Science of Water : Concepts and Applications

194 The Science of Water: Concepts and Applications
FIGURE 6.27 Whirligig beetle larvae.
FIGURE 6.28 Water strider.
Short front legs used
for getting prey
Middle legs used
like paddles
Abdomen
Antenna
the surface in aggregations of up to thousands of individuals. Unlike the mating swarms of mayfl ies,
these aggregations serve primarily to confuse predators. Whirligig beetles have other interesting
defensive adaptations. For example, the Johnston’s organ at the base of the antennae enables them
to echolocate using surface wave signals; their compound eyes are divided into two pairs, one above
and one below the water surface, enabling them to detect both aerial and aquatic predators; and they
produce noxious chemicals that are highly effective for deterring predatory fi sh.
(6) WATER STRIDER (“JESUS BUGS”; ORDER: HEMIPTERA)
Compound eye
Thorax
Rear legs used
for steering
It is fascinating to sit on a log at the edge of a stream pool and watch the drama that unfolds among
small water animals. Among the star performers in small streams are the water bugs. These are
aquatic members of that large group of insects called the “true bugs,” most of which live on land.
Moreover, unlike many other types of water insects, they do not have gills but get their oxygen
directly from the air.
Most conspicuous and commonly known are the water striders or water skaters. These ride the
top of the water, with only their feet making dimples in the surface fi lm. Like all insects, the water
striders have a three-part body (head, thorax and abdomen), six jointed legs, and two antennae. It has
a long, dark, narrow body (see Figure 6.28). The underside of the body is covered with water-repellent
hair. Some water striders have wings; others do not. Most water striders are over 0.2 in. (5 mm) long.
Water striders eat small insects that fall on the water’s surface and larvae. Water striders are
very sensitive to motion and vibrations on the water’s surface. They use this ability to locate prey.