Invertebrates

The invertebrates used to complete biotic classification are known as indicator species. Some creatures will only live in pollution free waters whilst others will tolerate quite severe pollution. By turning the bed material and scooping through the water with a fine net or sieve such creatures can be collected and examined so as to give a good indication of water quality. As you descend through the water classifications so the species will cease to live in the river, thus:

Water Quality	Invertebrate	Description
Clean water only	Stone-fly nymph	A brown coloured crawler. Two long tails.
Clean water only	Mayfly nymph	Brown mottled swimmer. Three thin tails.
Slight pollution	Caddis-fly larvae	Crawler,cased in plant matter, grit or sand.
Slight pollution	Freshwater shrimp	Grey-brown swimmer. Many six jointed legs
Quite bad pollution	Water Louse	Grey-brown crawler. Like a wood louse.
Quite bad pollution	Bloodworm	Bright red swimmer. Loops and unloops.
Bad pollution	Sludgeworm	Brown-red crawler. Crawls in the mud.
Bad pollution	Rat-tailed maggot	Grey, with long breathing tube.