river fly

The Blue Winged Olive Nymph

– Serratella ignita, the invertebrate formerly known as Ephemerella ignita

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The Riverfy Partnership fly life monitoring project trains volunteers to identify eight key aquatic invertebrate species. Among the four groups of insects commonly referred to as Mayflies which fly life monitors are trained to identify is the Ephemerellidae family which consists of just two species in Britain the fairly widespread Blue Winged Olive Serrattela ignita and the rarer Yellow Hawk or Yellow Evening Dun, Ephemerella notata. Both species are found in West Wales.

As our project has developed it's becoming clear that volunteers are having trouble in distinguishing between the Baetidae nymphs which regularly feature in their samples and those of the more pollution sensitive Ephemerellidae family - a situation that isn’t helped by the fact that the two families are commonly referred to as “Olives” by anglers which can be confusing. The picture below shows what we're actually after, it's easy to see how they're confused with Baetidae -

Ephemerillidae, a Blue Winged Olive nymph	Baetidae, probably a Large Dark Olive
From the web site of the Tillingbourne Riverfly Monitoring group link

As the presence of Ephemerellidae is a more significant indicator of good water quality than the more pollution resistant Baetidae it’s important for fly life monitoring groups to learn to identify this family correctly – it isn’t too difficult with a bit of patience and the use of a hand lens.

You’ll be unlikely to come across Ephemerellidae - between October and March in Wales, so although it’s always best to keep an open mind, experience will more than likely inform you that this is one that isn’t worth searching for in your winter sample. Nymphs will begin to grow to a sufficient size to be caught in the standard issue 2mm mesh of our nets from around March onwards as they begin to grow towards hatching from around the month of April.

Note the bristly tail and distinct banding -

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Images above used with the kind permission of Guillaume Doucet http://guillaume.doucet.free.fr/

One of the simplest ways to distinguish between Ephemerellidae and Baetidae is their rocking, clumsy swimming profile which contrasts to the more direct and rapid movements of the latter which gives them the name “Agile Darters”. Without the benefit of a video to demonstrate that aspect of their behaviour it remains something to look out for, explore further and learn to recognise from your own experience.

 Of several distinguishing features which identify Blue Winged Olives the most pronounced and easily visible feature is their stripy tails and legs. This alternating black and white banding is just about visible to the naked eye with larger specimens and readily apparent when viewed with a hand lens. Although this attribute makes identifying the BWO fairly easy it's a marking which isn't shared by its relative the Yellow Hawk which is pale and undistinguished in appearance.

What these two species do share though is a set of upright gills which run along the upper surface of their abdomens which differs significantly from the alignment of the gills on Baetidae - their gills are set at 90° to the side of the abdomen.  

The four pairs of gills which are visible on the Ephemerellidae hide a fifth pair and these are situated beneath the pair of gills which lie closest to the tail.

By kind permission of the Tweed Foundation - note the scorpion-like defensive position held by the subject, a distinguishing behavioural trait.

Here's a picture of a member of the Baetidae family to compare to the pictures above -

Note that the general shape of the Baetidae is more streamlined and elongated than the chunky and squat look which BWOs have.

A further identifying feature that BWOs have is the presence of pairs of spines that are located on each segment of its abdomen, running between the gills. These spines give the BWO the latin name Serratella. The picture below gives a good view of the spines -

Image with the kind permission of Igor Baryshev.

Just to hammer the message home,here are some more excellent images images of BWO nymphs which clearly show the alignment of the gills and spines -

http://www.biopix.dk

Both images above by kind permision of Neils Sloth, http://www.biopix.dk

And .......if you're really having trouble tying one .....