Water bug must take in a store of air at the surface and keep it beneath their wings when swimming underwater.
Pond skaters, Water striders, Gerridae
The pond skaters, left are in the Gerridae family. There are about 500 species worldwide and 10 British species. They range in length from 0.2 mm - 35 mm.
Legs. They feed on insects that have fallen into the water using their front legs to grasp their prey, their middle legs to row, and hind legs as rudders. Prey is detected by sight (they have fairly large eyes) and/or vibration. Although they will also eat dead insects they find floating on the water.
Pond skater feet feet are surrounded by water-repellent hairs, and these prevent the feet from piercing the water surface.
Wings. Some pond skaters have fully-formed wings and can fly; others are flightless.
In the UK they hibernate on land but near water over winter. Eggs are laid around May.
They are found on still freshwater ponds and lakes.
Water boatman, Notonectidae
There are 4 British species in this family. The water boatmen Notonecta sp. (right) are also known as back-swimmers because they spend most of their time swimming on their back on the underside of the water surface (see the drawing below). Their air supply is held in place by the wing cases and the short hairs on the body. This makes them very buoyant. If they wish to go deeper under water they must hold on to vegetation - usually with their short front legs - otherwise they will float to the surface.
They use their forelegs to catch prey, and their hair-fringed hind legs, which are twice as long as the other legs, for swimming rather like oars. If they are put in a tank which is lit from below they will swim normally.
The prey is detected by sight and by sensing surface ripples. The prey is often bigger than they are. Some of the larger species can pierce human skin. The adults are strong fliers.
Notonecta glauca is found throughout the UK and is about 1.5 cm long. It is particularly fond of tadpoles and water beetle larvae and other insects which are detected by sight and vibration. It will also take fish fry, so is considered a pest in trout hatcheries. It has large eyes, and these along with the sensitive hairs on its legs make it a fearsome predator. It avoids acidic, peaty water.
Mating takes place any time from mid-winter to May. Eggs are laid singly from February onwards in the stems of water plants. Nymphs take around 2 months to mature.
Lesser waterboatman, Corixidae
There are 35 British species in this family, but identification to species level is difficult. The lesser water boatman Corixa sp. (left) swims normally, i. e. with their back uppermost. Fully grown adults range in size from 12 - 16 mm.
They are mainly herbivorous. Most adults are good fliers. When underwater the air supply is stored in the space between the wings and the abdomen. They are usually found in still water. Male waterboatmen can stridulate by rubbing their front legs against a ridge on the side of their head. Eggs are laid singly, attached to plants or algae.
Nymphs breathe through their skin.
There are just 2 British species in this family, the water scorpion, left above and right, and the water stick insect, below.
The water scorpion, Nepa cinerea, (see above, left and right) got its name because of its long tail, but the tail is a breathing tube (actually two tubes stuck together), rather like a snorkel, not a sting. The body length is 20 - 23 mm, and the tail is usually around 10 mm long. It is dark brown and flat, so well camouflaged and is often mistaken for a dead leaf.
Habitat. It can be found in weedy, stagnant ponds, shallow lakes, fens and occasionally weedy streams, and tends to lurk around in vegetation waiting for prey to pass within reach.
Behaviour. It uses its front legs to catch prey, then sucks out the insides of whatever it has caught, usually other insects, tadpoles or small fish. It injects a powerful digestive enzyme into the prey to overcome it quickly. They are poor swimmers. Adults can fly, but rarely do so. It is about 3.5 cm in length including the tail. When taken out of the water it pretends to be dead, or crawls quickly away. The female lays her eggs underwater in the stems of vegetation just below the water surface.
Above left is the nymph which has a much shorter siphon.
Water stick insect, Ranatra linearis
The water stick insect (right) can be around 5 cm long, although its breathing tube tail can take up half this length. It is no relation to the true stick insects which are terrestrial.
Habitat. It can be found in weedy ponds usually in water less than 1 m deep.
Adults are active throughout the winter except in extreme cold. They are good swimmers and adults can fly. The first pair of legs is used to catch prey, but they will also eat dead insects it finds floating on the surface.
Eggs are inserted in plant stems.
Water cricket, Veliidae
There are 5 British species in this family. On the left is the water cricket, Velia currens. It is dark brown with 2 orange stripes down the back of adults.
In both adults and nymphs the underside of the abdomen is orange. The adult can be winged or wingless.
It is common on both still and running water. It feeds on insects and other small animals which land on the water surface. The female lays batches of eggs on floating vegetation.
Water measurer, water gnat Hydrometridae Family
This is one of the pond skater family. They are found world wide, there are approximately 100 species, but just 2 British species, and all are thin and stick-like.
On the right is Hydrometra stagnorum, the water measurer, water gnat, pond skater, adults are 9 - 12 mm long, glossy, dark brown and with an extremely long head. They are wingless even as adults.
It is found in still water, and feeds on small insects and water fleas. The female lays eggs singly on plants above the water level.