Prosthetops setosus Perkins & Balfour-Browne, 1994

Bilton, David T., 2013, Prosthetops wolfbergensis sp. nov. — a giant amongst the ‘ minute moss beetles’, with new data on other members of the genus (Coleoptera, Hydraenidae), Zootaxa 3666 (3), pp. 345-357 : 355-356

Prosthetops setosus Perkins & Balfour-Browne, 1994

Apparently relatively widespread in the Western Cape; recorded from rockpools, seepages and temporary pools on stony/sandy substrates.

Additional records: 19/ix/2009 Western Cape, Langeberge, stream along R324 road in Tradouw Pass, 10 km S of Barrydale—1 Ƥ; 19/ix/2009 Western Cape, Langeberge, temporary pool on sand beside R324 road 12 km S of Barrydale—1 3; 24/ix/2009 Western Cape, Groote Swartberg, small pools in seepage over gently sloping rock face beside R328 road ca. 2 km N of De Hell Jnct.—abundant; 25/ix/2009 Western Cape, small rockpools beside stream at Mont Rochelle, above Franschhoek, 1,100 m ( Fig. 12 B)—abundant; 25/ix/2009 Western Cape, seepage over gently sloping rock face beside stream above Franschhoekpas, ca. 500 m—abundant; 27/ix/2009 Western Cape, sandy/rocky temporary pond by R301 road ca. 1 km S of Bain’s Kloof Lodge in Bain’s Kloof Pass nr. Wellingtonabundant; 22/ix/2011 Western Cape, Cederberg, Matjiesrivier Reserve, Matjies River ca. 3 km N of Cederberg Oasis, riverside pool on shingle– 1 3; 23/ix/2012 Western Cape, Cederberg- rockpools below Wolfberg Arch @ 1,400 m—1 3.

Discussion

Members of the family Hydraenidae are often colloquially termed ‘minute moss beetles’ (e.g. Jäch et al., 2008), in reference to their small adult body size; most species averaging around 2 mm. At up to 3.36 mm maximum length from labrum to elytral apex Ochthebius (Enicocerus) granulatus Mulsant achieves the largest body size recorded in the family (Jäch et al., 2008; M.A. Jäch, pers. comm.). Although P. wolfbergensis sp. nov. displays a range of sizes, the largest paratypes slightly exceed O. granulatus in length from labrum to elytral apex, and are much longer when their exposed abdominal apices are taken into account, making P. wolfbergensis sp. nov. the longest hydraenid described to date.

Prosthetops species have been reported from a range of microhabitats, but based on both literature records (Perkins & Balfour-Browne, 1994; Perkins, 2008), and recent observations many species seem to be most commonly associated with rain filled rockpools, either on montane plateaux, or on rocky outcrops beside rivers. Whilst riverside rockpools are formed by a combination of stone and water erosion, on montane plateaux pools result solely from the chemical weathering effects of rainwater (Williams, 2006). Temporary rockpools of this nature are found in many parts of the world, including tropical Africa (McLachlan & Cantrell, 1980) and Western Australia, where they are known as gnammas (Bayly, 1992; 1997; Hendrich & Fery, 2008). Such pools are dry for much of the year, and are inhabited by an invertebrate fauna composed of a mixture of highly mobile, opportunistic species and specialists, the latter often having life history adaptations to cope with seasonal drought, such as dormant eggs, or desiccation resistant stages (McLachlan & Cantrell, 1980; Williams, 2006; Hendrich & Fery, 2008). In the larger rockpools it inhabited P. wolfbergensis sp. nov. co-occurred with P. setosus and Parasthetops nigritus Perkins & Balfour-Browne, 1994 , as well as Crenitis zimmermanni Knisch (Hydrophilidae) and Canthyporus lateralis (Boheman) and C. petulans Guignot (Dytiscidae) . Of these species, P. setosus and C. lateralis appear to be associated with rockpool habitats (Perkins & Balfour-Browne, 1994; Biström & Nilsson, 2006; D T Bilton, pers. obs.); other species being generalists. Observations of P. w o l f b e rg e n s i s sp. nov. at Stadsaal cave reveal that both adults and mature larvae can be active in what were previously dry rockpools within one day of fresh rain. In this respect they resemble larvae and adults of Ochthebius (Cobalius) lejolisii Mulsant & Rey , which can appear within 1 hour of the experimental wetting of dry supralittoral pools in SW England (D T Bilton, pers. obs.). Whilst adult beetles can potentially escape drought by dispersal to more permanent waters (which may explain some records of adult Prosthetops in other habitats), larvae do not have this option. Whether larvae of these insects have an ability to desiccate, as in the chironomid midge Polypedilum vanderplanki Hinton, 1951 (Watanabe et al., 2002; Sakurai et al., 2008; Gusev et al., 2010), or whether they survive the dry phase using some other mechanism (e.g. retreat to damp crevices below the pan) remains unknown, however. Adults of P. megacephalus , P. setosus and P. wolfbergensis sp. nov. have also been found abundantly in running seepages over gently sloping exposed rock, which may provide similar biofilms to those present in rockpools.

Figures

FIGURE 12. Habitats of Prosthetops species, Western Cape, South Africa. A) pools in Tsitsikamma National Park beside Salt River (P. pronotus); B) rocky outcrop with pools and seeps above Mont Rochelle, Franschhoek (P. seto s us). Photos D T Bilton.