Macroinvertebrate identification table

Very Sensitive Bugs

Caddisfly Larvae

Case Size: 4 – 50mm

Heads and legs emerge from protective case

Cases made from sticks, reeds, bark, leaf or even sand !


Stonefly Nymph

Size: Up to 30mm

2 long antennae

2 sets of wing pads

2-pronged tail

Can have feathery gills at the end of abdomen

Mayfly Nymph

Size: Up to 20mm

3 tails edged with hairs

2 antennae

Gills along abdomen

Can be confused with Damselfly Nymph

Sensitive Bugs

Water Mite

Size: Up to 5mm

4 pairs of legs

Round, often red bodies

Will be seen as tiny dots zinging around water


Freshwater Mussel

Size: Up to 40mm

Brown-black shell with two halves joined on one side

Soft body inside shell

Water Scorpian

Size: Up to 50mm plus tail

Long, leaf-shape body

Huge grasping forearms

Long, thin, tube-like tail used as a snorkel to collect air


Freshwater Shrimp

Size: 20-40mm

Five pairs of legs

Brushes on front pair of legs rather than pincers

Fan-shaped tail may be tucked under body

Dragonfly Nymph

Size: Up to 40mm

Big head and eyes

Wide body, tapered tail

Very camouflaged, often largest bug in sample

Fishing bait: ‘mudeyes’

Damselfly Nymph

Size: Up to 30mm

Slender body, large triangular head

Long tail with three fan-shaped gills

Colours green to brown

Tolerant Bugs

Freshwater Snail

Size: 3-20mm

Single coiled shell covering soft body

2 tentacles and eye-stalks

Slides on bottom or side of sample tray


Size: 4-11mm

Very large, fringed, paddle-like back legs

Swim backstroke in an upside down position

Light brown colour

Diving Beetle

Size: Up to 35mm

Egg-shaped body and fine antennae

Strong back legs with tiny hairs

Return to surface for air

Water Boatman

Size: Up to 12mm

Large back legs like flippers, front like scoops

Fine lines on wings

Large eyes wrap around sides of head


Size: Up to 20mm

Very flat, worm-like body

Arrow head shape on one end of tapering body

They glide rather than wriggle

Amphipod (Scud or Side Swimmer)

Size: 2-4mm

Rounded bodies often curled up on their side

Seven pairs of legs

Large antennae

Whirligig Beetle

Size: Up to 15mm

Front legs longer than back legs

Swim in small circles on surface of water in groups

Picture here not to scale


Size: Up to 10mm

Often a light brown / cream colour

Move by thrashing, coiling and uncoiling


Size: Up to 40mm

Worm-like with a sucker on the mouth and one on the tail

No hairs, legs or body sections

Beetle Larvae

Size: Up to 15mm

Long tapered striped body

Crawls with 3 pairs of walking legs

Water Strider

Body size: Up to 20mm

Leg span: 70mm

Thin body, two long legs out each side

Skates on surface in fast, jerky movement

Water Flea

Size: 4-6mm

Single eye in middle of head

Large antennae used for swimming

Very Tolerant Bugs

Chironomid/Non-biting Midge/Bloodworm 1

Size: Up to 30mm

Often red coloured

Move by coiling and uncoiling

Aquatic Earthworm

Size: Up to 60mm

Wormlike segmented body

Snakelike movement in water

Fly Larvae

Size: Up to 10mm

Plump segmented bodies

Caterpillar motion

Mosquito Larvae

Size: Up to 10mm

Head wider than abdomen, no legs

Swims with a wriggling motion