Larval morphology of the water mite Hygrobates fluviatilis (Ström, 1768) (Acari, Hydrachnidia, Hygrobatidae)

Data on the morphology of the larval water mite Hygrobates fluviatilis (Ström, 1768) are given in the papers of Ullrich (1976), Wainstein (1980), Martin (2000) and Müller (2015). Martin (2016) has presented a detailed redescription of a larva of this species. However, Pešić et al. (2017) yield six well supported species by using together molecular and morphological evidence in order to resolve of diversity in the Hygrobates fluviatilis species complex: Hygrobates fluviatilis, H. arenarius Smit & Pešić, 2017, H. corsicus Pešić & Smit, 2017, H. marezaensis Pešić & Dabert, 2017, H. persicus Pešić & Asadi, 2017 and H. turcicus Pešić & Dabert, 2017. The larva described by Martin (2016) under the name H. fluviatilis turned out to be the larva of H. arenarius (Pešić et al. 2017). Unfortunately, Wainstein (1980) and Müller (2015) have not characterized female morphology whose larvae have been reared which complicates the identification of the species. The aim of this paper is to study the morphology of larva H. fluviatilis in detail and identify differences to the larva of the related H. arenarius . The females of Hygrobates fluviatilis were identified with the key of Pešić et al. (2017).

Data on the morphology of the larval water mite Hygrobates fluviatilis (Ström, 1768) are given in the papers of Ullrich (1976), Wainstein (1980), Martin (2000) and Müller (2015). Martin (2016) has presented a detailed redescription of a larva of this species. However, Pešić et al. (2017) yield six well supported species by using together molecular and morphological evidence in order to resolve of diversity in the Hygrobates fluviatilis species complex: Hygrobates fluviatilis, H. arenarius Smit & Pešić, 2017, H. corsicus Pešić & Smit, 2017, H. marezaensis Pešić & Dabert, 2017, H. persicus Pešić & Asadi, 2017 and H. turcicus Pešić & Dabert, 2017. The larva described by Martin (2016) under the name H. fluviatilis turned out to be the larva of H. arenarius (Pešić et al. 2017). Unfortunately, Wainstein (1980) and Müller (2015) have not characterized female morphology whose larvae have been reared which complicates the identification of the species. The aim of this paper is to study the morphology of larva H. fluviatilis in detail and identify differences to the larva of the related H. arenarius. The females of Hygrobates fluviatilis were identified with the key of Pešić et al. (2017).

Hygrobates fluviatilis (Ström, 1768) (Figs 1-11)
Material examined. Adult mites are collected by the author in streams of Samara and Yaroslavl Provinces, both located in the European part of Russia. Larvae of Hygrobates fluviatilis were reared by the author from six females: 2 females and 9 larvae from Samara Province, National natural Park "Samara Luka", Kuibyshev reservoir near village Koltsovo; 4 females and 32 larvae from Yaroslavl Province, Nekouz District, Ild stream near village Mar'ino. To obtain larvae, water mites were maintained in the laboratory (room temperature, natural day-night conditions). Each mature female was placed in a separate glass cylinder (10-15 mm in diameter and 15 mm in height). The duration of the embryonic period was 9-12 days.
Larva. Idiosoma flat. Dorsal plate elongate (L/W ratio 1.55-1.70), in unengorged larvae covering almost the whole dorsum, with slightly convex lateral margin, simple setae long and thick, but setae Fch much shorter than Vi, trichobothria Fp, Oi short, thin and equal in length (Fig. 1). Seven pairs of setae (Oe, Hi, He, Sci, Sce, Li, Si) situated in the soft membrane laterally of the dorsal plate; Hi longest, Li shortest, Hi, He and Sci nearly subequal in length and longer than other posterior setae situated in the membrane. Surface of dorsal plate with elongate cell-shaped reticulation.  Both pairs of coxal setae (C1 and C2) short, thin and equal in length; lateral setae (C3) slightly shorter than medial setae (C4) (Fig. 2). Transverse muscle attachment scar on coxa III present anteriorly to setae Pe. Urstigma relatively large and oval. Excretory pore plate large and broad (L/W ratio 0.32-0.40), slightly convex or nearly straight anteriorly, with muscle attachment scars anteriorly; excretory pore located posterior to bases of setae Ae. Anal setae (Ai, Ae) reduced and represent in form of alveoli only, distance between setae Ai-Ai shorter than distance between setae Ae-Ae. Setae Pi and Pe short, thin and equal in length. Setae Ci very long, inserted on long projections. Surface of coxal plates with elongate cell-shaped reticulation.
Capitulum (Fig. 3) elongate, with relatively wide base and narrow rostrum, anterior hypostomal setae much longer than posterior ones.
Basal segments of chelicerae fused to each other medially (Fig. 4), with straight or slightly convex lateral margins. Chela relatively small and crescent-shaped, with two small subapical teeth (Fig. 5).
Pedipalps stout (Fig. 6): P-1 very short, without setae; P-2 relatively large, with one single short dorsodistal seta; P-3 with a very long, thick lateroproximal seta and a short dorsodistal seta; P-4 with long lateral and short dorsal seta and a massive dorsodistal claw; P-5 small, with one rather long solenidion and six unequal simple setae, one of them very short.