Wetland Insects
by John Walsby
"Wetland Rushes
and Sedges and Land Management" discussed the importance
of wetland plants, especially rushes and sedges, in creating meadows
where water flow was regulated, bank erosion prevented and the
land run-off cleansed of silt and polluting chemicals that result
from horticultural, agricultural, residential and industrial activities.
Flourishing on the
run-off nutrients, the large plants and the microscopic algae
that grow over and among them, provide bountiful food supplies
for many small insects, worms, snails and crustaceans that also
enjoy the shelter given by the rushes. These creatures in turn
become a food resource for larger insects such as damsel flies
and dragonflies, and for fish and wetland wading birds.
The conditions are
also ideal for small semi-terrestrial creatures that include small
snails, hoppers and slaters and many insects that would be susceptible
to drying out in more open habitats. They can move about easily
through the moist but airy layers of living stems and rotting
vegetation just above the water line and as the water level gradually
rises or falls during wet and dry periods they can remain in the
conditions that suit them best by moving up or down just a few
centimetres.
Some adult insects
- mostly predators and scavengers - live on the water surface
and others stay below it but need to rise regularly to breathe
air. Meanwhile the larvae of many wetland insects are fully adapted
to an aquatic life. They have gills for extracting oxygen underwater
and some also have red blood pigments, like the haemoglobin that
we have for transporting oxygen in our own blood, to enable them
to respire in places where oxygen levels are low.
The tall stems of marshland
plants make it easy for the emerging adults of insects with aquatic
larvae to climb clear of the water or damp ground below, to dry
their wings in preparation for an aerial life. For many, like
mayflies and midges, this lasts only a day or two, sometimes less
than a few hours; just long enough to find a mate, copulate, fly
to a new site and lay eggs for the next generation.
The standing water
and sluggish seepages of wetlands are usually well colonised by
very small, free swimming crustaceans, such as cyclops (copepods),
seed-shrimps (ostracods) and water fleas - that are best observed
through a strong magnifying glass or low power microscope. Like
the more familiar mosquito larvae, or wrigglers, that are also
common in these quiet waters, they feed on microscopic single
celled plants and animals (freshwater plankton) and on minute
particles of suspended detritus - the decaying remnants of larger
animals and plants.
Most aquatic larval
insects live on or in the soft mud below. There they feed on settled
detritus, filamentous algae, diatoms and single celled protistan
organisms (ciliates, flagellates and amoebae) that flourish in
the organic broth that covers the bottom.
They include the worm-like
larvae of midges and craneflies, burrowing mayfly nymphs, and
the larvae of caddis flies that are easily recognised by the homes
that they live inside and drag along. These tubular protective
cases are made either from hollow plant stems or twigs, or by
gluing plant fragments or small stones on to a parchment lining
and frequently they are difficult to spot as caddis cases until
you notice one moving across the bottom in still water.
All these crustaceans,
insect larvae and microscopic life forms play a major part in
cleansing run-off water and are important links in the food chain
because they help reprocess the finest particles of organic decay.
As they grow on this resource, they turn it into animals that
are large enough to become food for quite large predators such
as fish and wading birds.
