Common Backswimmer Notonecta glauca (Linnaeus 1758) (Hemiptera: Notonectidae)1
Notonecta glauca, the common backswimmer (Figure 1), is an aquatic insect in the family Notonectidae. Insects in this family are commonly referred to as backswimmers or greater water boatman. Notonectids propel themselves through the water with their ventral side (belly) facing upwards, hence their common name of backswimmers (Figure 2). Notonectids can inflict wounds to humans with their proboscis (mouthpart), but this is very rare and often is a result of rough handling.
Although commonly collected in Europe (Soós et al. 2009), the common backswimmer can range from parts of northern Africa to western Siberia and northwestern China (Berchi 2013). Notonecta glauca is typically found in inland freshwater ponds, although they can be found in eutrophic (water excessively enriched in nutrients) freshwater bodies near the sea (Kjærstad et al. 2009). Many other Notonecta species occur in North America north of Mexico (Torre Bueno 1905). It is unclear if this species could become established if introduced to Florida, but its current distribution includes several locations with similar climates and habitats.
Notonecta spp. have white oblong eggs. These eggs can be found attached to aquatic vegetation (Figure 3). Adult Notonecta from Great Britain lay eggs in the fall and or spring (this varies by species), and there is only one generation a year (Briers 1998).
Like many true bugs, nymphs of this species look like small adults. They do not have fully developed wings as nymphs, and they have much shorter abdomens. They are often more uniform in color and their pronotum is not darker than the rest of the body, as seen in the adults. British species of Notonecta have been reported to have five nymphal instars that develop over the summer months (Briers 1998).
The most easily identifiable feature of insects in the Notonectidae family are their long hind legs that are used to propel themselves in the water. Adult Notonecta spp. have a pale tan head and legs. The pronotum (area just behind the top of the head) is darker than the head and the elytra (hardened forewings) can be a range of tan colors. The eyes are large and dark red in color. Fully-grown adults measure about 16 mm (Reynaldi et al. 2011).
Notonecta glauca adults and immatures breathe by taking in air rather than dissolved oxygen from the water; however, their body is covered in hair-like structures called setae and microtrichia that aid them in their aquatic lifestyle. A backswimmer's entire body except the pronotum (area behind the head), head, and legs is covered in these hairs. The hairs create a film that traps air, allowing the insect to absorb oxygen while underwater and keep their bodies dry (Kuru et al. 2011). The setae tend to be larger and sparser in comparison to the denser patches of microtrichia. Only microtrichia are present under the upper side of the elytra. Due to the thickness of the microtrichia, air can be held in the film for up to 130 days. This air film is visible with the naked eye and appears as a silvery sheen on the body (Kuru et al. 2011).
The common backswimmer preys on a variety of other aquatic organisms, like Daphnia spp. (water fleas) (Giller and McNeill 1981). Notonecta spp. most commonly prey on other insect species, but have been observed to prey on fish eggs, fry, and tadpoles (González and Leal 2010). Notonecta glauca have been observed to prey on the larvae of the mosquito Culex pipens (Reynaldi et al. 2011).When hunting, the common backswimmer captures its prey by using its raptorial forelegs or middle legs, t arehese legs have grasping surfaces making it difficult for prey to escape (Giller and McNeill 1981). They are ambush predators that wait for an opportunity to strike unsuspecting prey (Ellis and Borden 1970, Giller and McNeill 1981).
The common backswimmer prefers environments with lush vegetation in which to hide itself. When aquatic plants are present, the common backswimmer will reside on the edge of the plant, usually mid-way up the plant, a preference not influenced by prey selection (Giller and McNeill 1981). Residing on the edge of the plant allows the common backswimmer to more easily observe its surroundings and snatch passing prey (Figure 4) (Giller and McNeill 1981). Due to its preference for vegetation and its low affinity for the surface, the common backswimmer does best in a complex environment where avoiding the surface and having ample vegetation to hide in will protect it from predation.
The common backswimmer sucks the hemolymph from its prey (bodily fluid in invertebrates that is comparable to blood). Notonecta glauca extracts food slower and has less of a response to surface activity to than other Notonecta species (Cockrell 1984). One reason for a comparatively lower response to surface activity is that they reside at lower depths rather than the water surface (Cockrell 1984). At air temperatures below 15°C (59°F), Notonecta glauca spends the majority of the time underwater. Above 15°C (59°F) it will spend more time above water than below the surface. At 5°C (41°F), it tends to stay completely submerged (Cockrell 1984). In highly oxygenated water and at higher temperatures, the common backswimmer prefers to stay submerged (Cockrell 1984).
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