Two different forms of cryptic species-complexes in mites of the Histiostomatidae (Astigmata) from bank mud and bark beetle-galleries and their significance for applied biodiversity research
The most distinct morphological character on the dorsal female is the unique big and bulging copulation opening (Figs. 3 C, 2 C), which was clearly depicted in Scheucher’s redescription. But she did not describe the pattern and arrangement of cuticula fields of the anterior stronger sclerotized proterosoma-shield in females (exists also in males, but I had not enough close-ups for detailed studies). It resembles very much the pattern of fields in the shield of Histiostoma palustre (Wirth, 2003 and Wirth 2004). I here adopted my nomenclature, which I introduced in these papers, where fields were named according to turtle shells: M (Marginalia), L (Lateralia) and (German) Z (Zentralia).
Like H. palustre, H. maritimum (3 female specimens studied) possesses 7 paired Marginalia, 4 paired Lateralia and 3 unpaired Zentralia (Fig. 4 B). Differences to H. palustre: M2 and L3 are not distinctly elongated to anterior, but with a similar shape than in the other fields. Z3 appeared in some specimens subdivided into two parts and thus looking (secondarily?) paired, one specimen showed a „normal“ unpaired Z3. H. maritimum has as unique feature laterally to the Marginalia 2 extra fields, without a given extra nomenclature. The patterns of cuticula fields on the proterosoma shield in females can support systematic reconstructions (Wirth, 2004). The proterosoma shield of H. litorale is only once photographed via SEM and is partly covered with dirt (unpublished, photograph in own collection). Arrangements are similar to H. maritimum and H. litorale, but shapes are generally different, it seemingly lacks L4. Z2 and Z3 are only hardly visible and appear more like a smooth area. The shield of H. n. sp. is not documented yet. The shield of the phylogenetically related H. feroniarum differs remarkably from all above mentioned with less existing symmetrical fields. I assumed less fields being a primeval character state (Wirth, 2004).
Scheucher (1957) presented no drawing of the dorsal side of the male of H. maritimum. She instead described the dorsum in words, but couldn’t see all complexity of structures. The dorsal male is here for the first time better described: The proterosoma shield extends posteriorly until reaching almost the border line of the hysterosoma (Figs. 5 A, B). It consists of uncountable many cuticula fields with unknown symmetrical arrangement. Dorsal setae sc i and sc e are located there in a typical histiostomatid arrangement and shape with both setae sc e laying posteriorly in a wider distance to each other than setae sc i. The elongated posterior shield is almost heart-shaped, but with a missing „tip“ of an imaginary heart symbol, and the two posterior „heart shanks“ being somewhat bent apart from each other (Fig. 5 B).
The dorsal hysterosoma is entirely covered by angular fields in uncountable numbers without a recognizable symmetrical arrangement, which was already described by Scheucher (1957). She additionally mentioned two smooth areas in a posterior position. I reconstructed the dorsal morphology based on two SEM preparations of males: there are three paired and one unpaired sharp outlined smooth areas, which I named s. a. 1 to s. a. 4 (Figs. 5 A, C). They are correlated with specific dorsal setae : s. a. 1 (unpaired) with both setae d1, all others paired: s. a. 2 with setae c1 to c3, s. a. 3 a with seta cp, s. a. 3 b with setae d2 and e2, s. a. 4 with seta e1.
The smooth area s. a. 3 is subdivided into two parts (a and b). The shape and arrangement of these areas are most characteristic for the male of H. maritimum. Part a with seta cp lays on the lateral edge of the hysterosoma, where it anteriorly and posteriorly bulges medianly, thus shaping a clearly visible „[“ on the left and „]“ on the right site. Part b with setae d2 and e2 is connected with the posterior bulge of part a and is shaped like a droplet, which is slightly turned medianly. Laterally of the droplet and close to seta e2 is the well visible opening of the opisthonotal gland located. A narrow seam of smooth area is additionally represented by the anterior edge of the hysterosoma outline. It remained without a separate nomenclature and it is anteriorly and posteriorly connected to area s. a. 2 (Figs. 5 A,B, C) .
Cupules are not visible on both body sites, seemingly being hidden in the honeycomb-shaped cover of dorsal cuticular fields.
Males being covered like honeycombs with angular fields, shaped by the dorsal cuticle, appear sometimes in species of different histiostomatids clades. In the closer relationship of H. maritimum, such a male morphology is rare. From the three species (in H. sp. unknown) of the wetland biotop of the former gravel pit area in Berlin, only H. litorale males show „honeycombs“, but being only hardly visible and difficult to describe from my SEM preparations with artefacts.