Nepomorpha 1

NEPOMORPHA

Water bugs (1)

HEMIPTERA > HETEROPTERA > NEPOMORPHA > Corixidae ('water-boatmen')

The NEPOMORPHA includes the true aquatic bugs that actually live in the water, rather than on it or in the vegetation around its margins. The NEPOMORPHA are freshwater species, separated from the marine shore bugs that are in a further infraorder LEPTOPODOMORPHA.

The group includes four small families with just one or two species; Nepidae (water scorpion and water stick-insect), Aphelocheiridae (river bug), Naucoridae (saucer bug) and Pleidae (pygmy backswimmer), and two larger, more well-known families; Notonectidae (with four species of backswimmer) featured on the next page, and Corixidae (with 33 species of water-boatmen) as featured here.

The common name of water-boatmen has led to confusion between the species of the Notonectidae, which were traditionally known as Greater water-boatmen, and the smaller Corixidae water bugs that have been and, in many cases still are, called Lesser water-boatmen. There's a misconception that the larger water bugs you see skittering across the surface of ponds are water-boatmen when, in fact, they're backswimmers. Physically, they look similar, as they all have a more-or-less oval shape and long legs with clusters of hairs that allow them to move effortlessly through the water. The name 'boatmen' originates from the hairs that have a paddle or oar shaped appearance rather than the shape of the body. In fact, the keeled body of a backswimmer is more boat-shaped than the flatter body of a boatmen. Another difference is that the Corixid bugs spend most of their time at the bottom of their specific habitat, whether it be a pond, lake, slow-flowing river, ditch, canal or even a water trough, feeding on aquatic plants and algae rather than continually moving through or across the water in search of insects as the predatory backswimmers do. This is why Corixids have modified front legs with spoon-shaped tarsal segments for scooping organic matter into their mouths, whereas Notonectidae species have standard front legs and a four-segmented rostrum that they use to bite prey.

Although some of the listed species are scarce or local to specific areas, most are regarded as common with reasonably widespread distribution. Around twenty have been recorded from my area of Dorset (VC9) and South Hampshire (VC11). Some can be distinguished from a combination of their length, general colour and, certainly in respect of the species within the Sigara genus, the number of pale lines on the pronotum. Those visual characters together with the habitat may be sufficient. Closer inspection, if possible, of other features such as the shape of the metasternal xiphus (the triangular body part between the front and mid coxae that can be pointed or blunt) will provide further evidence. But, in many cases, particularly with the mid-sized species that have extremely similar appearance and markings, positive identification from photos is unlikely.

The largest species is Corixa punctata at 13mm, Corixa dentipes is just a tad smaller and could be confused, but has not been recorded locally. Nor has Corixia panzeri, which is a little strange as it's one of the more common species elsewhere. The Hesperocorixa genus includes four species that can be distinguished from other Corixids by having a much darker appearance with narrow pale lines that extend more or less unbroken across the black body. The largest is Hesperocorixa sahlbergi at 8mm. It is one of the most frequently recorded species. Hesperocorixa linnaei is very similar, but has six pale lines on the pronotum whereas Hesperocorixa sahlbergi has between seven and nine. It can also be found locally as can the much smaller Hesperocorixa castenea. A further medium-sized, fairly common species is Callicorixa praeusta, which can be distinguished by a distinctive rectangular dark mark on its hind legs.

They're probably the only ones that I'm likely to encounter apart from eight or so Sigara species that are generally very difficult to separate from visual details that can be seen in photographs. The size helps, but most are within the range of 6mm to 8.5mm, so measurements need to be very accurate. Habitat is useful, but often inconclusive. Sigara dorsalis is by far the most common and from various features, such as having six pale lines on the pronotum and the pattern of the lines on the forewing, should be identifiable. Sigara distincta and Sigara falleni are very similar, both around 8mm long with seven or more pale lines on the pronotum. The side margin of the pronotum is different, described as asymmetric and truncate in the former and symmetric and pointed or rounded in the latter. The detail is difficult to judge so I had some photos checked by an expert particularly as Sigara distincta hasn't been officially recorded locally, whereas Sigara falleni is the fourth most frequent. Sigara semistriata is smaller, around 6mm, with seven to nine pale lines on the pronotum and an elongate xiphus. The details and identity of the individuals featured below have been carefully checked, but must all be regarded as tentative.