Caddisfly larvae case; Garold W. Sneegas photo
or caddisflies, is one of the largest groups of aquatic insects. More than 1,200
species are now known in North America north of the Rio Grande River. The larvae are
highly specialized for food acquisition. Some build cases from small pebbles,
leaves, or twigs held together by silk produced by the larvae. These cases help camouflage
the larvae and protect them from predators as they walk about grazing on fine particulate
organic material and diatoms. Other caddisfly species like the one below build silk nets
to trap detritus. Still others are predaceous, building and waiting within silk
tubes having external threads to detect the presence of prey.
Caddisfly nets; Ohio Dept. Of Natural Resources photo
particularly true in headwater streams, which are characterized by little or no other
nutrient sources, a closed shade-producing canopy, and low water temperatures. These cool,
shaded waters contain an active community of bacteria and a group of
macroinvertebrates known as shredders (many species of beetles, flies and caddisflies,
among others) which reduce the riparian litter to smaller particles which become the food
for other invertebrates downstream.
Further downstream as the stream widens and more
light penetrates the water, diatoms and other algae supplement detritus as a food
source. Here, other groups of invertebrates known as collectors (such as the
net-spinning caddisfly at left) become more abundant in the community, along with
invertebrates known as scrapers (including many mayfly species) that feed directly on the
algae, bacteria, and decomposing plant material.
At some point downstream, in the middle reaches well before it become riverine in
appearance, the stream's highest species diversities occur-- but not just of insects.
Depending on the part of the continent, fish populations of darters, minnows, sculpins,
and others, are at their greatest diversities as well.