Life on the edge

Our common skater Gerris lacustris is not alone on Britain’s waterways. A number of other insects share the surface habitat

• The common water measurer

(Hydrometra stagnorum) or marsh treader has a slender, elongated head and body with long antennae that look like an extra pair of legs. Found widely near the water’s edge, where plant stalks emerge, and on stagnant pools and bare mud, it moves slowly in search of small insect prey, detected through vibration—usually mosquito larvae and water fleas, which it spears on or below the surface, sucking out body fluids. It climbs stems both to find prey and to lay its eggs. The smaller

H. gracilenta is rare and found mainly on the Norfolk Broads

• A key element in the habitat food chain, the common water flea (Daphnia pulex) is one of some 80 species found in UK fresh waters and provides a diet for many predators, including fish. It reproduces without fertilisation and, if a pond starts to dry up, it produces egg sacs or ephippia, which drop to the bottom to await better conditions. These sacs are often distributed on waterbirds’ legs. It’s a member of the

Crustacea family and distant relatives range from lobsters to woodlice

• Smaller and stouter than the water skater, the water cricket (Velia caprai) is common across Britain. The related V. saulii is confined to Leicestershire and Rutland. Also known as the riffle bug or broad-shouldered water strider, the water cricket walks on water, feeds on fallen insects and is noted

for stealing the prey of others. It has two special features: a noxious chemical defence known to cause brown trout to spit it out unharmed and ‘expansion skating’, whereby the insect ejects saliva to lower the water surface tension, allowing it to travel twice as fast as normal

• Although Britain’s most remarkable water spider, Argyroneta aquatica, lives under the surface in an air-filled compartment woven from its own silk, the widely spread, chunky raft spider Dolomedes fimbriatus (left) lurks on the water’s edge, rides the surface tension and feels for vibrations, pouncing on water skaters and other prey. If endangered, it can submerge for several minutes. The female tends to attack any male that approaches. The larger

D. plantarius is endangered in the UK

• Swimming rapidly on the surface in circles and often in groups when searching for food, the whirligig beetle Gyrinidae latreille can dive instantly when threatened. Oval in shape and shiny black with orange legs, it is uniquely equipped with two pairs of compound eyes, one for seeing above water and the other below. Of about 1,000 found worldwide, two types are present in the UK