Rhombognathus Trouessart, 1888

Bartsch, Ilse, 2015, The genital area of Halacaridae (Acari), life stages and development of morphological characters and implication on the classification, Zootaxa 3919 (2), pp. 201-259 : 228-229

Rhombognathus Trouessart, 1888

Type species. Pachygnathus notops Gosse, 1855 .

Adults. In most of females GP and AP contiguous or partly fused, in a few species these plates separated. GO in posterior part of GP; with 1–45 pairs of slender pgs around GO and two pairs of sgs, rarely a single pair (e.g. in R. peltatus Viets, 1939 , cf. Bartsch 1996f: fig. 31). Acetabula internal, in general three pairs almost equal-sized gac present, rarely two pairs (e.g. in R. intermedius Schulz, 1933 , R. neotenus Abé, 1996 , R. peltatus Viets, 1939 ) ( Abé 1996a: fig. 50A; Bartsch 1975b: fig. 8). In two species ( R. amplus Bartsch 2013 , R. bulbosus Bartsch 2005 ), a single pair of gac distinctly recognizable ( Bartsch 2005c: fig. 22, 2013e: fig. 1h). Ovipositor short, both at rest and everted, at rest hardly extending beyond GO. When everted, ovipositor basally with pair of delicate papillae (may be vestigial), apically with five pairs of sclerotized, palmate genital spines, each ending with three to nine spikelike tines ( Bartsch 1975a: figs 8a, 9b, 1975b: fig. 46, 1975c: figs 4, 18, 2000: fig. 21C). In males GP and AP generally fused, also in species in which females have separate plates. Male GO often smaller and in a more ventral position than female GO. Area representing GP with 7–25 pairs of pgs, in many species setae plumose and arranged trapezoidally, else setae smooth. Each genital sclerite with two small sgs, these generally setiform, rarely plumose ( Abé 1996a: figs 25B, 29B, 33B, 37B, 43D, 46D, 59E; Bartsch 2013e: fig. 1d). GO normally with three pairs of internal gac, except for e.g. the five above mentioned species, in these, as in their females, two pairs or a single pair of acetabula recognizable ( Bartsch 1975b: figs 9, 24, 1975c: fig. 27, 2003c: fig. 12I, 2013e: fig. 1c). Acetabula in males often more contiguous than in females and situated near posterior end of GO (except e.g. R. leurodactylus Krantz, 1976 , cf. Abé 1996a: fig. 43D and C). All stages, adults and juveniles, with tube-like epimeral pores.

Juveniles. Majority of species with three nymphal and one larval stage. Tritonymphal GP and AP either fused or separated, GP and AP of deuto- and protonymph generally fused. Tritonymphal GP with two pairs of pgs, one pair of sgs and, in the majority of species, with three pairs of equal-sized internal gac. Tritonymphs of R. amplus and R. bulbosus with two large pairs and a median short, slender pair of gac ( Bartsch 2005c: fig. 37, 2013e: fig. 3c), these nymphs of R. cyrtonotus Bartsch, 2000 , R. intermedius , R. neotenus and R. peltatus with two pairs of gac ( Bartsch 1975b: fig. 21, 1975c: fig. 24; Abé 1996a: fig. 53A). Deutonymphal GP with two pairs of internal gac, equal-sized apart from e.g. R. amplus and R. bulbosus . Deutonymphs of these two species with minute anterior and tube-like posterior pair of acetabula ( Bartsch 2013e: fig. 3e). Deutonymphal GP without pgs and sgs. Protonymph with single pair of gac; pgs and sgs lacking. Larvae with pair of tube-like epimeral pores.

Remarks. In a few species, the deutonymphal stage seems to be suppressed. Of both R. intermedius and R. peltatus numerous individuals, collected in different seasons, have been studied, larva, protonymphs (with fivesegmented leg IV and single pair of gac), tritonymphs (with two pairs of pgs and one pair of sgs) and adults were present in the samples but no deutonymphs ( Bartsch 2003c). In these two species adults have no more than two pairs of gac. Because of the reduced number of gac in the tritonymphs of R. cyrtonotus Bartsch, 2000 and R. neotenus , these species, too, are expected to have lost the deutonymphal instar.

In the majority of the deuto- and tritonymphs the two or three pairs of acetabula are similar in size, but in R. amplus and R. bulbosus the size is different and the situation resembling that of Isobactrus species of the northern temperate zone.

The genus Rhombognathus is spread world-wide, it includes more than 100 species ( Bartsch 2009a, d, 2013e; Abé & Fernandes 2011, Abé & Etamadi 2014). The species are algivorous, hence restricted to the photic zone.