by 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.