Megisthanus Thorell 1882
Creators: name: Seeman, Owen D.
Publisher: name: Zenodo
Year: 2019
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Arachnida
Mesostigmata
Megisthanidae
Megisthanus

description: Megisthanus Thorell, 1882
Megisthanus Thorell, 1882: 48.
Type species: Megisthanus caudatus Thorell, 1882, by original designation.
Diagnosis. As per Seeman (2017).
Description. Female. Colour orange-brown or dark brown; large, idiosomal length 1.9–3.75 mm.
Dorsum. Dorsal shield ovate (most species; Fig. 1A), strongly tapered posteriorly (M. caudatus) or lyriform (M. jacobsoni Warburton, 1926, M. leviathanicus, M. manonae sp. nov.; Fig. 6A); secondary sclerotisation absent (Fig. 1B) or present (Fig. 1A), added progressively during adult stage; hypertrichous, sometimes heterogeneous in setal form and length (Fig. 11). Soft cuticle with numerous setae, which may be on small platelets. Lyrifissures absent or unapparent. Sigillae often in four paired groups (anteromedially two pairs; medially one pair K-shaped sigillae; posteromedially one pair) and one unpaired sigillum posteriorly.
Venter. Tritosternal base elongate, just over twice as long as wide, laciniae pilose, at least twice as long as base (Fig. 1C). Presternal region membranous, paired presternal shields reduced to anterior, weakly sclerotised straplike shields; setae st1 flanking tritosternum, positioned on more sclerotised, medial margin of each presternal shield. Sternal shield with 2–3 pairs of setae (st4 present or absent/displaced posteriorly to hypertrichous region of shield); fused to endopodal and ventral shield laterad sternogynal shields (Fig. 3A).
Genital opening with strongly thickened margin laterally and posteriorly. Sternogynal shields separate or weakly fused posteriorly, with strongly thickened posterior margin, each shield bearing one pair of lyrifissures and zero to seven setae; shields triangular (Fig. 3A) or medial margins eroded, forming C-shaped shields (Fig. 17A). Internal genitalia variable between species, but always comprising pyriform, free vaginal heads and a V- or Ushaped structure with medial porose areas (possibly derived from vaginal arms, which are fused posteromedially; Fig. 3C). Other elements possibly derived from external genitalic elements: a small medial mesogynal shield usually present, fused anteriorly with thickened posterior margin of genital opening and sometimes fused with vaginal arms posteriorly (Fig. 3C); latigynal element usually highly reduced or absent, but sometimes well developed (e.g., M. leviathanicus, M. manonae sp. nov.; Fig. 7B). Membranous sac extends posteriorly from genital opening (Fig. 1D).
Ventral shield long, narrows medially at level of coxa IV, somewhat expanded posteriorly; with several pairs of setae around shield margin, usually with additional setae within shield margin (Fig. 13A). Endopodal shield free from ventral shield and peritrematal-metapodal-exopodal shield. Ventrianal shield (Fig. 13C) free from ventral shield, rounded, subtrapezoid to subtriangular, posterior margin straight or curved; bearing several pairs of setae and pores; lyrifissures appear absent, possibly difficult to distinguish from other pores. Opisthogastric and lateral soft cuticle with several setae on small platelets.
Peritreme extends to coxa I (Fig. 1C); peritrematal shield broad, connected to dorsal shield anteriorly, extending posteriorly into metapodal region and fused with endopodal shields; anterior margin of peritrematal shield with three pairs of long barbed setae.
Gnathosoma. Gnathotectum triangular, reticulate, with strong keel. Subcapitulum with h1, h2, h3, and palpcoxal setae in usual arrangement (Fig. 4A). Subcapitular gutter with five rows of rudimentary fine denticles; anterior subcapitular gutter (between setae h1 and h3) flanked by longitudinal row of tiny denticles. Five processes discernible from tip of hypostome: labrum with distal part sparsely barbed, terminating in forked tip; paired hypopharyngeal processes with fringed edges; and paired broad hypostomal processes, hyaline proximally, each with medial bushy extension. Corniculi large, often with medial tooth (Fig. 4A). Palp with usual setation for Antennophorina (Evans 1963b); trochanter with ventro-distal spur, femur with tiny distal spur.
Chelicerae. Large, heavily sclerotised, denticulate, with numerous lobes and excrescences as shown in Seeman (2017). Fixed digit with 10–17 teeth; cheliceral seta scale-like, hyaline; arthrodial process a proximal lobe and distal corona of cilia; adaxial surface with several smooth lobes proximally, and several fringed lobes dorsoproximally, fringed lobes coalesce distally into a ciliate membranous process that extends just past cheliceral tip; dorsally, ciliate process overlapped by lobe of fixed digit; pilus dentilis not apparent (possibly extraordinarily modified into ciliate process). Movable digit with 8–13 teeth; proximal part with small ventral fringe of cilia; medial part with three long dendritic excrescences (Fig. 16D); one complex, folded fringed excrescence arises from base of distal dendritic excrescence, extending past tip of movable digit.
Legs. Leg I without claws or ambulacrum. Setal counts for legs I-IV (coxa to tarsus): 2–6– 12–12–13–42, 2–5– 10–12–10 –19; 2–5–7–10 –9–19; 1–5–8–10 –10–21. Femora I–IV (2, 2/2, 2/2, 2), (2, 2/1, 2/2, 1), (1, 2/1, 2/1, 0), (1, 3/1, 2/1, 0); genua I–IV (1, 3/2, 3/1, 2), (2, 3/1, 3/1, 2), (2, 3/1, 2/1, 1), (2, 3/1, 3/0, 1); tibia I–IV (3, 3/2, 1/2, 2), (2, 2/1, 2/1, 2), (2, 2/1, 2/1, 1), (2, 2/2, 2/1, 1). Setae not highly modified: several barbed and/or thickened, especially dorsal setae on femur II–IV; and several dorsal setae much longer than others. Ambulacra II–IV with slender setalike paradactyli. Coxa I with two posterolateral distal spines; trochanter I with small distal spine; trochanter II with large distal spine; femur II with small ventral, distal spur; femur III often with distal spur; trochanter IV with large distal spine; femur IV with three ventro-distal spurs and with (Fig. 17D) or without (Fig. 3E) dorsal projections bearing setae pd2; setae ad2 on small or rudimentary projection; seta ad3 on small to large proximal projection.
Male (major). Similar in size to female or sometimes much larger than female. Idiosomal length 1.70–3.95 mm, body length (including gnathosoma, to tip of gnathotectum) reaching 5.05 mm. Dorsum. Dorsal shield ovate and appearing bare (setae greatly reduced), sometimes with short setae near margins; secondary sclerotisation similar to female. Soft cuticle around shield with small setae. Sigillae similar to female or more extensive, with two anterior pairs, and mid-lateral and mid-posterolateral sigillae merged, plus unpaired caudal sigillum.
Venter. Presternal area similar to female. Genital opening circular, positioned between coxae III (Fig. 3F), surrounded by thickened ring of cuticle that often (possibly always) bears several pores. Sternoventral shield with setae st2–3 anteriorly, setae st4, if present, often difficult to discern among other setae; setation often hypertrichous, especially around genital opening; sternoventral shield with several loosely paired setae around posterolateral margin (Fig. 2B); posteriorly with two discs of porose cuticle bearing an anterior unmodified pair of setae and a highly modified pair of setae centrally: these setae weakly sclerotised, broad, appearing hollow, with flared tip (Fig. 17F). Endopodal shield fused with sternal part of shield just anterior to setae st3, free from ventral shield (Fig. 3F). Ventrianal shield similar to female but sometimes larger. Peritreme and peritrematal shield similar to female.
Gnathosoma. Similar to female, but more robust, hypostomal setae often with fewer barbs or smooth (Fig. 15B). Corniculi larger than female, often lacking medial tooth (Fig. 15B). Palp often with larger spurs than female, sometimes spur on palp genu.
Legs. Similar to female but spurs and spines sometimes larger.
Male (minor): Dorsal shield ovate or, if female with lyriform shield, then shield narrows slightly; secondary sclerotisation present or absent. Dorsum hypertrichous, with setal form similar to that of female (Fig. 2C). Venter similar to major male, but often smaller structures, especially porose discs on ventrianal shield. Gnathosoma similar to major male, except corniculi always retains medial tooth as in female. Legs similar to male.
Remarks. The above description is based mostly on the Australian species, but also Megisthanus floridanus Banks, 1904 and Megisthanus papuanus Womersley, 1937, for which specimens were examined. Information from published descriptions was also referred to, but only those that have sufficient and reliable detail in their descriptions. These are: Megisthanus afer Stoll, 1897; Megisthanus berlesei Hunter & Costa, 1970; Megisthanus caudatus Thorell, 1882; Megisthanus doreianus Thorell, 1882; Megisthanus gigantodes Stoll, 1893; Megisthanus hatamensis Thorell, 1882; Megisthanus jacobsoni Warburton, 1926; Megisthanus moaifensus Oudemans, 1905; Megisthanus obtusus Kramer, 1894; Megisthanus orientalis Oudemans, 1905; Megisthanus postsetosus Karg, 1997; and Megisthanus remilleti Hunter & Costa, 1970. Although the descriptions of Oudemans (1905) are rudimentary, the illustrations can be found at the Wikimedia Commons for the Oudemans Mite Collection: https:// commons.wikimedia.org/wiki/Category:Oudemans_Mite_Collection_at_Naturalis_Biodiversity_Center
description: Published as part of Seeman, Owen D., 2019, The megisthanid mites (Mesostigmata: Megisthanidae) of Australia, pp. 1-40 in Zootaxa 4563 (1) on pages 2-5, DOI: 10.11646/zootaxa.4563.1.1, http://zenodo.org/record/2600858