Sessiluncus G. Canestrini 1898
Creators: name: Kazemi, Shahrooz
name: Nasr, Abdelrady Korashy
name: Ramadan, Mahmoud Mohamed
name: Klompen, Hans
Publisher: name: Zenodo
Year: 2021
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Arachnida
Mesostigmata
Ologamasidae
Sessiluncus

description: Genus Sessiluncus G. Canestrini, 1898
Sessiluncus G. Canestrini, 1898: 486.
Gamasellus (Sessiluncus).— Berlese 1905: 168.
Sessiluncus.— Vitzthum 1931: 142; 1943: 758; Ryke 1962: 160; Lee 1970: 175; Bregetova 1977: 311; Nasr & Afifi 1986: 17; Castilho et al. 2016: 22.
Type species: Gamasus heterotarsus G. Canestrini, 1897, by monotypy.
Diagnosis (female and male). Holonotal shield free from ventrianal shield, no indication of fusion line between podonotal and opisthonotal shield regions; bearing 35–39 pairs of short to moderately long and usually acicular setae, smooth or slightly pilose, with 21 pairs of pore-like structures, including four pairs of glandular openings, gd1, gd5–gd6, and gd8 or gd9; with one pair of pit-like cuticular structures, “ pcs ”, between setae J1–J2. One pair of presternal plates, free or fused to sternal shield. Ventrianal shield free, capturing metapodal platelet. Peritrematal shield free posteriorly, with two crenate lateral projections at level of coxae II–III (Figures 2, 13, 26, 40). Peritreme straight or sinuous. Anterior margin of gnathotectum subtriangular, denticulate. Male spermatodactyl strongly recurved near the base (Figures 4, 30). Deutosternal row between setae h3 denticulate medially, extending laterally into smooth ridges (Figures 17, 31, 54). Pretarsus I sessile. Femur I with 13 setae; genua I and III with 13 and 8 setae, respectively; genu and tibia IV each with 8–10 setae. Some leg segments in male and female may bear modified setae or cuticular spurs.
Description (adults). Medium size mites (440–810 long). Idiosoma suboval, covered by a holonotal shield, occasionally slightly extending ventrally (a narrow band of dorsolateral soft cuticle present in S. reticulatus, bearing 1–2 pairs of setae posterolaterally in addition to two pairs of setae on ventrolateral soft cuticle), and free from ventrianal shield; shield usually with one pair of small, short and irregular projections anterolaterally, flanking a narrow sclerotised band bordering shield anteriorly, sometimes divided into small platelets/strips (Figures 1, 3, 9, 25, 26, 53). Holonotal shield bearing 35–39 pairs of short to moderately long acicular setae, smooth or slightly pilose, rarely finely flatted near the tip (S. oculatus, Figure 46), setae z1 usually absent (maybe present in S. heterotarsus); with 21 pairs of pore-like structures, comprising 17 pairs of poroids (id1–id2, id4–id6, idm1–idm6, is1, idx, idl1– idl4), and four pairs of gland pores, gd1, gd5–gd6, and gd8 or gd9. One pair of pit-like cuticular structures, “ pcs ”, present between setae J1–J2. Tritosternal base short; laciniae free, pilose. Presternal platelets well sclerotised, subrectangular, flanking base of tritosternum, free or fused to sternal shield. Sternal shield in female large, fused to endopodal elements between coxae I–IV, rarely free from endopodal between coxae III–IV (as in undescribed species from Nepal (Lee 1970) and Iran (Kazemi collection, description in progress); shield fused to continuous exopodal strip behind acetabula I–IV via endopodal element between coxae I–II, bearing gland pore gvb; sternal shield extending posteriorly and internally into the genital chamber as a horse-shoe shaped structure (Figures 28, 39). Male sternal shield usually fused posteriorly to the continuous exopodal strip via parapodal element (Figures 2, 26). Sternal shield of female with four pairs of setae and three pairs of poroids and that of male with five pairs of setae and three pairs of poroids; gland pore gv1 indistinct. Epigynal shield flanked by sternal and ventrianal shields, convex anteriorly and truncate posteriorly, with one pair of setae. Postgenital platelets absent. Ventrianal shield large, free, usually bearing six, rarely five or nine pairs of ventral setae in addition to circumanals; gland pore gv2 distinct or indistinct, gv3 absent (Figures 2, 13, 27). Metapodal platelet fused to ventrianal shield. Soft cuticle laterad ventrianal shield with 0–2 pairs of setae. Peritrematal shield free posteriorly, but anteriorly fused to dorsal shield at level of coxa II; with two crenate lateral projections; poststigmatal region well developed, sometimes extending well behind anterior margin of ventrianal shield, with one poroid and one distinct, large gland pore (Figure 2, 13, 26, 40). Peritreme relatively narrow, straight or sinuous, extending to midlevel of coxa II or up to level of coxa I. Gnathotectum anterior margin usually with a median subtriangular prong flanked by one pair or a few shorter lateral projections (Figures 41–44), occasionally projections less distinct (Figures 29, 53). Corniculus horn-like, simple (Figure 30) or with a lateral prong or tooth submedially (Figure 55). Movable digit of chelicera in female bearing three teeth, fixed digit usually with four teeth, rarely with two, three (?) or five teeth. Movable digit of chelicera in male with two teeth, rarely one, fixed digit with 3–5 teeth. Male spermatodactyl strongly recurved near the base (Figures 4, 30). Deutosternal groove with 8–9 rows, the row between setae h3 narrowly denticulate, extending laterally into smooth ridges, 5–6 posterior rows usually markedly wider; with two smooth or denticulate posterolateral transverse ridges connected to deutosternum groove (Figures 17, 31, 54). Leg I longer than legs II–IV, and not attenuated; legs I and II usually thicker than legs III–IV; pretarsus I sessile, not pedunculate, its base broadly fused to tarsus (Figures 5, 48). Lateral and median lobes of pulvilli on legs I–IV rounded apically, lateral lobes longer than median lobe. Tarsi II–IV each with three small apicoventral projections (two lateral and one median), median one occasionally denticulate apically (Figures 10–12). Setation of femora I–IV 13 (2 3/1, 2/3, 2), 11 (2, 3/2, 2/1, 1), 6 (1, 2/1, 2/0, 0), 6 (1, 2/1, 1/0, 1), those of genua I–IV 13 (2, 3/2, 3/1, 2), 11 (2, 3/1, 2/1, 2) or rarely 12 (2, 3/2, 2/1, 2) [S. reticulatus Loots (1980)], 8 (2, 2/1, 2/0, 1), 9 (2, 2/1, 3/0, 1) or 10 (2, 2/1, 3/0, 2) or rarely 8 (2, 1/1, 3/0, 1) [S. reticulatus Loots (1980)], and those of tibiae I–IV 14 (2, 3/2, 3/2, 2), 10 (2, 2/1, 2/1, 2) or rarely 11 (2, 2/2, 2/1, 2) [S. reticulatus, Loots (1980)], 8 (2, 1/1, 2/1, 1), 9 (2, 1/1, 3/1, 1) or 10 (2, 1/1, 3/1, 2) (e.g. as in S. hungaricus), or rarely 8 (1, 1/1, 3/1, 1) (S. oculatus) or (2, 1/1, 2/1, 1) [S. reticulatus Loots (1980)], respectively. Some leg segments in female (especially trochanter IV) may bear modified setae or cuticular spurs (Figures 23, 35, 49–51). Male with at least one large ventral spur on femur II (modified seta av) and III, seta av on genu and tibia II usually knob-like (Figures 6–8, 37) (more details in Table 1).
The following species (and country of type locality) are considered as members of Sessiluncus: S. abalaae Datta & Bhattacharjee, 1991 (India); S. aegypticus Nasr & Afifi, 1986 (Egypt); S. bengalensis Bhattacharyya, 1977 (India); S. calcuttaensis Bhattacharyya, 1977 (India); S. cavensis Willmann, 1940 (Bosnia and Herzegovina); S. colchicus Bregetova, 1977 (Russia); S. femoralis Bhattacharyya, 1977 (India); S. heterotarsus (G. Canestrini, 1897) [described as Gamasus heterotarsus] (Papua New Guinea); S. hungaricus Karg, 1964 (Hungary); S. indicus Bhattacharyya, 1977 (India); S. leei Datta & Bhattacharjee, 1991 (India); S. oculatus Vitzthum, 1935 (Tahiti, French Polynesia); S. reticulatus Loots, 1980 (Seychelles); S. yongchunensis Ma & Zhang, 2013 (China).
Remarks on the genus. The genus Sessiluncus can be easily distinguished from all other ologamasid genera by a combination of characters described in the genus diagnosis above, especially the sessile pretarsus I (ambulacral stalk almost absent) that is broadly fused to the tarsus basally, while the pretarsus I is pedunculate (ambulacral stalk well developed) in most other ologamasid genera (when ambulacrum is present), except in a few species described in Gamasellevans (Loots & Ryke 1967). Additionally, the peritrematal shield in all examined species of Sessiluncus has two lateral crenate projections that are not present or not described in other ologamasid genera (partially present in a Rykellus sp.). Also, if the structure of the deutosternal row between setae h3 (narrowly denticulate medially and extending into smooth ridges laterally) is not unique within the family (based on the published data) it is certainly rare (again, partially present in the same Rykellus sp.).
We recognise 14 valid species in the genus Sessiluncus. Another species that was previously considered as a member of the genus is S. holostaspoides (G. Canestrini, 1884) (e.g. by Bregetova 1977; Castilho et al. 2016; Beaulieu et al. 2019). Laelaps holostaspoides was described by Canestrini in 1884, but without illustrations. Berlese (1892) redescribed L. holostaspoides, including information almost identical to that provided by Canestrini (1884), but he added an illustration of the venter of this species. Berlese’s (1892) illustration shows a pedunculate (not sessile) ambulacrum I. A sessile pretarsus is the main diagnostic feature of Sessiluncus. Therefore, we excluded this species from Sessiluncus.
Seta z1 appears to be absent in most of the known species of Sessiluncus¸ but it is listed in the original descriptions of S. hungaricus and S. aegypticus. Notably, based on examination of the holotype of S. hungaricus, seta z1 is absent in this species (pers. comm. of the senior author (SK) with Jason Dunlop, Curator of the collections Arachnida and Myriapoda in Berlin Museum). Examination of some specimens assigned to S. hungaricus collected in Iran confirms Dunlop’s observation. This seta was also absent in all examined specimens of S. aegypticus. Seta z1 was labeled (and symbolised by a circle) on the left side of the illustration of the dorsal shield of S. heterotarsus in Lee (1970). However, the right side of the illustrated shield does not show any seta on the position of z1, suggesting that z1 may be absent on that species too. Lee (1970), in the description of the genus, wrote that the z -series has six pairs of setae (z1 present), an idea that was probably based on the original description of S. hungaricus (Karg 1964) and his own redescription of S. heterotarsus.
The most recent work on the family Ologamasidae is a catalogue presented by Castilho et al. (2016). In their diagnosis of the genus Sessiluncus, they stated that: (1) sternal setae st1–st4 aligned longitudinally; but they are not in a longitudinal line in most Sessiluncus species. This is most conspicuous in S. abalaae, S. aegypticus, S. heterotarsus, S. hungaricus, S. indicus and S. oculatus, with the distance between the st1–st1 and st3–st3 pairs markedly shorter than the distance between st2–st2, and (usually) st4–st4 (distance between st3–st3 and st4–st4 is subequal in S. aegypticus); (2) “exopodal shields near coxae II–III–III divided at median level of coxa III”; but they are fused in all species into a continuous strip and also fused to the parapodal plate. They also noted that the sternal shield is fused to the endopodal platelets near coxa IV; that is correct for the described species of the genus. However, Lee (1970) stated that free endopodal platelets mediad coxae III–IV were present in “an unnamed species from Nepal, dep. BM (NH)”. The same situation was observed in an undescribed species of Sessiluncus collected from western Iran (SK, in prep.). Also, Castilho et al. (2016) did not present any information about modified leg setae or cuticular spurs, the absence of z1, the lateral crenate projections in the outer margin of the peritrematal shield, the pit-like cuticular structures between setae J1–J2, the male recurved spermatodactyl, and variation in the setation on some of the leg segments. Our revised diagnosis above may therefore be useful.
Relationship of the genera of Ologamasidae are highly unstable. Lee (1970) described several subfamilies under Rhodacaridae s.l., including Sessiluncinae as a new subfamily comprising nine genera. On the other hand, Antony (1986) placed Sessiluncus in Gamasiphinae in addition to eight other genera. The two classifications show very little overlap, except for placing Gamasellevans Loots & Ryke, 1967 close to Sessiluncus. Other genera that are similar to Sessiluncus are Gamasellopsis Loots & Ryke, 1966 and Antennolaelaps Womersley, 1956, both placed in Sessiluncinae by Lee (1970).
The species placed in Gamasellevans may not form a monophyletic group, with different species showing distinctly different patterns of fusion of the ventral shields and variability in sessile vs. pedunculate pretarsi I. However, all Gamasellevans share a few characters that are different from Sessiluncus: (1) female epigynal shield bearing a subtriangular hyaline margin extending over the sternal shield to the level of setae st1 or st2 (vs. anterior hyaline margin in Sessiluncus narrow, never extending to the level of setae st3); (2) arthrodial process of male chelicera modified to a broad and long membranous process (as long as movable digit or longer) tapering at the end, with fine bristles on the surface (vs. similar structure absent in male chelicera of Sessiluncus, arthrodial process plumose or a small rounded hyaline process bearing short extensions); (3) peritrematal shield barely developed, poststigmatal region of the shield very short (vs. peritrematal shield well developed in Sessiluncus, poststigmatal region of the shield also well developed); (4) exopodal IV well developed, free or fused to ventrianal shield, and capturing the large poststigmatal gland pore (vs. exopodal IV may or may not be developed, free from ventrianal shield, and enlarged poststigmatic gland pore inserted on post-stigmatal region of peritrematal shield in Sessiluncus); and (5) spermatodactyl originating from distal third of the movable digit as a curved or coiled structure, but never strongly recurved (vs. spermadactyl originating on movable digit medially, strongly recurved near the base, then extending straight in Sessiluncus). Although Loots & Ryke (1967) mentioned the fusion line of the podonotal and opisthonotal shield as a generic character for Gamasellevans (also used in Castilho et al.’s (2016) key to the genera), the line is not complete or distinct in some species described by Loots & Ryke (1967), such as in G. spermadactylus Loots & Ryke, 1967, G. evansi Loots & Ryke, 1967 and G. reticulatus Loots & Ryke, 1967. In fact, Loots & Ryke (1967) used the presence/absence of the fusion line on the holonotal shield in their species key to separate G. spermadactylus from G. epigynalis Loots & Ryke, 1967.
The genera Sessiluncus and Gamasellopsis are quite similar. Both have separate holonotal and ventrianal shields, and the general shape of the ventral idiosomal shields in these mites is largely identical. However, members of Sessiluncus can be distinguished from Gamasellopsis based on the following characters: (1) ambulacrum I sessile (vs. ambulacrum I pedunculate in Gamasellopsis); (2) genu I with 13 setae, including av1–av2 (vs. 12 setae (av2 absent) in Gamasellopsis); (3) male spermatodactyl strongly recurved (vs. gently curved apically in Gamasellopsis); and (4) poststigmatic section of the peritrematal shield free (vs. relatively broadly abutting exopodal strips laterad acetabula IV in Gamasellopsis) (Loots & Ryke 1966). Loots & Ryke (1966) described Gamasellopsis to accommodate four new species, all of them from Africa. Later, Petrova & Tascaeva (1968) described Gamasellopsis puchus from specimens collected in China. Ma & Zhang (2013) transferred it to the genus Sessiluncus, but we question this because the poststigmatic region of the peritrematal shield in G. puchus is fused to the posterior part of the exopodal strip behind coxa IV. Unfortunately, Petrova & Tascaeva (1968) did not consider pretarsus I: sessile, as in Sessiluncus, or pedunculate, as in Gamasellopsis. Castilho et al. (2016) considered Gamasellopsis puchus as incertae sedis because of the following features: “it has separate pre-sternal plates, it has two pairs of setae in the opisthogastric soft cuticle rather than one, and it lacks separate endopodal plates III–IV. Its leg chaetotaxy is undescribed. All the described species of Gamasellopsis are from South Africa ”. We agree with Castilho et al. (2016) that the morphology of G. puchus was insufficiently described by Petrova & Tascaeva (1968), but disagree with their reasons to exclude it from Gamasellopsis, because: (1) presternal platelets can be either fused or free in some ologamasid genera, including the genus Sessiluncus; (2) based on Petrova & Tascaeva (1968, figure 6-1), there is only one pair of setae on the opisthogastric soft integument in the species, not two (another pair are located on the lateral margins of the dorsal shield extending ventrally), just like all African species of Gamasellopsis; (3) the endopodal platelets between coxae III–IV in females of two species of the African Gamasellopsis are free from the sternal shield (G. curtipilus Loots & Ryke, 1966 and G. longipilus Loots & Ryke, 1966), but they are fused to the sternal shield in two others (G. magoebaensis Loots & Ryke, 1966 and G. vandenbergi Loots & Ryle, 1966). Consequently, we follow Petrova & Tascaeva’s (1968) opinion about the species placement under the genus Gamasellopsis. However, description of the legs of G. puchus, especially the shape of the pretarsus I, would be useful to strengthen its taxonomic position.
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Antennolaelaps also resembles Sessiluncus by sharing features such as podonotal and opisthonotal shields completely fused, with no visible fusion line, ventrianal shield free posteriorly, sternal shield fused to all endopodal elements, all exopodal elements fused as a continuous strip and joined to parapodal platelet, metapodal platelets incorporated into ventrianal shield, and peritrematal shield free from the exopodal strip. However, in addition to the different shape of the pretarsus I, sessile in Sessiluncus and pedunculate in Antennolaelaps (if present), these two genera can be separated by the following characters: (1) peritreme with deeply crenate outer margin (vs. almost smooth margin in Sessiluncus); (2) genu III with nine setae, including two ventral setae (vs. 8 setae in Sessiluncus, including only one ventral seta); (3) male spermatodactyl gently curved anteriorly (vs. strongly recurved in Sessiluncus); (4) usually a small narrow, free platelet bearing the cribral spicules present behind ventrianal shield in Antennolaelaps (vs. similar platelet absent in Sessiluncus); and (5) a pair of crenate projections on the outer margin of the peritrematal shield at level of coxae II–III which are present in Sessiluncus species, but appear absent in Antennolaelaps. Presumably, based on the similarity of these genera, Domrow (1957) illustrated a specimen as S. heterotarsus, although he was aware that his specimens differ “in the armature of leg II and in pretarsus I from the illustrations of S. heterotarsus of Vitzthum (1926) ”. Later, Lee (1970) transferred S. heterotarsus sensu Domrow (1957) to the genus Antennolaelaps noting that it might be conspecific with A. testudo Lee, 1970, a new species of Antennolaelaps described in his paper.
description: Published as part of Kazemi, Shahrooz, Nasr, Abdelrady Korashy, Ramadan, Mahmoud Mohamed & Klompen, Hans, 2021, Review of the genus Sessiluncus (Acari: Mesostigmata: Ologamasidae), description of male and redescription of female of Sessiluncus aegypticus, and notes on some morphological characters of the genus, pp. 271-299 in Zootaxa 5061 (2) on pages 273-278, DOI: 10.11646/zootaxa.5061.2.3, http://zenodo.org/record/5649474