Orthacarus Zachvatkin, 1949

Uusitalo, Matti, 2025, A review of the family Alycidae (Acari, Acariformes) from North America, Zootaxa 5602 (1), pp. 1-92 : 31

Orthacarus Zachvatkin, 1949

Type species: Orthacarus tremli Zachvatkin, 1949 from Ukraine; by original designation .

Differential diagnosis. This genus is similar to Amphialycus Zachvatkin, 1949 in having robust and elongated chelicerae, a finely striated anterior pair of eyes, and a densely setulated empodia. Orthacarus differs from Amphialycus by having sparsely ciliated dorsal setae, a tiny and elongated body, and the prodorsal setae reduced in size. Other diagnostic features are as follows: naso is absent; prodorsal setae vi are very close to each other; setae in are pubescent or not; four palpal eupathidia are fused or basally very close to each other; legs II, III and IV are short and strongly bent; and the anus is caudal ( Figs. 16, 50J, K, L, 54, Uusitalo 2010: figs. 82–86, Uusitalo et al 2020: figs 76–83). The other genera of the tribe Alycini have either robust and non-elongated ( Alycus and Odontoalycus ) or beak-like ( Pachygnathus ) chelicerae.

Remark. The character states, suggesting a more edaphic way of life, was supposed to be unique to a single, cosmopolitan species O. oblongus . Orthacarus was not supported phylogenetically in Uusitalo (2010) and was therefore tentatively considered as a subgenus of Amphialycus ( Uusitalo 2010, Uusitalo et al. 2020). However, observations of a new species with robust and non-elongated chelicerae (e.g. Odontoalycus with 20 teeth) suggested that the basic shape of the mouthparts, like robustness and elongation, may also have appeared several times in evolution (also present in Petralycus ), and thus is not enough to justify oblongus being in the genus Amphialycus . The same combination of edaphomorphic character states is present in an undescribed species ’clu’ from South Africa ( Fig. 54). However, the preliminary analysis missed, for example, the exceptional mutual closeness of setae vi, the shape of legs and the location of anus, which in combination is unique to the two species of Orthacarus , and together with other edaphomorphisms, it seems justified to reinstate the old genus name. The undescribed species ’ clu ’ from South Africa ( Fig. 54) is in the Collection of the late Prof. Pieter Theron’s and has been temporarily lost in removal of the collection; therefore, its description is unpublished so far (Dr. Edward A. Ueckermann, in litt.).