Isobactrus Newell 1947
Creators: name: Bartsch, Ilse
Publisher: name: Zenodo
Year: 2015
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Arachnida
Trombidiformes
Halacaridae
Isobactrus

description: Isobactrus Newell, 1947
(Figs 53 –60)
Type species. Aletes setosus Lohmann, 1889.
Adults. Female GP delicate and often in form of pair of crescentic plates on either side of GO, not or only slightly longer than GO and not distinctly delimited from striated integument. Female genital area with three (rarely four) pairs of pgs, anterior pair in striated integument, anterior to GO, following two pairs level with and posterior to GO, respectively, in margin or outside GP. Anterior and posterior edge of genital foramen often enforced by internal pregenital and postgenital sclerites (Fig. 53). Genital sclerites with one to two pairs of sgs. Female with two or three almost equal-sized internal tube-like gac (Figs 54, 56), or two pairs of distinct, tube-like and one pair of small gac (Fig. 53; Viets 1939b: fig. 21; Newell 1947: figs 55, 80; Bartsch 1974b: fig. 2b; Abé 1996a: figs 4A, 8A, 16A, 20A, 1996b: fig. 2A) (the posterior markings illustrated in Newell 1947: fig. 59 are no acetabula). Ovipositor short, both at rest and when everted. Genital spines well-sclerotized, rather wide and palmate, generally ending with several tines; 10–11 spines present, four in anterior and six to seven in posterior position (Bartsch 1975a: figs 10b, 11, 1975b: fig 103, 2000: fig. 1D; Newell 1984: fig. 42). GP in male larger than in female, with about 30–100 perigenital setae, these often arranged in an inner and outer ring around GO and inserted on GP, rarely in striated integument outside GP. Setae generally slender and smooth, in a few species some of anterior setae of inner ring short, bristle-like or spiniform, namely in I. hutchinsoni Newell, 1947 and I. uniscutatus (Viets, 1939) (Fig. 60; Newell 1947: fig. 76). Genital sclerites with four (rarely three) pairs of sgs, these setiform, rarely flattened and foliate or spur-like. Acetabula generally slightly smaller than in females and situated in posterior part of GO or even posterior to GO (Fig. 60). As in female, male GO with either two to three pairs of large almost equal-sized gac or two large pairs and one anterior small pair (Fig. 56; Viets 1939b: fig. 21; Newell 1947: fig. 76, 1984: fig. 37; Abé 1996a: figs 4B, 16B; 1996b: fig. 2B; Bartsch 2003a: fig. 2M). Acetabula generally internal, in two species (I. hutchinsoni, I. uniscutatus) two pairs of external acetabula present (Newell 1947: fig. 76; Bartsch 1979b: fig. 13, 2003c: fig. 3F).
Juveniles. With four juvenile stages, one larva and three nymphs (proto-, deuto-, tritonymph). Nymphal genital and anal plate in some species separated in others fused. Tritonymph with a pair of small sgs and two pairs of pgs, one pair in striated integument anterior to GP, one pair on or close to GP. Genital region with two or three pairs of almost equal-sized acetabula (Fig. 55, 57; Newell 1947: fig. 81; Bartsch 1972: fig. 18G, 2005c: fig. 14, 2009b: figs 5F, 7G; Abé 1996b: fig. 2C) or with pair of large anterior acetabula, small, short middle pair, more or less adjacent to anterior pair of gac, and slender posterior pair (Bartsch 1972: fig. 21D, 1975b: figs 81, 100, 121; Abé 1996a: figs 16C, 23A). Deuto- and protonymph without sgs. Deutonymph with single pair of pgs situated level with gac and on or in margin of GP. Majority of deutonymphs with two pairs of gac, either equal-sized (Abé 1996b: Fig. 2 D) or with small anterior and large posterior pair of gac (Bartsch 1972: fig. 21C, 1975b: figs 82, 101, 120; Abé 1996a: figs 4C, 23B), or with single pair of gac (Fig. 58; Newell 1947: p. 80; Bartsch 1972: p. 191, fig. 18D). Protonymph with single pair of internal gac; pgs and sgs lacking (Fig. 59; Bartsch 1972: fig. 21B, 1975b: figs 83, 102, 119; Abé 1996a: fig. 23C, 1996b: fig. 2E). Larva with pair of epimeral pores (Bartsch 1974b: fig. 1a, 2c).
Remarks. At present 29 species are known. The species live in a zone more or less regularly emerged; all feed on algae. The genus is spread world-wide but three different lineages have been separated on the basis of their character combinations (Abé 2001; Bartsch 2010a). The species of each lineage are concentrated within a geographical area which roughly can be attributed to the temperate northern Atlantic and Pacific Ocean (ca 75°N to 25°N), the tropics (ca 25°N to 25°S) and the temperate southern Pacific and Indian Ocean (ca 25°S to 55°S). The different number and shape of the genital acetabula are in concordance with these three groups. Adults and tritonymphs from the southern temperate zone have two pairs of acetabula, those from the tropics three pairs, and those from the northern temperate zone generally two pairs of large-sized and one pair of small acetabula. Exceptions are the North Atlantic species I. uniscutatus and I. hutchinsoni which have two pairs of acetabula; populations of both can survive in low saline brackish water. The deutonymphs of the southern temperate zone have a single pair of gac, those from the tropics two equal-sized pairs and species from the northern temperate zone a large and a minute pair of acetabula, again with the exception of I. uniscutatus which bears a single pair of acetabula.
description: Published as part of Bartsch, Ilse, 2015, The genital area of Halacaridae (Acari), life stages and development of morphological characters and implication on the classification, pp. 201-259 in Zootaxa 3919 (2) on pages 220-222, DOI: 10.11646/zootaxa.3919.2.1, http://zenodo.org/record/245246