These are some of the guilds to which we might assign insect larvae and nymphs. A lot of the food that is eaten grows right in the stream, like algae, diatoms, nymphs and larvae, and fish. This food that originates from within the stream is called autochthonous . Most food in a stream, however, comes from outside the stream. Leaves fall from bushes and trees.
Worms drown in floods and get washed in.
Leafhoppers and caterpillars fall from trees. Adult mayflies and other insects mate above the stream, lay their eggs in it, and then die in it. All of this food from outside the stream is called allochthonous . Leaves that fall in are not ready to eat. They must be processed.