Small World Discoveries

HEMIPTERA > HETEROPTERA > NEPOMORPHA > Notonectidae ('backswimmers')

Previously known as Greater water-boatmen, backswimmers can, as their name suggests, often be seen on the water surface swimming upside down. They are highly evolved predatory bugs with a short powerful rostrum that can inflict a painful bite if they're handled carelessly. Prey includes small fish, tadpoles and a variety of aquatic invertebrates.

They are similar in appearance to the previously described Corixidae water-boatmen, but can be separated by their larger size and general colouration, plus their habits and predatory behaviour. Their body is streamlined and convex with fringed hind legs for swimming. Although primarily aquatic, these bugs fly well and so can disperse easily to new habitats.

Four species occur in Britain, all in the genus Notonecta.

Notonecta glauca is the most common and widespread species, occurring in various habitats, but mostly smaller ponds, canals and ditches rather than larger lakes. Adults overwinter with eggs being laid early from February to early May. Once hatched, there are five larval stages with the instars taking at least two months to fully develop, with the new adult generation appearing from around July or August. They're around 13-16mm long with a pale brown, sometimes lightly mottled body with an ovate dark mark at the inner angle of the corium, pale head with large red eyes, pronotum with obtuse front angles, dark scutellum and powerful oar-like rear legs. Their general appearance can be somewhat variable as they can often look silvery due to air becoming trapped in a fine layer of bristles.   

Notonecta obliqua is the least frequently recorded of the four species with rather scattered distribution, and with only one record from Dorset (VC9) and, similarly, from South Hampshire (VC11) in the past twenty years. It is more tolerant of cold water and replaces Notonecta glauca at higher altitudes. Elsewhere it is mainly associated with acidic waters and bog pools. It's the easiest species to identify due to the paired cream streaks on an otherwise mainly black body. It averages around 15mm long.

Notonecta viridis was previously considered a mainly southern coastal species but, in recent years, has spread both inland and northwards, and can now be found in various non-saline habitats. It is a little smaller than Notonecta glauca and Notonecta obliqua, but not enough to be noticeable unless you're comparing different individuals side by side. It could be confused with pale-coloured Notonecta glauca, but has a more-defined, somewhat star-shaped, mark at the inner angle of the corium rather than an irregular oval mark. They are generally paler with greener legs and underside, but the defining feature is the shape of the front angles of the pronotum that extend forwards slightly finishing beside rather than just behind the eye. It's a difficult detail to see unless you have a good side-on photo such as the one below.

I was extremely lucky to collect all three species from the same location during my very first attempt at finding any aquatic species to photograph in the new mini aquarium that I'd set up at home. They were kept overnight, but safely released the next morning. Given that Notonecta oblique has only been recorded once in the county, it really was an usual, if not unique discovery, that provided sufficient material for this page. It will now be interesting to see how long it takes to find the next one.

The remaining species is Notonecta maculata, which has an orange-brown ground colour with extensive fine dark marbling. It's relatively common and widespread, but is a species that favours barren waters that have hard surfaces, such as dams, bridges or pontoons rather than typical ponds or more natural habitats. It can also be found in concrete cattle troughs, which may well be the best place for me to find one as I don't have any suitable locations in the area that I can think of.

The following individuals were all found at Janesmoor Pond, Fritham, New Forest | March 23.