Back-swimmers
by John 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.
