Gnathosoma, Krefft, 1871

Abé, Hiroshi, 2021, Taxonomic review of the subfamily Lohmannellinae (Acari: Trombidiformes Halacaridae) with morphological comparisons among genera, Zootaxa 4980 (2), pp. 201-255: 242-243

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Form of rostrum

The outline of the rostrum is stable at the generic level, and that is nearly triangular or parallel-sided in the majority of halacarids. In addition to the typical rostrum, some peculiar forms are also found in a few genera. Outside of Lohmannellinae  , Australacarus  and Colobocerasides  have a styliform rostrum, and Halixodes  and Parhalixodes  have a rostrum with terminal barbs.

In Lohmannellinae  , Lohmannella  , Porolohmannella  , Scaptognathides  , and Xenohalacarus  have a parallelsided elongate rostrum. Scaptognathus  exclusively has a spatulate rostrum, which is a unique characteristic in halacarids and is considered a derived state. The form of the rostrum seems to relate its length relative to the gnathosomal base. In most halacarid genera, the rostrum is approximately the same length as the gnathosomal base or shorter than the base. In such a case, nearly triangular rostrum can be often seen. On the other hand, when the rostrum is much longer than the gnathosomal base, parallel-sided rostrum may be prone to occur, as can be found typically in Lohmannella  , Porolohmannella  , Xenohalacarus  , and Pseudosoldanellonyx  .

Number and position of rostral setae

Some setae are always found on the rostrum and/or gnathosomal base. Although, as exceptions, more than five pairs of setae exist on the gnathosoma  in some species of Copidognathus  and Thalassarachna  , halacarids generally have a maximum of four pairs of rostral setae, two of which are placed on the terminal tip of the rostrum, and the others on the proximal part of the rostrum or gnathosomal base. Generally, at least one pair of rostral setae is placed on the gnathosomal base, except for some genera that have all rostral setae only on the rostrum. Outside of Lohmannellinae  , such a case is found in Anomalohalacarus, Arenihalacarus  , Camactognathus, Halacaroides  , Halacarus  , Mictognathus  , Halacarellus  , Acarochelopodia  , Actacarus  , Rhombognathus  , Rhombognathides  , Isobactrus  , Metarhombognathus  , and Corallihalacarus  .

In Lohmannellinae  ,a pair of rostral setae is placed on the gnathosomal base in Porolohmannella  , Scaptognathides  , and Xenohalacarus  . Alternatively, rostral setae are absent from the gnathosomal base in Lohmannella  and Scaptognathus  .

Number of palpal segments

The original function and construction of the palpi resemble the walking legs and the articulations probably correspond to joints of the leg segments ( Van der Hammen 1989). Typically, the palpi are simple sensory appendages equipped with a variety of setae that assist in locating food, and a modification of the palpal structure is sometimes observed in Acari ( Krantz 2009)  . The ancestral number of actinotrichid palpal segments is five, but the number of palpal segments is reduced in many actinotrichid species ( Van der Hammen 1989). A great number of halacarids have four free segmented palpi, and the number of segments is unchanged within a genus. Outside of Lohmannellinae  , the diminution of one or two palpal segments is found in Atelopsalis, Coloboceras  , Colobocerasides  , Rhombognathides  , Metarhombognathus  , Simognathus  , Acaromantis  , and Enterohalacarus  . As an extreme case, the palpi are completely absent in Spongihalacarus  .

In Lohmannellinae  , Scaptognathides  and Scaptognathus  have only two palpal segments as the result of a fusion of P-2, P-3, and P-4, and the trace of a boundary between P-2 and P-3 can be recognized on rare occasions. A similar reduction is recognized in Acaromantis  . As noted by Van der Hammen (1989), the reduction of the number of palpal segments is considered an advanced state in halacarids.

Form of chelicera

Although ancestrally, the actinotrichid chelicera is composed of three segments ( Van der Hammen 1989), the chelicera of Actinotrichida has a bipartite structure in consequence of the lack of a distinct basal article due to a reduction ( Evans 1992). The chelicera is considerably divergent in form, and its modifications can be seen in the reduction of the fixed digit and the development of the movable digit into a sickle-, hook-, or stylet-like structure ( Cunliffe 1954; Woolley 1961; Evans 1992). In halacarids, the chelicera generally consists of the base, stem, and two digits, of which the fixed digit is reduced to a membranous structure and the movable digit is furnished with some minute denticles on its dorsal edge. The form of the chelicera is also related to the form and length of the rostrum, and the majority of halacarids have a nearly sickle- or hook-like chelicera; however, some conspicuous modifications are found in some genera.

In Lohmannellinae  , the elongate sickle- or hook-like chelicera is found in Lohmannella  , Porolohmannella  , Scaptognathides  , and Xenohalacarus  . On the other hand, a long, styliform chelicera is typical in Scaptognathus  . This unique cheliceral form will be modified from the common sickle- or hook-like chelicera.