Aturus Kramer, 1875

Gerecke, Reinhard, 2014, Studies on European species of the water mite family Aturidae Thor (Acari: Hydrachnidia), Zootaxa 3841 (1), pp. 1-46 : 3-6

Genus Aturus Kramer, 1875

In contrast to the situation in Kongsbergia (see below), palp morphology is rather uniform in most species. While males are often highly distinct due to an extreme sexual dimorphism of IV-L (for an example, compare Figs 13 B- C with Fig. 13 F), occasionally also III-L, in females not even the distinction of species groups is possible in general and attribution of sexes remains a big problem, also if morphological details are studied under high magnifications at the SEM (Güntzel 1977). As a general rule, Aturus species were described from the male sex only, and females were attributed to a selected species when they were found together with the corresponding males. Differences in colouration or idiosoma size and proportions may provide additional help in associating males to females when several species are found coexisting. However, until present day we are not well orientated on significance of morphological characters in females and their intraspecific stability, and attribution of females is highly uncertain in several species, or they are known from the male sex only. In the following treatment, this problem cannot be brought to a general solution, but some steps are possible: The rather rich material, especially from Sicily, allows to get an overview on the composition of the Aturus fauna on the island. In several isolated areas, the presence of only one species is highly probable, and the morphological analysis of females there gives a rather well-founded idea about diagnostic characters and their variability.

In males, a rich variety of diagnostic characters is found both on the dorsal idiosoma and on III-L and IV-L, the latter being highly modified as a grasping organ. While on III-L, modifications are found only in a few species and are not very extensive (a low sexual dimorphism in shape of segments 3–4 and their setae), sexual dimorphism of IV-L is a basic diagnostic character, with considerable changes in male IV-L-3–5, following a general ground pattern ( Figs 1 A–D, 2 A–B): (1) IV-L-4 distally enlarged, bearing a group of modified distal setae—as a rule, on the anterior surface two to three long, blade-like, on the posterior surface a higher number, often slightly flattened—these setae along with setae of IV-L-5 forming a characteristic bristle; (2) IV-L-5 bowed, with a smooth ventral surface in the centre; (3) in proximal part of IV-L-5 two groups of modified setae—near segment base several stronger, flattened, occasionally leaf shaped, slightly more distally more slender, in basal part slightly enlarged setae in a line; (4) at distal segment end of IV-L-5 a group of ventral setae, enlarged in basal part, fine and slender, often whip-like distally.

The variety of morphological adaptations on the dorsal shield of males is very large: Setae of Dgl and Lgl may be simple, thickened or bi- to trifurcated, in addition, a fur-like cover of fine setulae may be developed on the central dorsal shield ( Fig. 2 C), and extended setal fields are developed on the lateral parts of the ventral shield ( Figs 1 E, 2C–D). These are often bent dorsally in the posterior part of idiosoma and bear groups of densely-arranged, differently adapted setae. Most characteristic club-shaped or balloon-like inflated setae flanking the dorsallydirected gonopore, often in one or two pairs ( Figs 1 E, 2C), and rows of long, curved, proximally inflated, in the following called "bulbous" setae ( Fig. 2 D).

As in males, the female genital field is characterized by a gonopore in terminal position (the excretory pore consequently being shifted to the dorsal surface, Fig. 1 F), flanked by the strip-shaped longitudinal genital plates which bear 5–20 pairs of acetabula arranged in one or two lines. As is seen in the SEM photographs ( Figs 1 G, 2E), acetabula are often internally subdivided into several subunits and have therefore an irregular surface. The interspace between coxal and genital field is in mature specimens generally covered by secondary sclerotization. Here lie two pairs of Vgl, with their position probably of diagnostic value for species recognition: in the secondary sclerotization ( Fig. 1 G), at the posterior border of the primary sclerotization, or embedded into the latter ( Fig. 2 E).

In this treatment, species names are given to females when they were found associated with males of the species in question and their morphological features are in agreement with the set of character states considered as characteristic. As this set of characteristics may overlap with the conditions typical for other species, females not found together with males are treated as "uncertain records" requiring confirmation by detection of the male sex.

Figures

FIGURE 1. Aturus intermedius (I 181), SEM photographs. A. – E. male; A. IV-L posterior; B. IV-L- 4 – 5 anteroventral; C. IV-L- 4 – 5 posteroventral; D. IV-L- 4 – 5 ventral; E. posterior dorsum with balloon-like perigenitalsetae; F. – G., female; F. posterior dorsum; G. genital field posterolaterally. Scale bars: A: 50 µm; B – G 20 µm.

FIGURE 2. Aturus crinitus (I 181), SEM photographs. A. – D. male; A. IV-L- 4 – 5 anteroventral; B. IV-L- 4 – 5 posteroventral; C. dorsum; D. posterolateral idiosoma; E. female genital field posterolaterally. Scale bars: A – B, E: 20 µm, C – D 50 µm.

FIGURE 13. A. – B. Aturus rotundus, male. A. posterior venter; B. IV-L- 4 – 6 anterior. C. – H. Aturus scaber; C. – E. male; C. IV- L anterior; D. palp; E. dorsum; F. – G. female; F. IV-L- 4 – 6; G. dorsum; H. venter. Scale bars = 100 µm.