Through the looking glass

Trainee Ecologist, Genevieve Dalley, introduces us to the world of freshwater invertebrates.

Through the looking glass

Water is a mysterious thing. When looking at it, most people see nothing more than a reflection. But, if you slip past the surface, there is a whole other world waiting, with its own stories and an alien set of characters.

Here, there really are fairies, dragons and monsters of the not-so-deep...

Blue-tailed damselfly (Ischnura elegans).

Emperor dragonfly larvae (Anax imperator).

Water Scorpion (Nepa cinerea).

My new job as a Trainee Ecologist, specialising in freshwater invertebrates, allows me to delve into this secret world. And I would like to bring others with me.

So, let me introduce you to a lesser-known star of the stream. This is the story of the Orange-striped stonefly.

The orange-striped stonefly. Taken at Dovestone RSPB reserve by Ken Gartside. Photo via www.WildaboutBritain.co.uk.

Stoneflies (Plecoptera) are ancient insects, with the first fossils dating from 250 million years ago. They live as larvae in freshwater, sometimes the formidable top predators of their habitat, until they reach the correct size and emerge as flying adults. However, unlike more familiar insects, such as butterflies and moths, stoneflies do not have a pupal stage between young and adult.

Stoneflies require high oxygen levels and so are very sensitive to organic pollution in freshwater, which reduces the amount of oxygen available to them. They are therefore very good indicators of water which does not contain organic pollution. Interestingly, many species are not so sensitive to heavy metal pollution, with healthy populations found downstream of disused lead mines.

The Orange-striped stonefly (Perlodes microcephalus) is widespread throughout Europe. It was previously believed that all stoneflies identified in Britain as P.microcephalus were the same species. However, recent work has revealed that Britain in fact has its own species, the British Orange-striped stonefly (Perlodes mortoni), now classified as a new, separate species found only in the UK. This exciting discovery means that the distribution and conservation status of both P.microcephalus and P.mortoni in Britain is thrown into question.

The two species look very similar. It is not yet possible to tell the larvae apart. In fact, the only way to distinguish between them is by differences between the structure of the eggs and the length of the wings on an adult male. P.mortoni adult males are flightless, with their wings reduced to stubs, while P.microcephalus males have a range of wing lengths, some with fully formed wings. To confuse things further, females of both species have fully formed wings.

These stoneflies can be seen as adults between March and July. However, the peak emergence period is May, so now is the best time to see them. Orange-striped stonefly adults are quite big (males: 13-18mm, females: 16-23mm) and, as the name suggests, the adults have a very visible orange stripe running from the top of the head and across the prothorax (the first body section after the head). You can look for them beside rivers and streams, sheltering under rocks, crevices and other structures such as bridges.

Adult male Periodes mortoni, the British orange-striped stonefly. Photo via www.riverflies.org.

It is important to learn more about the species so we can protect them. If you come across an Orange-striped stonefly it would be very helpful if you could collect it and send it to the Riverfly Recording Scheme. For more information on this, visit the Buglife website, the Riverflies Trust website, or contact: rrs@riverfly.org.

By doing this, you will be giving vital information on the status of the species in Britain and helping to conserve this beautiful and important animal.