Family NOTONECTIDAE

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Family NOTONECTIDAE

Compiler and date details

October 2010 - Dr G. Cassis, University of New South Wales, Sydney, New South Wales

Introduction

The Notonectidae, or backswimmers, are a nepomorphan group of aquatic bugs presently comprising 11 genera and 400 species (Truxal 1979; Slater 1982; Zoological Record 1980–1994; Henry 2009; Polhemus, J.T. pers. comm.).

Notonectids are medium sized, strongly convex dorsally, flattened ventrally and tapered caudally. The head is convex, transverse and opisthognathous, with a distinct anteclypeus. The eyes are large, reniform and occupy most of the head. Ocelli are absent. The antennae are short, 3 or 4-segmented, not visible from above, and are housed beneath the eye. The labium is short, stout and 4-segmented. The pronotum is subtrapezoidal, the anterior margin is obtuse and the posterior margin is usually emarginate. The scutellum is large and triangular. The forewing is roof-like and differentiated into a clavus and corium, with the membrane coriaceous and without veins. The fore- and midlegs are adapted for grasping. The hind legs are adapted for swimming and are long, flattened, oar-like and densely fringed with hair. The tarsi are usually 2-segmented, although Anisops Spinola species have 1-segmented foretarsi. The pretarsi have two claws which are greatly reduced in the hind legs. Metathoracic glands are present in the Notonectinae and absent in the Anisopinae. In the notonectines the glands open mid ventrally and have paired lateral accessory glands. The larvae of both subfamilies lack dorsal abdominal glands. The ventral surface of the abdomen has a prominent, median, hairy keel, and reflexed, hair-fringed lateral margins, which form two longitudinal channels that function as air chambers. The male genitalia are either symmetrical or asymmetrical. The females have a well-developed laciniate ovipositor (Truxal 1979; Slater 1982).

Notonectids are most noted for swimming on their backs, like the Pleidae and Helotrephidae. They are most commonly encountered in still waters of ponds and lakes, with a few species known from streams. They are usually found near the water surface. Like other nepomorphans, notonectids are predaceous, feeding mostly on other aquatic arthropods but occasionally on small fish and other aquatic vertebrates. The biology of some backswimmers has been reviewed by Hungerford (1920) and Truxal (1979). Lansbury (1968) gave a detailed summary of the biology of Enithares Spinola.

The Notonectidae are divided into two subfamilies, the Anisopinae and Notonectinae, with the latter divided into three tribes: Aphelonectini, Notonectini and Nychiini (Štys & Jansson 1988). Zettel (1993) recently synonymised the Aphelonectini with the Notonectini. The Anisopinae are readily separated from the Notonectinae by the presence of a pit at the anterior end of the hemelytral commissure. The Anisopinae are represented by the Old World genera Anisops Spinola, Paranisops Hale and Walambianisops Lansbury and the Western Hemisphere genus Buenoa Kirkaldy. The Notonectinae are represented by two tribes, the Notonectini (Aphelonecta Lansbury, Enithares Spinola, Enitharoides Brooks and Notonecta Linnaeus), and Nychiini (Martarega White, Neonychia Hungerford and Nychia Stål). Keys to the genera and suprageneric groups have been provided by Lansbury (1968) and Poisson (1966).

The Australian fauna includes members of both notonectid subfamilies, six genera and 43 species. The Anisopinae are represented by 32 species of Anisops. The genus is widely distributed in Australia and found in all states and territories. A number of ubiquitous tropical species are also found in Melanesia and the Oriental Region. Brooks (1951) revised the genus Anisops, described 11 new species and provided keys to species and comprehensive descriptions. The anisopines are also represented by two endemic monotypic Australian genera: Paranisops and Walambianisops. In several reviews of notonectids, Lansbury (1964, 1969, 1975, 1984, 1991, 1995a, 1995b) described new species, and gave keys and detailed distributional data. The Notonectinae are represented in Australia by eight species. The Notonectini are represented by Enithares (five species) and Notonecta (two species). Lansbury (1968) revised the genus Enithares in the Oriental Region, with reference to Australian species. The two Notonecta species are possibly synonymous (Polhemus, J.T., pers. comm.) but are retained as separate species in the Catalogue pending further examination. Lansbury (1985) reviewed Nychia, recognising only one species, N. sappho Kirkaldy, which is found in tropical Northern Territory and Queensland, and the Oriental Region. Sweeney (1965) gave a detailed account of the distribution of notonectids in south-eastern Australia. Lansbury (1981) summarised the distribution of the continental fauna, adding an anecdotal account of their affinities. Andersen & Weir (2004b) provided a key to the Australian notonectids.