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The whirligig beetles are water beetles, comprising the family Gyrinidae that usually swim on the surface of the water if undisturbed, though they swim underwater when threatened. They get their common name from their habit of swimming rapidly in circles when alarmed, and are also notable for their divided eyes which are believed to enable them to see both above and below water. The family includes some 700 extant species worldwide, in 15 genera, plus a few fossil species. Most species are very similar in general appearance, though they vary in size from perhaps 3 mm to 18 mm in length. They tend to be flattened and rounded in cross section, in plain view as seen from above, and in longitudinal section. In fact their shape is a good first approximation to an
ellipsoid An ellipsoid is a surface that may be obtained from a sphere by deforming it by means of directional scalings, or more generally, of an affine transformation. An ellipsoid is a quadric surface;  that is, a surface that may be defined as the z ...
, with legs and other appendages fitting closely into a streamlined surface. Whirligig beetles belong to the beetle suborder
Adephaga The Adephaga (from Greek ἀδηφάγος, ''adephagos'', "gluttonous") are a suborder of beetles, and with more than 40,000 recorded species in 10 families, the second-largest of the four beetle suborders. Members of this suborder are collecti ...
, which also includes ground beetles and diving beetles.


Description

Whirligig beetles are most conspicuous for their bewildering swimming. Their coloration is not showy and they can be quite hard to see if they are not moving or are under water. However, most species are handsomely coloured with a sombre lustre of steely grey or bronze. Their
integument In biology, an integument is the tissue surrounding an organism's body or an organ within, such as skin, a husk, shell, germ or rind. Etymology The term is derived from ''integumentum'', which is Latin for "a covering". In a transferred, or ...
is finely sculpted with little pits; it is hard and elastic and produces a water repellent waxy outer layer, which is constantly supplemented. Among other functions, the lubricant layer and smooth outline make the beetles remarkably difficult to hold on to if caught; they slip from between one's fingers like a fresh orange pip. The antennae are unusual among beetles, being short and plump, and placed about at water level. The compound eyes are remarkable for each being divided into a higher part that is above water level when a beetle is floating passively, and a lower part that is below water level. In this respect they recall the horizontally divided eyes of the four-eyed fishes (''Anableps''), which also live at the surface of the water. The middle, and more especially the hind legs are adapted for swimming ( natatory): they are greatly flattened and fringed with bristles that fold to aid swimming action. In contrast the front legs are long and adapted for grasping food or prey. In males the front tarsi have suckers, which are used to hold onto the slippery female during mating.


Behavior and morphological adaptations

The Gyrinidae are surface swimmers for preference. They are known for the bewildering and rapid gyrations in which they swim, and for their gregarious behavior. Most species also can fly well, even taking off from water if need be. The combination constitutes a survival strategy that helps them to avoid predation and take advantage of mating opportunities. In general the adults occupy areas where water flows steadily and not too fast, such as minor rapids and narrows in leisurely streams. Such places supply a good turnover of floating detritus or struggling insects or other small animals that have fallen in and float with the current. The positions that individuals occupy within a group are determined by a number of factors, thought to include hunger, sex, species, water temperature, age, parasite level and stress level. Research underway on their behavior is directed at investigating the significance of
chemical defense Chemical defense is a life history strategy employed by many organisms to avoid consumption by producing toxic or repellent metabolites or chemical warnings which incite defensive behavioral changes. The production of defensive chemicals occurs in ...
in relation to their position in the group. Such studies are of interest in research into aspects of
nanotechnology Nanotechnology, also shortened to nanotech, is the use of matter on an atomic, molecular, and supramolecular scale for industrial purposes. The earliest, widespread description of nanotechnology referred to the particular technological goal ...
because the beetles' motion may be expected to provide insights into how groups of robots might coordinate movements.In particular the beetles make behavioral trade-offs that affect their choices of positions within a group. For example, relatively hungry beetles go to the outside of a group, where there is less competition for finding food, but higher risk of encountering predators. Males are also more likely to be found on the outside of groups (although grouping is not known to be relevant to mating behavior in this family). The economies that the beetles can gain by suitably adjusting their positions within the group, are important when individuals swim against the flow of a stream. By swimming behind other beetles they can take advantage of forward-moving drafts. Such action is called drafting. The determination of forward/backward positioning within a group has been found to be affected in a complex manner by a combination of water speed, sex of the beetle, and the type of predator (bird or fish) that a beetle has most recently observed. The beetles could use the waves generated by their moving as a sort of radar to detect the position of object on the water surface around them. This technique could be used to detect prey or to avoid colliding each other. The adult beetles carry a bubble of air trapped beneath their
elytra An elytron (; ; , ) is a modified, hardened forewing of beetles (Coleoptera), though a few of the true bugs (Hemiptera) such as the family Schizopteridae are extremely similar; in true bugs, the forewings are called hemelytra (sometimes alterna ...
. This allows them to dive and swim under well-oxygenated water for indefinite periods if necessary. The mechanism is sophisticated and amounts to a physical gill. In practice though, their ecological adaptation is for the adults to scavenge and hunt on the water surface, so they seldom stay down for long. The larvae have paired plumose tracheal gills on each of the first eight abdominal segments. Generally, gyrinids lay their eggs under water, attached to water plants, typically in rows. Like the adults, the larvae are active predators, largely
benthic The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from ancient Greek, βένθος (bénthos), meaning ...
inhabitants of the
stream bed A stream bed or streambed is the bottom of a stream or river (bathymetry) or the physical confine of the normal water flow ( channel). The lateral confines or channel margins are known as the stream banks or river banks, during all but floo ...
and aquatic plants. They have long thoracic legs with paired claws. Their mandibles are curved, pointed, and pierced with a sucking canal. In this they resemble the larvae of many other predatory water beetles, such as the
Dytiscidae The Dytiscidae – based on the Greek ''dytikos'' (δυτικός), "able to dive" – are the predaceous diving beetles, a family of water beetles. They occur in virtually any freshwater habitat around the world, but a few species live ...
. Mature larvae pupate in a cocoon that also is attached to water plants.


Taxonomy

Whirligig beetles were previously grouped with other aquatic members of the
Adephaga The Adephaga (from Greek ἀδηφάγος, ''adephagos'', "gluttonous") are a suborder of beetles, and with more than 40,000 recorded species in 10 families, the second-largest of the four beetle suborders. Members of this suborder are collecti ...
such as
Dytiscidae The Dytiscidae – based on the Greek ''dytikos'' (δυτικός), "able to dive" – are the predaceous diving beetles, a family of water beetles. They occur in virtually any freshwater habitat around the world, but a few species live ...
, as members of the group "Hydradephaga". However based on molecular evidence they are currently thought to be the earliest diverging lineage of the
Adephaga The Adephaga (from Greek ἀδηφάγος, ''adephagos'', "gluttonous") are a suborder of beetles, and with more than 40,000 recorded species in 10 families, the second-largest of the four beetle suborders. Members of this suborder are collecti ...
, and to have evolved their aquatic ecology independently from other adephagans. Cladogram after Vasilikopoulos et al. 2021


Internal taxonomy

Taxonomy after * Spanglerogyrinae ** '' Angarogyrus'' - Early Jurassic-Early Cretaceous, Asia ** '' Spanglerogyrus'' - North America * Heterogyrinae ** '' Mesogyrus'' - Late Jurassic-Early Cretaceous, Asia ** '' Heterogyrus'' - Madagascar ** '' Cretotortor'' - Late Cretaceous-Paleocene (Asia) ** '' Baissogyrus'' -
Zaza Formation The Zaza Formation is a geological formation located in Buryatia ( Russia). It dates to the Lower Cretaceous period. It is Aptian in age and consists of sandstones, siltstones, marls and bituminous shales, deposited in a stratified lake. It is s ...
, Russia, Early Cretaceous (
Aptian The Aptian is an age in the geologic timescale or a stage in the stratigraphic column. It is a subdivision of the Early or Lower Cretaceous Epoch or Series and encompasses the time from 121.4 ± 1.0 Ma to 113.0 ± 1.0 Ma (million years ago) ...
) * Gyrininae ** Dineutini *** '' Cretodineutus'' -
Burmese amber Burmese amber, also known as Burmite or Kachin amber, is amber from the Hukawng Valley in northern Myanmar. The amber is dated to around 100 million years ago, during the latest Albian to earliest Cenomanian ages of the mid-Cretaceous period. The ...
, Late Cretaceous (
Cenomanian The Cenomanian is, in the ICS' geological timescale, the oldest or earliest age of the Late Cretaceous Epoch or the lowest stage of the Upper Cretaceous Series. An age is a unit of geochronology; it is a unit of time; the stage is a unit in ...
) *** '' Cretogyrus'' - Burmese amber, Cenomanian *** '' Dineutus'' *** '' Enhydrus'' *** '' Macrogyrus'' (including '' Andogyrus'' ) *** '' Mesodineutes'' - Darmakan Formation, Russia,
Danian The Danian is the oldest age or lowest stage of the Paleocene Epoch or Series, of the Paleogene Period or System, and of the Cenozoic Era or Erathem. The beginning of the Danian (and the end of the preceding Maastrichtian) is at the Cretaceou ...
*** '' Miodineutes'' - Germany, Miocene *** '' Porrorhynchus'' ** Gyrinini *** '' Aulonogyrus'' *** '' Gyrinoides'' *** '' Gyrinus'' *** '' Metagyrinus'' ** Orectochilini *** '' Gyretes'' *** '' Orectochilus'' *** '' Orectogyrus'' *** '' Patrus'' ** '' Chimerogyrus'' - Burmese amber, Cenomanian *''Incertae sedis'' ** '' Anagyrinus'' - Insektenmergel Formation, Switzerland, Early Jurassic,
Hettangian The Hettangian is the earliest age and lowest stage of the Jurassic Period of the geologic timescale. It spans the time between 201.3 ± 0.2 Ma and 199.3 ± 0.3 Ma (million years ago). The Hettangian follows the Rhaetian (part of the Triassic ...
** '' Gyrinopsis'' - Insektenmergel Formation, Switzerland, Early Jurassic, Hettangian


References

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External links


Gyrinidae
Tree of Life {{Authority control Aquatic insects