trocanters are short and triangular. The middle and hind legs have large, immovable coxae which have posterior slots along which the small triangular trocanters can move in a lateral arch to provide a thrust for paddling, the femora and tibiae are short and flat, the tarsi 5-segmented, flattened and variously expanded to open like a fan when swept back against the water, the fourth segment (at least) is fringed externally with long swimming hairs, and the terminal segment is small with relatively long and curved claws.

A key to the British species can be found HERE.