Water-boatmen
by John Walsby
There are a few species
of insects that spend their whole lives in or on the water and
rarely fly. While the adults are able to fly well they rarely
use their wings except to escape to another pool should the one
they are living in dry out.
Water-boatmen, back-swimmers,
water measurers and pond skaters are all examples and are common
insects of our ponds, lakes and quiet stream backwaters. They
are all bugs related to more familiar examples, such as the cicada,
aphid and the green vegetable bug.
Bugs are insects with
piercing mouthparts inside a tube that looks like a hypodermic
needle through which they suck up plant or animal juices. Bugs
do not develop through larval and pupal stages as many other insects
do but instead, nymphs - wingless miniatures of the adults - emerge
from the eggs.
The water-boatman,
Sigara arguta, is a plant feeder, sucking sap from water weeds
below the surface. Supported and protected below water, these
plants have soft stems that are easy to penetrate and feed from.
Like any insect, the water boatman has three pairs of legs but
from above only two pairs are visible. The forelegs are tucked
away underneath and are only used to manipulate and steady the
weed while the bug thrusts in its feeding proboscis.
The hind legs are very
long and flattened and their surface areas are expanded by broad
fringes of hairs along the edges. This equips them to function
as a pair of strong oars to row the bug through the water.
The hooked middle pair
of legs are used to grasp weeds under water when the water-boatman
is not swimming, for a bag of air trapped around its abdomen by
a coat of hairs makes it slightly buoyant. The water-boatman breathes
from this air supply and when the oxygen in it becomes low, it
floats to the surface to trap a fresh supply of air.
To find these insects,
watch for their jerky swimming strokes. If you can catch one,
you will see it is an attractive insect with a dappled meshwork
pattern covering the back.
