In the margins and meanders of our chalk streams, where there is a slower flow, amazing insects swim across the surface. Lesser water boatmen may be small – up to 1.5cm long – but their unique characteristics make them a memorable sighting. There are actually several species in their family (Corixidae), and while they are hard to tell apart with the naked eye, their common features can help you to spot them. When you next visit a chalk stream, keep an eye out for these fascinating animals!

Pictured below from left to right (click to expand): Lesser water boatman from the side, with its air bubble visible / lesser water boatman from above.

The first thing to look for is a brown, boat-shaped body, although a close look at most species will reveal golden yellow stripes. Much of this patterning is actually on the wings, which are usually kept folded against the body. Another clue is a pair of long hind legs; their oar shape and ‘fringe’ of hairs are handy for both swimming and floating on the surface. There will be another long pair that are often used for standing on the stream bed, and a short pair just under the head. See if you can spot the large, reddish-brown eyes, which have a compound structure that combines many tiny lenses to create a wide view.

Despite living in ponds, lakes, and rivers, lesser water boatmen don’t have gills. Instead, they have a clever trick for taking longer dives: their undersides are covered in water-repelling hairs, which trap a bubble of air against their bodies. This bubble acts like the tank of a scuba diver, feeding the insects oxygen through small holes – or ‘spiracles’ – in their exoskeletons. This comes in handy when foraging for food, which for most lesser water boatmen species will be plants, algae, and dead organic material, although some are carnivores that hunt live prey.

Pictured below from left to right (click to expand): Common backswimmer or greater water boatman (Notonecta glauca) / Micronecta scholtzi. 

You may be wondering… are there greater water boatmen? Indeed there are, although they are a different family of insects (Notonecta) and are usually referred to as ‘backswimmers’. As the name suggests, these species swim upside down, so their air bubbles are more visible from above. This can make them appear silver in colour, but they are actually light or dark brown with reddish eyes. Unlike most of their ‘lesser’ relatives, backswimmers eat small insects, fish, and crustaceans. They like to swim just under the surface and sneak up on their prey from below.

There is also the lesser or pygmy backswimmer (Plea minutissima) and several pygmy water boatman species (Micronecta), which are in their own insect families. Despite being just 2-3mm long, one of the latter is famous for being surprisingly noisy. Like crickets and grasshoppers, the males of many water boatmen species ‘sing’ or stridulate by rubbing their legs along ridges on their bodies. Micronecta scholtzi has been recorded at 99.2 decibels – that’s as loud as a car horn! Thankfully, water absorbs most of the sound, but you may still hear it if you listen carefully.

Cherishing our chalk streams

Insects like water boatmen are a key part of our chalk stream ecosystems. As food for fish, bats, birds, and amphibians, these small creatures are crucial to the health of many other animal populations. If you’d like to safeguard their chalk stream homes, why not check out the Watercress and Winterbournes scheme? We're protecting, enhancing, and celebrating seven Hampshire chalk streams that feed the famous Rivers Test and Itchen.