Last updated 09:57, Thursday, 08 May 2008

This week we start a new series of articles by The Wildlife Trusts, Cumbria

HAVE you ever had a look at what lurks on the bottom of a local stream? Ever wondered what it takes to live there? Or how creatures move around or even stay still in moving water?

Some, such as blackfly larva, use hooks whilst leeches use suckers to anchor themselves to plants or stones. The caddis larva uses stones to weigh itself down, while freshwater shrimps burrow to avoid fast currents.

There are other variations to contend with. Warm water is thinner, providing less support for tiny creatures. Upland streams can be acidic due to the soil, which many species cannot live in. Minerals in water are important – snails require calcium carbonate for shell growth.

How do creatures avoid being seen, both as predator and prey? Some hide under rocks. Others, like damselfly nymphs, blend in with surrounding vegetation. Some lie hidden in the mud. The freshwater shrimp, moves quickly and in a seemingly random fashion.

There’s a fascinating world of lesser-known creatures, which live with these challenges and are easy to find. Get your wellies on and bring one or two things to help you find the ‘beasts of the shallows’. Take a net, a tray (used ice cream tubs, white ones are ideal), a glass jar and a magnifying glass. You want to avoid polluted streams, which are lacking in life as well as unpleasant.

What might you find crawling along the bottom or lurking under stones? There are various insect nymphs which are around 1-2cm and have three pairs of legs spread out from their sides, jointed bodies and appear to have long tails – this last feature helps in identification. These crawling juniors are the eternal teenagers and very different to the more familiar winged adults.

Two ‘tails’ and it’s a stonefly nymph, with a voracious appetite as a nymph yet as an adult some don’t eat at all. Three ‘tails’ with gills sticking out on each side of the lower body and you’ve got a mayfly nymph. In Cumbria, more than two thirds of all the British species of stonefly and mayfly can be found.

Three ‘tails’, without bits sticking out of the lower sides and you’ve got a damselfly nymph. As an adult, these are the thin, brightly coloured quick fliers, with bodies shaped like matchsticks. Damselflies stay as nymphs for two to three years, but on becoming an adult only live for 40-50 days; stonefly and mayfly adult lives are even briefer.

A stocky body, with no obvious tail and you’ve got a dragonfly nymph. They lie in wait, sensing prey with antennae or eyes and move slowly towards them before using their unique weapon, a mask. This is an L-shaped appendage, which with mouth closed is tucked away as the lower jaw. When prey is within range it drops the mask and fires it forward where spikes on the end impale the victim.

Other things you might find include shrimp, water boatmen, leeches (not all are bloodsuckers) and water snails. The existence of different species gives an indication of water quality. The three nymphs described indicate healthy water.

So get out and take a look at the world at the bottom of the running water around us – and prepare to be amazed.