When it comes to our local chalk streams, few animals are more important than invertebrates. As a major food source for fish, birds, and mammals, their presence – or absence – directly impacts other wildlife populations. Moreover, because of their key role in these ecosystems, invertebrates act as ‘canaries in the coal mine’ for the health of the habitat at large.

This is particularly true of certain invertebrates, known as riverflies or the 'EPT species'. The three constituent insect groups - mayflies, stoneflies, and caddisflies - all spend prolonged periods in the water, and don't roam far from their place of origin. They are also especially sensitive to environmental pressures, with specific sensitivities varying between species.

These traits make riverflies invaluable for detecting problems, including chronic ones that may be missed by spot water sampling. The disappearance of a certain species can warn of harmful pesticides, algae-feeding fertilisers, low flow, or excess sediment. Let's take a closer look at these fascinating flies and their vital role in protecting our chalk streams.

Mayflies (Ephemeroptera)

Mayflies - sometimes called 'up-winged flies' - number around 50 species across the UK. They start their lives as eggs, which the females attach to stones on the stream bed. The speed of their development is dictated by the water's temperature, but is usually complete in about 30 days.

In certain species, however, this timeline is slowed significantly. Just before a blue-winged olive egg hatches, for instance, it enters a dormant period called a diapause. It can remain this way from autumn to early spring, when the rising temperature kicks off its larval phase.

As larvae, mayflies typically have three tails and fluffy gills along their lower bodies. Although these insects are known for their short lifespans, for some species this life stage can be two years long. During this time, they moult their exoskeletons up to 50 times as they grow larger.

Adulthood for mayflies is unique, since unlike other insects they have two adult forms: the dull-coloured ‘dun’ (or sub-imago) and the clear-winged ‘spinner’ (or imago) that follows. Some adults retain their three tails, while others keep only two. Their wings are held upright, butterfly-style, above their bodies.

Stoneflies (Plecoptera)

Of the roughly 35 stonefly species in the UK, most live up to their name by liking stony habitats. Their eggs have distinctive patterns that vary by species, which can prove useful since female stoneflies are hard to identify. To lay their eggs in a stream, they either drop them from above or dip their bodies in the water.

As larvae, stoneflies usually have stout bodies, two tails, a pair of antennae, and sturdy legs ending in claws. This stage lasts for around one year, during which time they moult their exoskeletons up to 35 times. Unlike most mayflies, they would rather crawl out of the water than swim up to the surface.

This grounded approach continues into adulthood, when they are most often found on the bank or nearby vegetation. Despite gaining wings, they tend not to be strong fliers and instead prefer to jump, swim, or crawl. For some species this has evolved into considerably shortened wings.

One unusual stonefly trait is their mating strategy, which involves communication via ‘drumming’. Males strike their abdomens on a solid surface, and if females are nearby they will show their interest by drumming back. The rhythms are so unique that a species can be identified by sound alone.

Caddisflies (Trichoptera)

The UK hosts a whopping 200 species of caddisfly, which you may have heard referred to as 'sedge flies'. Females vary in how they bring their eggs to the water: some lay them on stones, some drop them from above, and others leave them on plants to be washed in by the rain.

Caddisflies are perhaps best known for their larval stage, when many species construct protective cases. Made by spinning silk around stones, shells, sand, and leaves, these shelters come in all shapes and sizes. They can also be portable, allowing the larvae to move about in relative safety.

Unlike the other riverflies, caddisflies only moult their exoskeletons about five times during their development. Instead, they have two additional stages: one where they build an underwater cocoon to pupate in, then another where they swim to the surface in a thin waterproof skin.

Adults caddisflies have hairy wings that fold into a 'house roof' shape when at rest. Attracted by lights and preferring to fly after sundown, they are often mistaken for their close relatives - the moths. While they share a fluffy quality, however, caddisflies have a pair of palps in place of a long tongue.

Get involved

If you want to keep tabs on these amazing creatures, there are several ways to do so. Much of the monitoring in Hampshire is done through citizen science projects, which combine expert support with the passion of local volunteers. This enables far more extensive data collection than would otherwise be possible, and a much more complete picture of wildlife populations.

Depending on the recording system being used, you might count adult riverflies by setting up a light trap or surveying a stream bank. You might also do a 'kick sample', which means disturbing the stream bed with your foot so that insects are swept into your net. The net's contents are then emptied into a tray, where they can be identified to family, genus, or species level.

To learn more, check out the following organisations:

• The Riverfly Partnership host several monitoring projects, including the Anglers' Riverfly Monitoring Initiative (ARMI) and Urban Riverfly.
   
• WildFish perform in-depth monitoring through their SmartRivers hubs. For an overview of how this works, you can watch our recorded online talk.
   
• If you're in northern Hampshire, you can help monitor riverflies on the Test and Itchen headwaters through the Watercress and Winterbournes scheme.