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Back-swimmers


by John Walsby


drawing of backswimmer by J. walsby

Throughout the country the back-swimmer, Anisops assimilis, is found in most lakes and large ponds. It is a bug - related to cicadas and the green vegetable bug - with mouthparts for piercing and sucking. But it is specialised for a life spent swimming upside down underwater. Its body is streamlined, roundly blunt at the front - where large, reddish eyes seek out prey - and pointed at the back.


Like other predators, the back-swimmer must be able to move rapidly to catch its prey. It propels itself by strong strokes of its oar-like hind-legs. These are very long and have marginal fringes of fine hairs that form the paddle "blades". The front two pairs of legs are folded away neatly against the body and are only extended to grasp prey.


Quite big insects, such as moths, accidentally caught in the surface film, are attacked and it catches insects laying eggs on water plants or on the water surface. When the eggs of other insects hatch it also feeds on their aquatic larvae. It is therefore useful in controlling midge and mosquito numbers.


The back-swimmer feeds by thrusting its sharp proboscis into captured prey, like a hypodermic needle and then injects an anaesthetic poison to stop the prey struggling. The prey's juices are then sucked out through the proboscis tube.


The back-swimmer breathes underwater from an air bubble trapped around the bug's rear end by a layer of fine hairs. Periodically it breaks through the water surface to change the air. The bubble size is just right to keep the insect neutrally buoyant so it neither floats up nor sinks when it stops swimming but can hang motionless at any depth in the water.


Male back-swimmers make chirping sounds underwater by rubbing bristles on the front legs against the body to attract females. They mate underwater and attach their eggs to water plants. As with all bugs, minute wingless versions of the adults hatch from the eggs and grow through successive nymphal stages to the winged adult form.


As ponds are isolated water bodies which may dry out in summer, winged adults may, just once, leave the water to fly off in search of another pond to assure the species dispersal and survival.


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