On a summer walk you approach a pond, and as you draw near you notice a number of dimples on the water surface. With your next footfall the dimples spring to life as insects descend, swimming jerkily into the depths. If you crouch down and keep still you will see that as they float slowly to the surface again they do so upside down, their legs touching the surface and the hind part of the abdomen just breaking the surface film.
The water boatman is easy to identify and is a familiar sight to anyone who has indulged in a little pond watching. But like many familiar insects it has a life-style and adaptations that make it well worth a closer look. In this article, therefore, we put the spotlight on waterboatmen or backswimmers as they are sometimes called.
The water boatmen belong to the family Notonectidae.They are represented in Britain by four species of Notonetca. Notonecta spends its whole life in water and most species insert their eggs into plant tissues, though one (N. maculata) lays them on submerged stones. The nymphs hatch, then undergo 5 moults before becoming adults. During the nymphal stages the colour is a distinctive green, making them easy to spot as they hop around at the bottom of the pond net. The adult insect finally attains the usual brown colouration above and dark brown or black underneath. Adults can fly well enabling dispersal prior to egg laying or escape from a drying pond. In appearance the insect is keeled on the back and triangular in cross-section. The hind legs are held out very much in the same way as oars on a rowing boat, hence the common name. The four front legs are raptorial, being able to catch and hold prey, for these creatures are fierce predators. Two wing cases cover a single pair of wings, though the cases are not as hard as those of beetles and are referred to as hemielytra. Differences in pattern on the hemielytra enable species to be identified, though this is not easily done in the field as Notonecta will make a painful stab through the skin of the unwary naturalist.
The Notonecta are true bugs, feeding by piercing the prey and sucking out body fluids. The usual insect arrangement of mouthparts has three pairs of jaws. In Notonecta a tube-like snout or rostrum is formed by an elongated pair of third jaws, the first two pairs of jaws lying inside this rostrum. These first two pairs of jaws are referred to as stylets and form two narrow channels. They are longer than the rostrum and can be extended when the insect feeds. It does this by piercing its prey with the two pairs of stylets and sucking out any liquids inside. Thus the waterboatman feeds exclusively on a liquid diet. A frog tadpole captured by Notonecta will shrink in front of your eyes to resemble a punctured balloon as the body fluids are removed.
Prey is in part determined by the stage of development. Before the first moult (instar 1) Notonecta will feed mainly on tiny crustaceans such as Cyclops or very small Mayfly nymphs, whereas adults will take larger prey, for example tadpoles and beetle larvae.
Notonecta species hang upside down from the surface film or 'perch' the same way on objects lower in the water body, waiting for prey. Capture is made by the fore and middle legs; the oar-like hind legs being used for swimming. More will be said about the 'waiting position' later on in relation to respiration and sensory organs. However three main factors influence the water boatman's choice of hunting technique.
The rate at which fluid is ingested decreases the longer a bug sucks on its prey, until it is eventually depleted. Therefore in conditions of high prey density frequent attacks of short duration appear to be the best technique. This gives the largest intake of fluids but obviously expends most energy. Where prey is less frequent, a longer time is spent feeding at each capture and fewer attacks are made.
High prey density is more likely to occur amongst the submerged plants of a pond, so hunting may be preferable at a lower depth. However, long periods away from the surface, actively attacking several prey, are physically demanding in terms of oxygen consumption. Notonecta is air-breathing and must return to the surface to replenish its reserves.
Temperature plays a large part in the hunting technique used. The amount of available oxygen dissolved in water increases as the temperature decreases. So through much of the summer Notonecta feeds more at night than during the day. Cool nights mean deeper, longer, more active dives can be made.
Techniques also vary according to species ; Notonecta glauca has been shown to favour the mid-water, low attack frequency technique, whereas N. maculata favours near-surface, short duration attacks. As N. glauca is very much more common, one could speculate that the former technique is more successful although there are doubtless many other contributory factors.
Insects breathe through openings (spiracles) along their abdomen. In Notonecta these openings are all contained in two grooves which travel the length of the underside of the abdomen. The grooves are covered by a line of hairs on each side which make a kind of roof to the groove. These hairs are waxy and unwettable so that the grooves are always filled with a trapped air bubble. Further waxy hairs protrude from the last segment of the abdomen. When resting at the surface these terminal hairs ensure that the air bubble joins the surface air and gases are exchanged. This replenishes the oxygen supply, whilst the fore legs attach to the surface film to maintain position. When the animal dives it can use the oxygen in the bubble, breathing through the spiracles as a terrestrial insect would. In the winter, during inactivity and high aquatic oxygen concentration, the insect can survive constantly submerged as enough oxygen dissolves into the bubble directly from the water. This mechanism is sometimes therefore called a 'physical gill'.
Most people will be of the view that Notonecta, hanging inverted from plant or water surface, is in a good position to see passing prey. It has large eyes, a sign of an actively hunting insect, and the field of vision from each eye partly overlaps to give some degree of stereoscopic sight. Although it doubtless does use it's eyes, the truth, as so often with insects, is more complex.
Consider the environment in which Notonecta liNovember 18, 2011 16:25asiest conditions in which to rely on vision alone. It has, in fact, been shown that the role of vision is secondary to that of mechanoreceptors. These are sensory hairs that are located on the feet and joints of all six legs as well as on the last three segments of the abdomen.
The motion of aquatic animals results in pressure waves that travel through the water. The sensory hairs are sensitive to these waves and are able to detect prey moving through the water or struggling as it drowns in the surface film. The sophisticated adaption does not end here, however.
By selective removal of these hairs it has been shown that the hairs on the legs respond to low frequency waves, up to about 50 Hz, whilst those on the abdomen react to high frequency waves of 50 to 300 Hz. Notonecta seems to need combinations of both frequency ranges to set the prey-catching behaviour in motion. Inanimate objects, which are moved or are drifting, fortunately do not produce the required combination of waves, thus they do not set up the prey-catching response, saving on energy expenditure. Notonecta adults and larger nymphs produce waves at around 40 Hz, so avoiding tussles between members of the same species. The only pitfall in this clever arrangement is that small Notonecta nymphs produce higher frequency waves and thus are frequently cannibalised by the adults.
Predation and Parasitism
Fish are the greatest threat to adult Notonecta, and N. glauca seems to me, based on purely on observation, to be the most likely species to find in water bodies inhabited by fish, though in Suffolk no species is totally absent from any particular type of freshwater.
Notonecta nymphs are of course more susceptible to predation, especially in their earlier stages. Other water bugs and beetles form the greatest invertebrate threat and smaller fish take them too.
There is circumstantial evidence that chemical defences are used and certainly laboratory examination has shown that chemicals such as hydroxybenzaldehyde and benzoic acid are secreted when these bugs are attacked. According to literature and field observations however, these substances seem to give little protection against predation.
Notonecta taken in the field often show signs of parasite attack. This can take the form of the eggs or larvae of various internal parasites, but mite larvae are also frequently to be found on the outside of water bugs, particularly under the wing cases, on the legs or head. Examination of live water bugs and observations of their frequent and unsuccessful cleaning motions suggest that the parasite infestations are at the very least an annoyance. In severe cases impeded movement may result and doubtless the worst infestations weaken the insect sufficiently to contribute to its early demise. The underwater world is not a pleasant place.
Notonecta in Suffolk
Of the four British species of Notonecta, three continue to be recorded in Suffolk with Notonecta glauca (see photo at top) easily the most widespread and common. N. maculata and N. viridis are less often found though not at all rare. This reflects national trends, N. glauca is also the most recorded species in Britain, common in all lowland areas. N. maculata and N. viridis have also been less often recorded nationally than glauca, and the present figures show a general distribution in the south of the British Isles (Huxley, 1999).
There are also 3 archive records in Suffolk for Notonecta obliqua. The species is sparsely but very widely distributed across the British Isles. It was found at two sites on the River Stour in the 1950's, but no new records have been made in Suffolk for many years. N obliqua is more common in the west of Britain and more often in acidic, peaty pools on moorland often at higher altitudes and so these old records must be viewed with suspicion.
Some books worth reading for further information
- Dolling, W.D. (1991). The Hemiptera. Oxford University Press, Oxford.
- Huxley, T (1999). Distribution of Aquatic Bugs (Hemiptera-Heteroptera) in England, Wales & Scotland: 1st Feedback Report.
- Kirby, P. (1992 ). A review of the scarce and threatened Hemiptera of Great Britain. UK Nature Series No. 2. JNCC, Peterborough.
- Savage, A. A. (1989). Adults of the British Aquatic Hemiptera Heteroptera; Freshwater Biological Association. Scientific Publication No. 50. Ambleside.
Southwood, T. R. E. & Leston, D. (1959). The Land and Water bugs of the British Isles. Warne, London.