by John Walsby
Many insects lay their
eggs on or near freshwater and have aquatic larvae and pupae.
The midge, mosquito, mayfly, dragonfly, damselfly, stonefly and
caddisfly are all insects that spend their larval lives in water.
They feed by various methods, scraping algal films from stones,
straining minute organisms from the water, chewing rotting leaves
and other debris washed down from the surrounding bush or preying
on other small animals.
The larvae of mosquitoes
breathe air through small snorkels that they periodically poke
through the surface film. Other larvae have external gills to
take dissolved oxygen from well aerated water into the tubular
air systems inside their bodies.
Caddisflies are particularly
interesting as many of them carry protective shelters and they
include carnivores, herbivores and scavengers of riverbed debris.
Some build their mobile homes from secreted parchment which hardens
to form a leathery case. Some collect sand grains or small chips
of wood and cement them together to make miniature brick or shingle
tubes. One species makes a spiral tube that looks rather like
a snail's shell.
Yet other caddis fly
larvae make their homes in existing tubes especially hollow pieces
of twig.This makes them very difficult to see on the streambed
for when we first look over the bank, they look like all of the
other pieces of waterlogged stick lying on the bottom. This is
a good self protective measure for animals that live in an environment
where many insect larvae are snapped up by predators.
To find the occupied sticks, you must sit perfectly still on the bank. The insects are very sensitive to movement but probably see detail very poorly so will not see you if you keep motionless. Soon their heads and front legs will emerge as they resume foraging for food, towing their stick caravans along behind.