Hydrachna sp.

Arjomandi, Elham, Zawal, Andrzej, Hajiqanbar, Hamidreza, Filip, Ewa & Szenejko, Magdalena, 2019, New record of a parasitising species of Hydrachna (Acari, Hydrachnidia) on water beetles Eretes griseus (Fabricius, 1781) (Coleoptera, Dytiscidae, Dytiscinae, Eretini), ZooKeys 865, pp. 31-38 : 32-34

Hydrachna sp. Figs 3-10 , 11

Description.

The idiosoma are oval, with the integument striated, and the dorsal plate is very large, covering the whole idiosoma of unengorged larva, the integument pointed and with a concave anterior edge ( Figs 3 , 11 ). There are four pairs of setae on the dorsal plate (Mp1, Lp1, Lp2, Hu). The basal bodies of Mp2 on dorsal plate invisible; setae Mh1, Mh2, Mh3 located on soft integument ( Fig. 4 ). There are three pairs of coxal plates located on the proximal half of the idiosoma, and all are wider than long. Median edges of coxa I and III almost the same length and two time longer then coxa II. The anterior coxa bears two setae, the medial coxa is without seta, and the posterior coxa has one seta. The excretory pore plate is very large and is located behind of coxal plates ( Figs 5 , 11 ). Gnathosoma short, strongly tapering forward; gnathosomal sucker large, discoid with corrugated borders ( Figs 5 , 6 ). Pedipalps relatively short and thin: femur stocky with strongly convex ventral margin and one seta; genu with two setae and concave ventral margin; tibiotarsus relatively long with two claws the same size, weakly bent, five tibiotarsal spines, four of them pinnate ( Fig. 7 ). Trochanters of all legs with one seta, all femora with four setae and with one swimming seta on I and II and two swimming setae on III femora. Genu I with five setae including two swimming setae, genu II and III with four setae including one swimming seta. All tibiae with five setae including one swimming seta, and with one solenidium. Tarsi each have 14 setae including two swimming setae, tarsi I and II have one solenidium, and tarsi I and III have one eupathidium ( Figs 8-10 ).

Measurements.

In µm, n = 3. Dorsal plate: L/W 250 –254/162– 157; coxal plates: Cx-1 L 40-45, Cx-2 L 20-22, Cx-3 42-44; excretory pore plate L/W 17 –18/16– 17; gnathosoma; L/W 173 –176/138– 140; diameter of sucker ring 71-73; pedipalpal segments (P-1-3) L: 8-9, 36-38, 39-42; leg segments L: I-leg 1-5: 18-19, 37-39, 32-34, 38-40, 67-69; II-leg 1-5: 20-21, 32-34, 29-30, 37-39, 68-70; III-leg 1-5: 28-29, 29-30, 27-28, 38-40, 61-64.

Remarks.

The larva of Hydrachna sp. is most similar to larvae of H. processifera described by Wainstein (1980) as a H. inermis ( Aykut et al. 2018). It is similar in the shape of coxal plates, the discoidal hypostomal sucker, the tibiotarsus relatively long with two claws the same size, weakly bent; five tibiotarsal spines the same size. It is different by the presence of a eupathidium on tarsus leg-2; localisation the Mh1, Mh2, and Mh3 setae outside of the dorsal plate on soft integument, and the presence of a very large excretory pore plate. The last two features are very strange and different from all other species of Hydrachna . These differences indicate the probability of a separate subgenus to which the described larva would belong.

Thor (1916) split the genus Hydrachna into five subgenera: Hydrachna s. str., Anahydrachna , Diplohydrachna , Schizohydrachna , and Monochydrachna ; subsequently he synonymised Monochydrachna with Hydrachna s. str., and Schizohydrachna with Diplohydrachna , and established two more subgenera: Rhabdohydrachna and Scutochydrachna ( Thor 1925). Davids et al. (2007) stated the differences between these subgenera were not clear and he abolished the division into subgenera.

At the current level of research, we propose to leave the taxonomy of the genus Hydrachna without sub-division, indicating the existence of greater morphological differentiation. Relationships within the genus of Hydrachna should be recognised on the basis of molecular studies and a decision on the possible splitting the genus into subgenera should be made. Up to now six species of Hydrachna were recorded from Iran ( H. cruenta , H. skorikowi , H. sepasgozariani , H. cf. v aillanti, H. sistanica , H. globosa lacerata), and two of them ( H. sepasgozariani , H. cf. v aillanti) belong to the Hydrachna processifera group of species ( Pešić and Saboori 2007; Pešić et al. 2012, 2014). Larvae were described only for H. cruenta , H. skorikowi , and H. globosa ( Wainstein 1980). The morphology of this larva and its parasitism on Dytiscidae show plausible grounds for it belonging to the H. processifera group of species and possibly to one of the two species from Iran ( H. sepasgozariani or H. cf. v aillanti) for which the larvae are still not described. On the other hand, the differences in morphology (localisation the Mh1, Mh2, Mh3 setae outside of dorsal plate, on soft integument and very large excretory pore plate) indicate that it could belong to another species.

Figures

Figure 11. Photographs of Hydrachna sp. from Eretes griseus A dorsal plate B ventral side C dorsal view D pedipalp.

Figures 3 - 10. Hydrachna sp. 3 dorsal plate 4 ventral side 5 gnathosoma ventral side 6 gnathosoma dorsal side 7 pedipalp 8 I-leg- 5 9 II-leg- 5 10 III-leg- 5.