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Gupta and Ghosh (1980) described Cunaxa bambusae, Cunaxa cynodonae Den Heyer (2006) erected Riscus for Riscus thailandensis. Den Heyer (2011) transferred Cunaxa bambusae and Cunaxa cynodonae to Riscus based on the redescriptions by Corpuz-Raros (2008). Den Heyer and Castro (2012) described Riscus austroamericanus.
Gnathosoma. Pedipalps 5-segmented, extend beyond the subcapitulum by at least the last segment, and end in a strong claw; apophysis absent. Basifemora and telofemora complemented with simple setae; these two segments fused, although a line remains visible and they can thus be differentiated. Subcapitulum complemented with 6 pairs of setae (hg1–4 and 2 pairs of adoral setae). Setae hg3 and hg4 both near the coxal bases of the pedipalps.
Idiosoma, dorsal. Female dorsal idiosoma has a sclerotized plate that bears 2 pairs of setose sensillae (at and pt) and 2 pairs of simple setae (lps and mps). Idiosomal shield covered by integumental papillae that form a reticulated pattern. Hysterosoma lacks a plate and bears 7 pairs of setae (c1–2, d1–h1). Cupule im present, usually laterad or in the proximity of e1.
Idiosoma, ventral. Coxae ill-defined. Coxae I and II fused; coxae III and IV fused. Coxae I–IV setal formula 3-1-3-1 (including paracoxal seta). Genital plates each bear 4 setae. Anal plates bear 1 pair of setae (ps1). 2 pairs of setae (ps2 and h2) associated with, but do not occur on, the anal plates. Cupule ih present in close proximity to h2. Integument between plates striated and bears 5 pairs of additional setae. Legs. Ambulacral claws on either side of a 4-rayed empodium present.
Key to adult female Riscus
(modified from Den Heyer and Castro 2012)
- Skvarla, M; Fisher, J; Dowling, A; 2014: A review of Cunaxidae (Acariformes, Trombidiformes): Histories and diagnoses of subfamilies and genera, keys to world species, and some new locality records ZooKeys, 418: 1-103. doi
- Gupta S, Ghosh S (1980) Some prostigmatid mites (Acarina) from Andaman and Nicobar Islands. Records of the Zoological Survey of India 77: 189–213.
- Den Heyer J (2006) Riscus, a new cunaxid genus from Thailand (Acari: Actinedida: Cunaxidae). Acarologia 46(3–4): 195–201.
- Corpuz-Raros L (2008) Additional species of Cunaxidae and first report of the subfamily Neobonzinae (Cunaxidae, Acari) from the Philippines. Asia Life Sciences 17(1): 71–89.
- Den Heyer J, Castro T (2012) New Neotropical cunaxinae species (Acari: Prostigmata: Cunaxidae). Zootaxa 3265: 22–42.