Environmental Engineering Reference
In-Depth Information
and fresh macrophytic leaves near the bottom
along the littoral zone. They are active swim-
mers and swift divers. Adult dytiscids range
from 1.4 to 3.8 mm in length. Although most
species are small to medium sized, some adults
can attain a length of 35 mm. The body is cov-
ered with an adherent layer of grease which
holds dust particles or detritus. They are usually
black or brownish in colour, sometimes marked
with dull yellow, orange or brown shades. The
hind coxae are very large and the second and
third legs are widely separated. The hind legs of
dytiscid beetles are very important and contrib-
ute mainly to swimming movements. The anten-
nae are very long and threadlike with 11
segments. Ten pairs of spiracles are present, the
fi rst two on the thorax, three to nine on the
abdominal segments and the tenth on the tip of
the abdomen. The spiracles open in subelytral
chambers and help in oxygen supply. During
submergence these beetles utilize the oxygen
from the tracheae and subelytral chambers. Dey
and Sengupta (
1993
) have recorded 16 species
from a few wetlands of Kolkata and surrounding
districts. More than 3,700 species are known
( Nilsson and Persson
1990
), of which 223 have
been recorded from India.
Adults are 1/4 to 1-3/16 in. long, ovate and
smooth and have shining black bodies. There is a
yellow margin around the front of the head and
sides of the thorax and wing covers. The anten-
nae are threadlike and prominent, with the hind
legs longest and fl attened to serve as oars in
swimming. They are attracted to light, often
migrate from one pond to another and are strong
fl iers. Unlike other water beetles which move
their legs alternately when swimming, these bee-
tles stroke them together like oars as the back-
swimmer bugs do. They can be collected by
sweeping submerged vegetation in streams and
ponds with a dip net. Both adults and larvae are
predaceous. Sharp hollow mandibles (jaws) are
used to inject paralyzing and digestive fl uids into
their prey and to draw out the liquid contents.
They attack aquatic insects and fi sh and have the
ability to regenerate lost parts to some extent.
Larvae crawl to shore to pupate in the soil, and
hibernation occurs in the water as an adult or lar-
vae. There is one generation per year, and adults
can live several years.
2. Family: Gyrinidae
The members of family Gyrinidae are com-
monly known as whirligig beetles. They are
found in freshwater ponds, lakes, open fl owing
streams, etc. When the gyrinid beetles are swim-
ming on the surface of the water, the dorsal por-
tion of the eye is in the air and the ventral portion
in the water. The fi rst abdominal sternite is
divided by hind coxae (suborder Adephaga);
short, clubbed antennae; and seemingly two pairs
of eyes, one above and one below the water level.
The forelegs are long and thin; the middle and
hind legs are short and paddle-like, not extending
beyond the margin of the abdomen (only front
legs visible in dorsal view); the body is elongate
oval and fl attened, usually 3 to 15 mm in length,
and the tarsi are 5-5-5. Vazirani has recorded 46
species under the family Gyrinidae from India.
Adults are 3/8 in. long or slightly longer, oval,
fl at and hard bodied with a shiny black bronze
sheen. The legs are brownish yellow. Eyes are
divided into two widely separated parts; the
upper one is oval and remains above the water,
and the lower one is somewhat smaller and
remains below the water. These fast-moving bee-
tles are often seen swimming in groups in endless
gyrations or circular “skating” movements, in a
spot on the water surface. The outer margins of
the wing covers are curved inward near the tip.
The legs are fl attened and fi tted for swimming,
and the antennae are quite short with the last seg-
ment enlarged. They are found in lakes, ponds
and streams. The adult scavengers and the larvae
are predaceous and aquatic. The larvae leave the
water to pupate on bordering plants and overwin-
ter as adults. There is one generation per year.
3. Family: Hydrophilidae
This is another major family of aquatic
Coleoptera. There are more than 2,500 known
species, and many new ones are being added
every year. Currently 171 genera are known.
The family is diverse not only in number of spe-
cies but also with respect to variety of form, size
and ecological strategies (Hansen
1991
).
Hydrophilids occur in all biogeographic regions.
The hydrophilids commonly termed as “water
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