Family PLEIDAE

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Family PLEIDAE

Compiler and date details

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

Introduction

The Pleidae, or pygmy backswimmers, are a family of truly aquatic, nepomorphan bugs, comprising three genera and 38 species (Slater 1982; Zoological Record 1980–1994; Henry 2009; Polhemus, J.T. pers. comm.). They are cosmopolitan and are most diverse in the tropics.

Pleids are very small, ranging in length from 2 to 3 mm. They are coleopteriform, ovoid, strongly convex and tapered caudally. They are generally pale in colour. The head is hypognathous. The eyes are large and ocelli are absent. The antennae are 3-segmented and not visible from above. The labium is 4-segmented and awl-shaped. The pronotum is large, quadrate, and either punctate or reticulate. The scutellum is triangular. The forewings are uniform, without a differentiated membrane, and fit closely over the abdomen. The hind wings are often shortened. The larvae have dorsal abdominal glands between terga III and IV. The metathoracic glands are sac-like and have a single midventral opening (Aldrich 1988). The coxae are open and pagiopodous. The hind legs are characterised by long hairs adapted for swimming. The tarsi are 3-segmented and the hind pretarsus has two claws (Slater 1982).

Pleids are predaceous and feed on mosquito larvae and other invertebrates, such as ostracods and Daphnia Müller species (Polhemus 1988). Pleids are usually found in quiet waters. Like notonectids, pleids swim upside down in a rowing manner. Carver et al. (1991) report that they are poor swimmers, preferring to walk on or through thick submerged vegetation. Australian pleids are most common in swamps and lakes.

Esaki & China (1927) first recognised pleids as a family and allied them with the Helotrephidae. Schuh (1986) and Carver et al. (1991) placed them in the Notonectoidea, which is followed in the Catalogue. Alternatively, Štys & Jansson (1988) placed the Pleidae, together with Helotrephidae, in the superfamily Pleoidea. Rieger (1976), in his phylogenetic analysis of the Nepomorpha, considered the Pleidae and Helotrephidae to be the most apomorphic nepomorphans. Lundblad (1933) reviewed the Pleidae and described one new Australian species. Drake & Chapman (1953) gave an account of the Western Hemisphere species and Polhemus (1984) updated the North American fauna, cataloguing it in a subsequent work (Polhemus 1988).

Carver et al. (1991) listed Australian pleids as belonging to the genus Plea. Štys & Jansson (1988), however, listed this genus as restricted to the Palaearctic Region. In Cassis & Gross (1995) the three species that occur in Australia are referred to Paraplea Esaki & China. P. brunni (Kirkaldy) is widely distributed in Australia, P. halei (Lundblad) is a temperate species known from Victoria and Tasmania, and P. liturata (Fieber) is a tropical species known from the Northern Territory, Java and New Caledonia. Andersen & Weir (2004b) provided a key for the Australian pleids.