Family NEPIDAE

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Family NEPIDAE

Compiler and date details

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

Introduction

The Nepidae, or water scorpions, are a cosmopolitan nepomorphan family comprising 15 genera and 268 species (Slater 1982; Zoological Record 1980–1994; Henry 2009; Polhemus, J.T. pers. comm.).

Water scorpions are medium to large sized bugs, ranging in length from 15 to 45 mm. The body shape varies from flattened and ovoid to cylindrical and parallel-sided. Nepids are usually dull brown in colour. The head is small, horizontal, and usually deeply inserted into the pronotum. The eyes are small and contiguous with the anterior margin of the pronotum. Ocelli are absent. The antennae are 3-segmented, short, inserted ventrally below the eyes, and not visible from above. The labium is 3-segmented, short, and often arcuate. The pronotum is subquadrate, and the anterior and posterior margins are often emarginate. The scutellum is triangular and large. The forewing is differentiated into a large clavus, corium, and membrane with weak veins. The legs are long, slender, and apparently poorly adapted for swimming. The forelegs are raptorial and the anterior femora are strongly incrassate and grooved ventrally, housing the tibiae. The tarsi are 1-segmented. The adults lack metathoracic scent glands, and the larvae lack dorsal abdominal glands (Staddon & Thorne 1979). The adult abdominal sternites are divided longitudinally. The subgenital plate is broad. Nepids possess long, slender, respiratory horns at the tip of the abdomen (Menke 1979; Slater 1982). The male genitalia are characterised by short conjuctiva and a stiff endosoma. Lansbury (1967) made a comparative morphological study of three Australian male nepids, with emphasis on the male genitalia. De Carlo (1968a) described the male genitalia of Austronepa angusta (Hale). De Carlo (1968b) described the female genitalia of some species of Ranatra Fabricius.

Menke (1979) and Sites & Polhemus (1994) reviewed the biology of nepids. They are mostly found in quiet waters. Ranatra species usually live amongst plant debris or submerged weeds. Nepa Linnaeus and Curicta Stål species are found in shallow, muddy pools or gentle streams with little vegetation (Menke 1979). Goodnomdanepa Lansbury species have been collected under rocks in shallow running water. Nepids are predaceous and are thought to feed on mosquito larvae, tadpoles, and a variety of other invertebrates. McKeown (1942) reviewed the little that is known of the biology of Australian nepids, except for Bailey's (1984, 1986a-d, 1988) detailed studies of Ranatra dispar Montandon.

Menke & Strange (1964) revised the suprageneric classification of the Nepidae, recognising two subfamilies, the Ranatrinae and Nepinae. Lansbury (1974) erected a new tribe, the Goodnomdanepini, and regarded them as intermediate between the two subfamilies. De Carlo (1967) raised the two subfamilies to family level, which was not followed by Menke (1979). Štys & Jansson (1988) summarised the nepid suprageneric classification as follows: Nepinae (Curictini and Nepini) and Ranatrinae (Austronepini, Goodnomdanepini and Ranatrini), which is followed in the Catalogue. The Curictini are monogeneric and are restricted to the Western Hemisphere. The Nepini are represented by six genera which are mostly found in the Eastern Hemisphere, with Nepa having a Holarctic distribution. The Austronepini and Goodnomdanepini are both monogeneric Australian endemic tribes. The Ranatrini comprise two genera: Cercotmetus Amyot & Serville which is found in the Oriental and Australasian regions, and Ranatra which has a cosmopolitan distribution. Keffer (1996) revised the New World genus Crucista Stål and Johnson and Masner (2004) revised the genus Thoron Haliday.

The Australian Nepidae comprise five genera and nine species. Menke & Strange (1964) revised the Australian nepids, providing a key to genera and a list of the five known species. Lansbury (1972) revised Ranatra from the Oriental Region, including descriptions of the three Australian species. Lansbury (1974) described Goodnomdanepa from the Northern Territory, and later described two additional species from tropical Australia (Lansbury 1978). Lansbury (1975) described a new subspecies Cercotmetus brevipes australis, which occurs in creeks of tropical Queensland and the Northern Territory. The Nepini is represented in Australia by one described species, Laccotrephes tristis (Stål), and several undescribed species (Carver et al. 1991). Hale (1924) indicated that Laccotrephes grossus (Fabricius, 1787) (as Laccotrephes ruber Linnaeus, 1758 in part) may occur in Australia. He based this on one specimen from South Australia, but indicated that it was possibly mislabelled; it was excluded from the Cassis & Gross (1995). Andersen & Weir (2004b) provided a key to all the Australian nepids. Five genera and nine species are recognised currently (July 2012).