Pond-skaters and
Water-measurersby John Walsby
Do you have pond-skaters
on your local pond ? Our flat bodied native species, Microvelia
macgregori is only 2-3 mm long but is very common. Like all skaters,
it has its legs spread wide and flat with their ends laid along
the water surface so that the skater's weight is broadly distributed.
With this wide legged gait, easily supported by the surface tension,
it skims across the water with shallow bounds, leaving a trail
of ever-increasing circles behind.
When most other insects
land accidentally on water, their legs or wings break the surface
tension and the become trapped. As they struggle to escape they
send ripples across the pond surface.
As well as supporting
and propelling these carnivorous bugs, the legs of pond skaters
also serve as vibration sensors. When the ripples from a struggling
victim pass under each leg it can work out where they originated
and move in for the feast. Generally many pond-skaters converge
to feed on the stranded insect.
A good if rather cruel
way to observe these bugs is to catch a winged insect, a small
moth perhaps, and release it below the water. It will float up
and then struggle at the surface. As the pond-skaters arrive you
will be able to see each one thrust in its long pointed proboscis
to suck out the juices.
The water-measurer,
Hydrometra risbeci, is also a predatory bug. At about 13mm long,
it is much larger than the pond skater. From its long head and
body protrude spindly antennae and legs and it looks rather sinister,
moving with measured steps along waterweed stems and across the
water's surface.
It keeps its piercing
proboscis tucked away beneath the long head, but unfolded it is
the length of the head. This great length enables it to feed on
animals just below the water surface as well as those caught in
the surface film.
These bugs are less
common than pond-skaters. See if you can find them in your local
pool or lake among the vegetation that grows around the margins.
