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Anglers' Riverfly Monitoring Initiative invertebrates guide

Cased caddisfly, Caseless caddisfly

The distinguishing feature of the Cased caddisfly (left) is the case carried by the nymph. Silk fibres hold together the materials of the case, which varied widely between species. The case may be formed of tiny stones or of various kinds of plant matter - some species even include a 'rudder' on the case made from a small twig (e.g. Halesus radiatus). The caddis nymph inside the case has a segmented body with three pairs of legs, although you cannot see these features unless you remove the nymph from its case. The Caseless Caddisly (right) has three pairs of legs and paired tail-like legs at the rear are important features to identify a caseless caddisfly. Certain true fly larvae can look similar but have no legs; certain beetle larvae can look similar but do not have the tail-like legs at the rear. One of the most common caseless caddisflies found on the Irwell is Rhyacophila dorsalis, which is unmistakable from its green colour.

Cased caddis Caseless caddis
Left image cropped from "Northern case-maker caddis, Pycnopsyche gentilis" by Bob Henricks. Right image cropped from "Freeliving caddisfly larva, Rhyacophila carolina" by Bob Henricks. Used under Creative Commons license.

Mayfly (Ephemeridae), Blue-winged olive (Ephemerellidae)

Mayflies (left) have feathered gills along the side of the body, usually seven pairs, and a pair of wing cases that become more distinct as the nymph gets older and larger. The most common blue-winged olive (right) found in the Irwell, Serratella ignita, is up to 14 mm in length. One of the easiest distinguishing features is the clear banding of colours on the tails. If you have a live specimen, the blue-winged olive shows an unmistakeable 'donkey kicking' way of swimming - this clearly distinguishes it from the olives (Baetidae) that swim straight.

Mayfly (Ephemeridae) Blue-winged olive (Ephemerellidae)
Left image cropped from "Mayfly nymph (rhithrogena sp.)" by Dave Huth. Right image cropped from "Spiny crawler mayfly, Serratella tibialis" by Bob Henricks. Used under Creative Commons license.

Stone clinger (Heptageniidae), Olive (Baetidae)

Stone clingers (left) have distinct broad flattened bodies and large heads, with eyes on top of the head, that make these nymphs easy to identify in the field without needing a hand lens. In the Irwell we find lots of the yellow may dun Heptagenia sulphurea, which has pretty yellow and dark markings. Olives (right) are slender nymphs with bodies tapering towards the rear. The gills along the side are plate-like and not feathery. Easily distinguished as live specimens from the blue-winged olives by the straight swimming motion (in contrast to the 'donkey kick' of the blue-winged).

Stone clinger (Heptageniidae) Olive (Baetidae)
Left image cropped from "Flatheaded mayfly, Heptagenia marginalis" by Bob Henricks. Right image cropped from "Small minnow mayfly, Baetis tricaudatus" by Bob Henricks. Used under Creative Commons license.

Stonefly (Plecoptera), Freshwater shrimp (Gammarus spp.)

Stonefly (left) are distinguished by their two tails and a lack of gills along the side of the abdomen. The two sets of wing cases are distinct and held out from the abdomen, in contrast to the related mayflies, stone-clingers and olives. The antennae are also long. Freshwater shrimp (right) are small crustaceans up to 25 mm - distinguish clearly by the curved body, which is flattened side to side.

Stonefly Freshwater shrimp
Left image cropped from "Stonefly Nymph" by Dave Huth. Right image cropped from "Mikroskop-Wasserbewohner (Mühlbach-Tachinger See)" by labormikro. Used under Creative Commons license.
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