Water Beetles

Water beetles overview

Nearly all water beetle adults are air breathing, so they have to rise to the surface for air which they often store under their elytra. Some rise tail first and others head first.

Note the rear legs of all these beetles, they are fringed with hairs and bristles to aid in swimming, and are unsuitable for movement on land.

Gyrinidae, whirligig beetles

Left and below is the whirligig beetle, there are 12 species in Britain and 700 in the world. The adults range in length from 3 - 15 mm long.

True to its name, this is the beetle you will see gyrating around on the surface of the water.

On the right are the mouthparts of a whirligig beetle. The mandibles are for biting/chewing, and are the equivalent of our jaws. The maxillae are accessory jaws; the maxillary palps are sensory for testing food, and the labium and labial palps are also sensory. The labium is the equivalent of our lower lip.

whirligig beetle mouth parts
Gyrinidae, whirligig beetle

Its eyes are divided into two parts; the lower part is used for looking down and through water, and the upper for looking ahead and in the air, so it can see above and below the water simultaneously. It can escape from danger below by flying and from above by diving into the water.

Flight. In order to fly the adult climbs up some vegetation, opens

its wing cases, unfurls its wings, then flies off. Quite often whirligigs will make a squeaky noise just as it is about to take off. And in flight the wing cases rub against the body making a humming or buzzing sound.

It uses its middle and hind legs like oars for swimming, and its fore legs for grabbing prey. The adults prey on small insects that fall on to the water surface.

Gyrinus natator

The most common whirligig in the UK is Gyrinus natator, and it should be a welcome resident in any garden pond as it feeds mainly on mosquito larvae.

Over wintering. In the U. K. Most adults do not survive the winter. However any adult whirligig seen in spring has managed to successfully overwinter by hibernating in the mud at the bottom of a pond.

The adults (above right) are 5 - 6 mm long, shiny black oval-shaped with yellow legs and short, clubbed antennae. They are found in slow-moving and stagnant water. The middle and hind pairs of legs are flattened and fringed with hairs. The abdomen extends a little beyond the elytra. The adults hibernate in mud at the bottom of ponds and streams.

The larvae look like small centipedes (see the drawing above) and though highly predatory, tend to lurk in vegetation or detritus in shallow water. They have a small head, 3 pairs of legs, 1 pair of gills on each abdominal segment except the last which has 4 hooks. When fully grown they can reach 15 mm long. The larvae pupate out of water by climbing up vegetation and spinning a cocoon.

The female lays her eggs on submerged plants.

Gyrinidae larva, whirigig beetle larva

Hydrophilidae, water beetle

Hydrophilidae family of water beetles

There are around 2 000 species in the world, 70 have been recorded in Britain. The larvae range in length from 1 - 48 mm.

The beetle on the left is a typical water beetle belonging to the Hydrophilidae family. It has the long palps and short antennae typical in this family. Not all the beetles in this family are aquatic - there are a few terrestrial species, although these tend to be found in damp, marshy places. The aquatic adults are easily recognised in water as their under-surface appears silver, though they are usually black. This is caused by the air supply held in position by hairs.

Some of the species are wholly carnivorous, some vegetarian, and others omnivorous.

They are much slower swimmers than the Dytiscidae. The way to tell the two families apart is to note the difference in sizes of their palps and antennae. In the Hydrophilidae the palps are longer, or at least as long as the antennae, and the antennae end in clubs.

Breathing. Adults collect their air supply through their antennae, and it is stored as a bubble attached to the underside of the body. The larvae take in air through a pair of spiracles in the tail (see the drawing of Hydrobius sp. below). The larvae tend to be slow moving.

In some species the female carries her eggs around in a silken case attached to her abdomen or hind legs.


Hydrobius sp. larva

Left is a typical Hydrobius larva.

On the right is a pupa from the genus Philydrus.

These adults are around 5 mm long. The female makes an egg cocoon which she attaches to floating plants.

The larvae often emerge from the water, and the pupa is always found out of water, usually hanging from moss with the final larval skin found beneath it.


Philydrus sp. pupa

Hydrophilus piceus (Hydrous piceus), the Great Silver water beetle

In some books it is also known as the Great Water Beetle which is the common name for Dytiscus marginalis. This is the second largest British beetle with an adult length of up to 4 cm. Most of the other British species in this family are less than 1 cm long as adults. The adults are black with red antennae. Even though they are large they can still fly, but tend to do so at night. They hibernate over winter in mud at the bottom of ponds. They are rather slow moving and mainly vegetarian.

The adults have needle-sharp spurs on their tibia, and a sharp spine on their underside which can cause injury if the beetle is not handled carefully.

The females construct egg cocoons of silk produced from a gland in the last abdominal segment. Each female lays about 50 eggs in the cocoon which floats. The eggs hatch after 2 - 6 weeks, depending on temperature.

The larvae are found in weedy ponds, especially those covered in duckweed, and they are carnivorous eating snails and tadpoles, they are also cannibalistic if they have the chance. They breathe through spiracles on the last abdominal segment. A fully grown larva can be as much as 7 cm long. They pupate in damp earth at the side of the pond. Sadly this beetle is much less common now than it used to be.


Hydrous piceus, the silver water beetle
Hydrobius fuscipes

Hydrobius fuscipes

On the left is an adult, and on the right the larva, it is common in stagnant water, weedy ponds and ditches and shallow lakes with muddy bottoms.

Adults are about 8 - 10 mm long, and have a blue, black or green metallic sheen and reddish brown legs. The antennae are often clubbed.

The larva grow up to 12 mm long, and are soft with short legs. They look a little like a maggot.

Hydrobius fuscipes larva
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