A group of large-eyed aquatic bugs living in ponds, lakes and canals, including the backswimmers, boat-flies, water boatmen, or wherry-men, with a world-wide distribution. The commonest of the five British species is Notonecta glauca, which can be seen in almost any stretch of sluggish water. It is rather more than ½ in. in length and of a pale brownish colour with darker thorax and undersides. Backswimmers are especially conspicuous for their long, paddle-like hind legs and their characteristic habit of resting upside down beneath the surface of the water.
Aqualung diver
The colloquial name «backswimmer» is particularly appropriate for this insect because it does actually swim upside down. Its very long hind legs – almost twice the length of the other two pairs – are fringed by a series of fine hairs, and with a few strokes of these oar-like legs the backswimmer can propel itself through the water at a remarkably fast rate. The hind legs are, in fact, used in unison, like oars, quite unlike the tripodal method of progression used by terrestrial insects. The wing-cases of the backswimmer join to form a ridge along the middle of the back, and when the insect is seen resting just beneath the surface with its long hind legs held out sideways the impression is of a keeled rowing boat with a pair of oars over the sides.
Backswimmers are extremely wary and at the slightest hint of danger will swim down from the top of the water. This needs great physical effort because the insect is very much lighter than water and would be continually and involuntarily rising to the surface, but for a powerful swimming action or a firm grip on some submerged object. This buoyancy is largely due to a bubble of air which the backswimmer always carries pressed to its abdomen by a series of bristles. Although fully aquatic, a backswimmer does not have gills. It must therefore get its air supply from outside, and this it does by rising periodically to the surface and sticking the tip of its abdomen out of the water. There is a channel formed by hairs on each side of the abdomen. These the insect opens at the surface, allowing air to flow in and then closes them again, trapping the air. These air-bubbles are therefore in direct contact with the backswimmer’s spiracles or breathing holes which are arranged along the sides of the abdomen. The spiracles are protected by a further fringe of hairs which allows air in and keeps water out. The bubble acts as a gill, absorbing oxygen from the water so that the backswimmer can stay submerged for a long time.
Although adapted to life in the water, and scarcely able to walk on land, backswimmers are strong fliers and can leave their natural element at any time; they are in consequence among the earliest colonisers of that fairly new feature, the water-filled gravel pit. During the Second World War, it was not uncommon to find backswimmers suddenly appearing in emergency water tanks, far from the nearest pond or river, surrounded by tall buildings. They would appear overnight, because they fly mostly at night, and are attracted to light, reflected from the water surface.
Voracious feeders
Backswimmers are extremely voracious feeders and it is always unwise to include them with other small forms of life in any aquarium. Mosquito and other fly larvae form a large part of their diet, but size alone does not always deter them. Large beetle larvae, tadpoles and even small fish are often attacked. The backswimmer’s method of hunting is to hang motionless at the surface of the water, immediately swimming towards anything that catches its attention. It has excellent eyesight, but primarily it discovers its prey by a form of vibration-location certain hairs on the hind legs can pick up the vibrations caused by small animals swimming nearby. Only when the backswimmer is within a few inches of its prey do the eyes play their part in securing its capture. Having captured its prey, the backswimmer then plunges its sharp rostrum (or beak) into the body of the victim, pumping in a toxic digestive fluid containing enzymes which rapidly break down the body tissues. The carcase is then held firmly by the prehensile fore-limbs while its internal tissues, now made fluid, are sucked out. The rostrum of the backswimmer can pierce human skin, and the toxic fluid pumped in may cause extremely painful symptoms. Fortunately, this only happens when the backswimmer is handled, and then only if handled carelessly, although the insect appears to be not over-particular what it attacks. In the days when cattle and horses drank far more commonly from village ponds, it was not unusual for backswimmers to attack their tender muzzles.
Life cycle
Mating in Notonecta usually occurs between about December in one year and late spring of the next. Batches of the elongated oval eggs are inserted into the stems of aquatic plants, such as Canadian pondweed, by the female’s ovipositor. The eggs hatch after several weeks, the larvae, which are at first wingless, escaping by means of the hole originally made by the female’s ovipositor. By late summer, these have become young adults and the older generation dies off. Only one generation is produced each year. Like all bugs, backswimmers undergo incomplete metamorphosis. That is, from the egg hatches a larva, which resembles the adult in form. Growth then proceeds by a series of moults.
Enemies
As a water insect, the backswimmer falls prey to the carnivorous animals present around its habitat. These include waterfowl, frogs, toads, and sometimes fish, such as trout or bass.
Seeing wrong way up
When we are standing or walking on firm ground we know we are the right way up partly because of our appreciation of the pull of gravity but also partly through our eyes. A passenger strapped in an aeroplane and unable to look out can be flying upside down and will be unaware of it. Anyone swimming underwater and caught in a strong turbulence can, if the water is murky, lose all sense of which way up he is.
A simple experiment that can be carried out on a backswimmer shows that their large compound eyes and well-developed sight play an important part in their orientation. Normally this insect rests just beneath the surface of the water, upside down. In a glass aquarium, with the top of the aquarium in darkness and a bright light shining from below the backswimmer will take up its position on the surface of the water with its lower surface directed down towards the light, so when we look at it from above it is in the position in which any other insect would be that was resting on the surface film. The backswimmer will try to live and swim in this position as long as light is maintained from below. This reaction to light is present in backswimmers even from a very early stage in their larval life.
