Water beetles

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.

Below is the whirligig beetle, in the family Gyrinidae, there are 12 species in Britain and 700 in the world, and 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. 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. It uses its middle and hind legs like oars for swimming, and its fore legs for grabbing prey. 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.

The beetle on the left is a water beetle belonging to the Hydrophilidae family. Not all the beetles in this family are aquatic - there are a few terrestrial species. The aquatic adults are easily recognised in water as their under-surface appears silver. This is caused by the air supply help in position by hairs. They collect their air supply through their antennae, and it is stored as a bubble attached to the underside of the body. The larvae tend to be slow moving. Adults are usually black.

It has the long palps and short antennae typical in this family. Some of the species are wholly carnivorous, some vegetarian, and others omnivorous. They are much slower swimmers than the Dytiscidae (below). 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.

Hydrous piceus, the Silver water beetle, right, 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 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 larvae are found in weedy ponds, especially those covered in duckweed, and they are carnivorous eating snails and tadpoles. 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. The adults are black with red antennae. The hibernate over winter in mud at the bottom of ponds.

On the left is a typical Hydrobius larva.

On the right is a pupa from the genus Philydrus. The 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.