Gyrinidae
Common name: Whirligig beetles
Number of species: 12
Size Range: 3-7mm
Tarsi: Strange.
Description:
This family, with 12 British species in Britain, are most familiar as groups of small dark elliptical beetles swimming in circles on the open water of ponds and streams. Scarcer species live amongst emergent vegetation, rather than circling openly by day. All are carnivorous, feeding on animals caught in the surface film. Larvae breathe through gills, rather than from the surface, which may explain the group’s predilection for deeper water bodies.
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, with legs and other appendages fitting closely into a streamlined surface. Whirligig beetles belong to the beetle suborder Adephaga, which also includes ground beetles and diving beetles.
Where are you most likely to find a whirligig beetle?
The whirligig beetle (family Gyrinidae) is a type of aquatic beetle that is found in calm bodies of water such as in some streams, rivers, lakes, and ponds.
What is unusual about the eyes of whirligig beetles?
Perhaps the most astounding feature of the whirligig beetles is the complex eye conformation with split compound eye structures forming dorsal and ventral eyes.
How fast is a whirligig beetle?
100 body lengths per second
Whirligig beetles (Gyrinidae) are the fastest-swimming insects. The one-centimeter long aquatic beetle can reach a peak acceleration of 100 m s-2 and a top velocity of 100 body lengths per second.
Whirligig beetles are most conspicuous for their bewildering swimming. They can be difficult to see if they are not moving or are under water. Most species are coloured steely grey or bronze. Their integument 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 difficult to hold on to if caught.
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 in relation to their position in the group. Such studies are of interest in research into aspects of nanotechnology 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. 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 inhabitants of the stream bed 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. Mature larvae pupate in a cocoon that also is attached to water plants.
You may have seen this little black beetle whirling around on the top of the water in a pond or slow-moving river. Their back legs are short and flat so act as the perfect paddle for shooting across the water surface on the hunt for small insects. They also dive underwater to catch other animals for a tasty treat.
Scientific NameSpecies in the beetle family Gyrinidae
FamilyGyrinidae (whirligig beetles) in the order Coleoptera (beetles)
DescriptionWhirligig beetles look like a blur as they gyrate endlessly around each other on the water. These beetles are oval, streamlined and usually blackish, sometimes bronzy or metallic. The forelegs are long and slender; the middle and hind legs are short, flattened, and fold tightly under the body. One common species is Dineutus americanus.
Whirligigs can be distinguished from all other beetles by their short, clubbed antennae and their two pairs of compound eyes — one pair above the water, and one pair below — which helps them to quickly and accurately capture their prey while also evading predators.
Like other beetles, whirligigs have membranous hindwings that are covered by forewings that have been modified into thick, heavy, shields. When the wings are closed, the two shieldlike forewings form a straight line down the back.
Larvae are pale, elongated, flattened, with 3 pairs of crawling legs and 8 pairs of featherlike gills protruding from the sides of the abdomen.
SizeAdult length: about ¼–¾ inch (varies with species).
Habitat and Conservation
Whirligig beetles occur in many types of aquatic habitats, including ponds, lakes and streams. They can swim almost as effortlessly underwater as they do on the surface, making them difficult to catch. Their wings are well developed, so whirligigs can fly to a new home if their pond or stream should dry up.
Food
These beetles and their larvae are carnivorous. The larvae eat other aquatic insects and invertebrates. The adults often feed on land insects that fall into the water. They are attracted to the waves caused by the struggling insect, and sometimes a group of whirligigs can be seen crowding around one of these meals, each trying to take bites from it. They also function as scavengers, eating dead creatures, too.
Life Cycle
Life CycleEggs are laid on the surfaces of submerged aquatic plants. The larvae are not as commonly seen as the adults and spend most of their time crawling on the bottom or sometimes swimming with sinuous movements. When grown, the larvae crawl out of the water and form pupas on nearby plants. The adults return to water and overwinter in mud and debris. In spring, they emerge from hibernation and form hunting groups.
Feeding and diet
Whirligig beetles feed by capturing and eating dead or injured insects on the water surface with their elongated front legs. The larvae are predators that crawl rather than swim. Like most predacious diving beetles, the adults find water by flying.
Other behaviours and adaptations
Whirligig beetles are among the most specialised beetles for living in and on water because all their legs are modified into flat paddles and their eyes are split to give both underwater and above-water vision. If a predator such as a bird approaches from above, the beetle dives below the water. If a predator such as a fish approaches from below, the beetle seeks shelter at the water’s edge.
