Diplonychus rusticus ( Fabricius, 1781 )

Polhemus, Dan A. & Polhemus, John T., 2013, Guide To The Aquatic Heteroptera Of Singapore And Peninsular Malaysia. X. Infraorder Nepomorpha-Famlies Belostomatidae And Nepidae, Raffles Bulletin of Zoology 61 (1), pp. 25-45 : 27-28

Diplonychus rusticus ( Fabricius, 1781)

( Fig. 2 )

length shorter than width of vertex. Pronotum, scutellum and hemelytral corium finely punctate.

Distribution. — Originally described from India, and subsequently reported from Sri Lanka, India, Burma, China, Japan, Taiwan, Thailand, Vietnam, Malaysia, Singapore, the Philippines (Mindanao), Sumatra, Java, Borneo and Sulawesi ( Chen et al., 2005). Fernando & Cheng (1974) reported this species from Singapore and the Peninsular Malaysian states of Johor, Negeri Sembilan, Perak, Penang and Selangor, with some of the Peninsular Malaysian records erroneously listed under the name Sphaerodema molestum (see below).

Discussion. — This relatively small, brown, ovate belostomatid is widespread in the Indo-Australian region, and shows a notable degree of intraspecific variation in size and colouration. It may be found along the margins of ponds and other lentic ecosystems, but does not generally inhabit lotic ecosystems unless current speed is very slow. The life history of this species was investigated in the laboratory by Su & Yang (1992), who found that the species could produce 3–4 generations a year.As with many belostomatids (although not Lethocerus ) males of Diplonychus exhibit parental care for egg masses which are carried on their backs. Such egg guarding may be essential for successful reproduction, given the results of Su & Yang (1992) who found that under laboratory conditions egg masses deprived of such care dessicated and failed to hatch.

Diplonychus rusticus is a beneficial insect whose positive effects are well documented. It is a demonstrated predator of immature mosquitoes, with individuals capable of consuming Nepa rustica Fabricius, 1781: 333

Diplonychus rusticus: Laporte, 1833: 18

Appasus marginicollis Dufour, 1863: 393 ; syn. by Distant, 1906: 36

Sphaerodema molestum: Fernando & Cheng, 1974: 41 , misidentification

Diplonychus indicus Ventaktesan & Rao, 1980: 299 ; syn. by J. Polhemus, 1995: 651

Material examined. — We have examined several hundred specimens of this species from Singapore and throughout Peninsular Malaysia, including the states of Johor, Malacca, Negeri Sembilan, Selangor, Pahang, Terengganu, Perak, Perlis, and Panang. Many long series have been taken at light, and others from standing water habitats ranging from reservoirs to duck ponds .

Diagnosis. — Body ovate ( Fig. 2 ), length 15.0–16.0 mm; maximum width 9.0–9.5 mm. Colouration medium brown, with lateral margins of pronotum and hemelytra contrasting paler brown. Head with eyes convergent anteriorly, head up to 87 larvae a day ( Saha et al., 2007). Yano et al. (1981) also noted that this species occurs in rice paddies, and considered it to represent a potential natural biological control agent for certain rice pests. In addition, it is a predator of the molluscan intermediate hosts of the parasitic trematode Fasciola gigantica which causes tropical fascioliasis, a disease of the liver that commonly affects livestock, and occasionally humans ( Reyes et al., 1970).

J. Polhemus (1994) discussed the history of nomenclatorial confusion surrounding this taxon name. A proposal to conserve the name for the sake of nomenclatural stability was submitted to the ICZN by Polhemus & Kerzhner (1995), and later approved by the ICZN (1996). We also consider previous Peninsular Malaysian records of the Indian species Diplonychus molestus (Dufour) , listed by Fernando & Cheng (1974) under the name Sphaerodema molestum , to be referable to D. rusticus .

Figures

Fig. 2. Diplonychus rusticus (Fabricius), male, specimen from Philippines, Luzon.