Idiosoma, Ausserer, 1871

Abé, Hiroshi, 2021, Taxonomic review of the subfamily Lohmannellinae (Acari: Trombidiformes Halacaridae) with morphological comparisons among genera, Zootaxa 4980 (2), pp. 201-255 : 238-242

Idiosoma

Condition of dorsal plates

Actinotrichida shows a great diversity of body plates, which seem impossible to reduce to a common plan ( Hughes 1959). However, it tends always toward the four dorsal and four ventral plates in halacarids, and this condition may be a primitive pattern. The faintly sclerotized opisthosoma, lacking well-formed tergites and sternites, is considered as an apomorphic condition in Acari ( Lindquist 1984) . As for Anomalohalacarus , Australacarus, Arenihalacarus, Halacaroides , and Parhalixodes , the diminution of the sclerotized area may relate to the body extension along the longitudinal body axis ( Abé 1998). In addition, a secondary reduction owing to the parasitic phase is postulated in Parhalixodes . Among idiosomal plates, the dorsal ones tend to be smaller than the ventral ones, and the ocular plates often get extremely smaller in size, especially in arenicolous taxa.

In Lohmannellinae , the outline and relative size of the ventral plates do not distinctly vary among genera, but those of the dorsal plates are rather diverging. Lohmannella and Porolohmannella do not show a size-reduction of the dorsal plates, instead seeming to hold the basic body plan of halacarids. On the other hand, Scaptognathides , Scaptognathus , and Xenohalacarus have smaller ocular plates, which might reflect adaptation to an interstitial habitat.

Number of dorsal setae

In Halacaridae , the number of dorsal setae is almost stable at the generic level. Bartsch (1992) suggested that a diminution of the number of setae resulted from a simple loss of a setal structure from the body surface, while an increase in setae is caused by the transformation from dorsal pores to setae. The presence of such a transformation is supported by SEM observation ( Abé 1998). Judging from the morphological comparison of halacarid genera, halacarid mites basically have six pairs of dorsal setae arranged according to a formula of one pair on the anterodorsal portion, two on each ocular and posterodorsal portion, and one pair on the adanal portion. However, the particular position of each dorsal seta highly depends on the developmental condition of the dorsal plates and membranous cuticle and greatly varies even within a specific level. Further, the lack or displacement of some setae on the dorsum is found in several taxa, e.g., Isobactrus , Rhombognathus , Mictognathus , Phacacarus , Corallihalacarus , Parhalixodes , Enterohalacarus , Limnohalacarus , and Ropohalacarus .

In Lohmannellinae , Lohmannella , Porolohmannella , Scaptognathides , and Xenohalacarus retain the basic formula of six pairs of dorsal setae, but Scaptognathus has seven pairs. Taking the general arrangement of dorsal setae and pores into consideration, one pair of dorsal pores placed on the ocular or posterodorsal portion will transform into setal structure.

Number of dorsal pores

A thorough examination of the number and characteristic distribution of dorsal pores in Halacaridae has not been carried out because of the difficulty in detecting them due to inconspicuous small size in some taxa. Morphological comparison of a large number of halacarid genera showed that halacarids basically have five pairs of gland pores on the dorsum. The number of dorsal pores is nearly stable in a genus. The first pair is present on the anterodorsal portion, the second pair is on membranous area or ocular portion, the third pair is on the ocular portion, and the fourth and fifth pairs are on the posterodorsal portion. However, the detailed position of the dorsal pores is fairly variable among species, as well as among genera. Wiles (1997) discussed the homology of gland pores (glands and glandularia in Hydrachnidia) among several water mite taxa, and suggested that it was possible to identify homologous gland pores based on the relative location on the body, but some tended to wonder or disappear.

In Lohmannellinae , Lohmannella and Scaptognathides have a basic number of gland pores on the dorsum. Porolohmannella lacks the second pair, and Xenohalacarus lacks the first pair. Scaptognathus has only three pairs of dorsal pores; either the third or the fourth pair from the basic plan may have been transformed into a setal structure, and the other may have degenerated or disappeared.

Number and condition of ventral plates

Generally, the ventral plate of halacarids consists of an anterior epimeral, right and left posterior epimeral, and genital and anal plates or a single genitoanal plate. Genital and anal plates are fused into a genitoanal plate in many halacarid genera. Although an anal plate is generally separated from a genital plate in the initial nymphal stages, these two plates are fused into a single genitoanal plate during ontogenesis. Therefore, the basic number of ventral plates is regarded to be four in adult halacarids ( Abé 1998).

In Lohmannellinae , all five genera have the typical arrangement of four ventral plates. However, the condition of the genitoanal plate in Scaptognathus is quite different from that in the other four genera. The genus Scaptognathus has a uniform, bipartite, or tripartite genitoanal plate consisting of a sclerotized part or a combination of sclerotized and membranous parts, which are referred to as pars sclerosum and pars membranosum, respectively, in Newell (1984). Such a condition of the genitoanal plate is unique in halacarids and considered to be an advanced state.

Number of ventral setae on the coxal area

The number of epimeral setae on the coxal fields is almost stable at the generic level. Halacarids generally have three pairs of setae on the anterior epimeral portion. A reduction of some of these setae is known in Thalassophthirius , Parhalixodes , and Acarochelopodia . Outside the subfamily Lohmannellinae , setal adjunction on this portion is known in Rhombognathus , Agauides , and Acanthohalacarus . On the posterior epimeral portion, about two thirds of the halacarid genera have four pairs of setae. Beyond Lohmannellinae , some additional setae are recognized in Arhodeoporus , Bathyhalacarus , Agaue , Bradyagaue , and Rhombognathus . In contrast, some setal reduction is detectable in Agauides , Australacarus , Colobocerasides , Rhombognathides , Isobactrus , Metarhombognathus , Phacacarus , Acarochelopodia , Parhalixodes , and the majority of freshwater genera.

In Lohmannellinae , Porolohmannella , Scaptognathus , Scaptognathides , and Xenohalacarus have typical chaetotaxy on the anterior epimeral portion. However, a pair of additional setae are present on the posterior margin of the anterior epimeral plate in some species of Lohmannella . Judging from their position, these setae will not be homologous with the adjunctive setae observed in Rhombognathus ( Abé 1998) . As for the number of setae on the posterior epimeral portion, Scaptognathus , Scaptognathides , and the majority of Lohmannella species typically have four pairs of setae. In contrast, Porolohmannella and Xenohalacarus have three pairs of setae in which only one seta, instead of two, is placed on each coxal field of the fourth leg.

Number of perigenital setae

Perigenital setae usually surround the genital foramen, and the number of these setae is usually different between sexes as well as among species. In males, the number of perigenital setae is highly variable at a specific level and frequently overlapping among genera. Therefore, the number of perigenital setae in males is not discussed here. In females, the number of perigenital setae is somewhat variable at a specific level, but the basic number is recognizable on the basis of their arrangement. More than half of the halacarid genera have three or four pairs of perigenital setae in females. Outside the subfamily Lohmannellinae , five or more pairs of perigenital setae are recognized in Halacarus , Agauides , Lobohalacarus , Porohalacarus , Agaue , Bradyagaue , Halixodes , Rhombognathus , Werthelloides , Parasoldanellonyx , Limnohalacarus , and Enterohalacarus . The notable excessive number is recorded in Enterohalacarus , which has more than 150 perigenital setae in females.

In Lohmannellinae , Porolohmannella and Scaptognathus basically have three pairs of perigenital setae, and Xenohalacarus has four or five pairs of perigenital setae in females. An excessive number of perigenital setae are found in Lohmannella , which has 10 to 80 perigenital setae in females. In contrast, females of Scaptognathides have only two pairs of perigenital setae, the smallest number in female halacarids. Considering that the reduction in the number of appendages is a main trend in the evolution of Acari , setal reduction on the genital area can be postulated in Scaptognathides .

Number of subgenital setae

The subgenital setae in females are completely lacking in one third of halacarid genera, and one or two pairs of subgenital setae are present in females of about one third of halacarid genera. Outside the subfamily Lohmannellinae , three or more pairs of subgenital setae are sometimes observed in females of marine genera Agauopsis , Bathyhalacarus , Thalassarachna , Halacarus , and Pelacarus , and freshwater genera Soldanellonyx , Parasoldanellonyx , and Limnohalacarus . Concerning the subgenital setae in males, the majority of halacarids have three or more pairs, and one third of genera have more than five pairs. Outside of Lohmannellinae , only two pairs of subgenital setae are exceptionally present in males of some species of Camactognathus and Actacarus .

In Lohmannellinae , Lohmannella has two to five pairs of subgenital setae in females. Scaptognathus generally has two pairs and exceptionally no or three pairs of subgenital setae in females. Scaptognathides has no or one pair of subgenital setae in females. The subgenital setae are absent in females of Porolohmannella and Xenohalacarus . As for the males, Xenohalacarus has five pairs of subgenital setae. Two to four pairs of subgenital setae are present in Lohmannella , and three pairs of subgenital setae are usually present in Scaptognathus and Scaptognathides . A few species in Scaptognathus have only two pairs of subgenital setae, and one species in Scaptognathides has no subgenital setae in males. Considering the number of subgenital setae in males and females, it can be said that Lohmannella retains a relatively high number of subgenital setae in halacarids.

Position of genital and anal foramens

Ancestrally, the genital opening is at the posterior border of the eighth segment of the chelicerate opisthosoma, and the anal opening is more or less terminally placed ( Van der Hammen 1989). The more ventral position of the anal opening is considered a secondary transformation in which a terminal telescoping of opisthosomal tergites carries this opening from a terminal to a ventral position ( Krantz 2009). The reduction in the number of segments of the idiosoma is a main trend in the evolution of Acari ( Sitnikova 1978) , and a ventrally placed genital region is widely shared among mite taxa and considered a primitive condition. In notostigmatid mites, the large anal opening is terminally situated and this condition is regarded as a retention of the primitive feature ( Evans 1992). In halacarids, the position of genital foramen is almost stable within a genus. All male and the majority of female halacarids have a genital foramen on the ventral surface of idiosoma , however, a few exceptions are found in some females. A terminally placed genital foramen is known in females in Actacarus and Rhombognathides . On the other hand, the anal foramen is located on the terminal end of the idiosoma in the majority of halacarids. Outside of Lohmannellinae , the ventrally situated anal foramen is found in some species in Isobactrus , Pelacarus , Werthella , Halixodes, Halacaropsis , and Enterohalacarus .

In Lohmannellinae , all five genera have a ventrally placed genital foramen, which is regarded as a basic state in halacarids. In contrast, a rather ventrally situated anal foramen is observed in Porolohmannella . This condition is an exceptional state in halacarids and is considered a secondary transformation.