Megisthanus southcotti, Seeman, 2019

Seeman, Owen D., 2019, The megisthanid mites (Mesostigmata: Megisthanidae) of Australia, Zootaxa 4563 (1), pp. 1-40 : 22-27

Megisthanus southcotti sp. nov.

Figures 12–15

Material examined. Australia, Queensland , ex Aulacocyclus fracticornis Kuwert, 1891 : Holotype female, Mossman Gorge , 31.i.1996, O. Seeman, ex beetle #66 (log 1, passalid 1, pr A) ( QMS 109486 ). In QM. Paratypes: 14 females, 7 major males, as follows. 1 female, same data as holotype ( QMS 109488 ) ; 1 female, same data except ex beetle #71 (log 2 passalid 1) ( QMS 109487 ) ; 1 major male, same data except ex beetle #220 (log 1, passalid 2, pr A) ( QMS109489 ) ; 1 female, Wongabel, Atherton , 28.i.1996, O. Seeman, ex beetle #119 (log 1, passalid 3) ( QMS109490 ) ; 1 major male, same data except ex beetle #86 (log 1, passalid 1) ( QMS109491 ) ; 1 female, 1 major male, Gadgarra, Atherton , 27.i.1996, O. Seeman, ex beetle #90 (log 2, passalid 7) ( QMS 109492–3 ) ; 1 major male, same data except ex beetle #92 (log 2, passalid 8) ( QMS 109494 ) ; 1 female, Oliver Ck, Cape Tribulation , 30.i.1996, O. Seeman, ex beetle #17 (log 3, passalid 2) ( QMS 109495 ) ; 1 major male, same data except ex beetle #14 (log 5, passalid 1) ( QMS 109496 ) ; 1 major male, same data except ex beetle #8 (log 2, passalid 1) ( ANIC 51- 006453 ) ; 1 female, Earthwatch Site, Cape Tribulation , 30.i.1996, O. Seeman, ex beetle #51 (log 2, passalid 3) ( ANIC 51-006452 ) ; 3 females, 1 male, 8 miles east Wondecla, 24.x.1943, R.V. coll., ex passalid ( J17309 –12). 2 females, Longlands Gap Rd, near Herberton, 10.i.1950, J.C. Brooks, ex small carabid ( J17307 –8). 1 female, Atherton Tablelands , 28.iii.1945, D.L. Pollier ( J17328 ). Specimens with J-numbers in SAM, others in QM except one paratype female and paratype male in ANIC .

Other material examined, Queensland , ex Austropassalus hultgreni Mjöberg, 1917: 1 female, Mt Lewis Rd, Mt Lewis , 30.i.1996, O. Seeman, (beetle #57, log 1, passalid 2). 1 female, same data, except beetle #68 (log 1, passalid 5).

Diagnosis. Female and male. Sternal setae st4 absent. Seta pd2 of femur IV on rudimentary projection. Dorsal shield finely punctate. Ventrianal shield widest at posterior level of anus. Female. Dorsal shield ovate, length 1.95– 2.05 mm, maximum width 0.85–1.05 mm, not including secondary sclerotisation. Most dorsal shield setae 90–120 in length, with fine but conspicuous barbs; some marginal setae thicker and longer; podonotum and opisthonotum densely setose. Sternogynal shields weakly fused posteriorly, each bearing 4–7 setae; internal genitalia with rudimentary latigynal element. Ventrianal shield a rounded trapezoid. Major male. Broader than female, dorsal shield length 1.95–2.20 mm, width 1.40–1.60 mm; setae on soft cuticle laterad dorsal shield short, length 15–30; 130–160 setae around genital opening; corniculi lacking medial inner tooth; setae h1-h2 smooth, h3 and palp coxal setae smooth or with 1–2 barbs.

Description. Female. Dorsum ( Figs 12A , 13B ). Idiosomal length 2.30 mm (2.20–2.30 mm), width 1.50 mm (1.25–1.50 mm), body length (including gnathosoma , to tip of gnathotectum) 2.80 mm (2.60–2.80 mm). Dorsal shield ovate, length 1.95 mm (1.80–2.05 mm), width 0.95 mm (0.85–1.05 mm); secondary sclerotisation absent or present (including holotype, Fig. 12A ), mostly posterolaterally and posteriorly; maximum length of dorsal shield including secondary sclerotisation 2.20 mm, width 1.10 mm. Dorsal shield hypertrichous, especially mid-dorsally, setation sparser at margins and off dorsal shield. Most dorsal shield setae with several conspicuous but fine barbs, length 80–100; anterior and lateral setae similar but longer, and several marginal setae thicker and much longer (up to 360); posterior dorsal shield setae slightly longer, length 90–120 ( Fig. 13B ). Dorsal shield cuticle with obvious but fine punctation. Setae in soft cuticle with small barbs, on small platelets; mediolateral setae close to dorsal shield long (170–320), other setae much shorter (50–115).

Venter ( Figs 12B , 13A, 13C , 14 A–B). Setae st1 sparsely barbed, length 140 (135–155). Sternal shield ( Fig. 14A ) reticulate, with 2 pairs of setae, 5–6 pairs of pores opening ventrally, without pores opening laterally; setae st2–3 sparsely barbed, st2 length 110 (100–110), st3 length 90 (75–95); st4 absent; shield medial length 60 (55–70).

Sternogynal shields ( Figs 14A ) weakly fused posteriorly, genital opening length 255 (255–265), width 285 (280–290), each sternogynal shield length 220 (220–240), weakly lineate-reticulate, each bearing 4 (4–7, asymmetry present) setae and one lyrifissure; anterior seta thicker, longer 90 (85–110) than posterior setae (50–65). Internal genitalia comprises three obvious elements (mg, va, vh) and rudimentary latigynal (lg) elements ( Fig. 14B ); rudimentary latigynal elements fused with thickened margin of sternogynal shield, with small porose area near attachment of vaginal arms with sternogynal shield.

Ventral shield ( Figs 12B , 13A ) extends 750 (725–820) posteriorly from posterior margin of genital opening; maximum width behind coxa IV 340 (340–370); shield with 9–12 pairs of setae around shield margin and 5–11 setae within shield and close to sternogynal shield; setae often expressed asymmetrically; setae smooth, length 40– 90; shield reticulate posteriorly and posterior to genital opening, remainder lineate-reticulate.

Ventrianal shield a rounded trapezoid ( Fig. 13C ), anterior width 350 (350–420), maximum width 530 (530– 570), length 320 (315–370); shield reticulate; 12–16 loosely paired smooth setae, length 55–90.

Peritrematal shield hypertrichous, post–peritrematal region extends 450 (450–520) behind coxa IV into metapodal region ( Fig. 12B ); setae laterad legs barbed, length 25–50; anterior margin of peritrematal shield with three pairs of long barbed setae, length 150–260.

Gnathosoma ( Fig. 15A ). Gnathotectum reticulate, medial length 680 (660–690). Subcapitulum with setae h1 240 (230–250), h2 155 (150–160), h3 245 (235–250), pc 135 (135–150), all setae barbed; medial subcapitulum with some fine striae, hypostome smooth. Corniculi length 230 (230–255) with medial tooth. Palp trochanter with ventro-distal spur, length 15 (12–15), palp femur with tiny distal spur, length 5 (5–8). Chelicerae with entire fixed digit length 610 (610–660), movable digit length 355 (350–370). Morphology as per genus description. Fixed digit with 12 teeth; cheliceral seta length 35–45 (not visible in holotype). Movable digit with 10–11 teeth.

Legs ( Fig. 14C ). Setation as per genus description. Selected setal measurements: genu II setae ad1, pd1 subequal, 85–100; seta pd1 on genu III very long, 220–300, much longer than ad1, 100–110, tarsus III ad3 moderately long, length 210–250, femur IV pd1 moderately long, 200–250, genu IV ad1 long, 375–410, pd1 long, length 435–510, tibia IV pd1 long, length 410–460 and tarsus IV ad4 long, length 320–340, pd3 long, length 315– 325. Femur IV with small projections bearing setae pd2, ad2, and slightly larger projection bearing setae ad3.

Male (major). Dorsum. Idiosomal length 2.20–2.40 mm, width 2.00– 2.30 mm, body length (including gnathosoma , to tip of gnathotectum) 2.60 mm. Dorsal shield ovate, length 1.95–2.20 mm, width 1.40–1.60 mm; secondary sclerotisation absent or present, expanding laterally and posteriorly, length of dorsal shield including secondary sclerotisation 2.05–2.30 mm, width 1.50–1.75 mm. Dorsum hypertrichous. Setae on dorsal shield tiny, smooth, length <5 excepting few anterior and posterior setae up to 30 in length. Soft membrane with setae smooth, length 15–30.

Venter ( Fig. 12 C–D, 14D). Setae st1 barbed, length 125–140. Sternoventral shield reticulate, length from posterior margin of genital opening 860–940. Sternal setae st2 and st3 sparsely barbed, length 115–130 and 115– 135, respectively; sternoventral shield ( Figs 14D ) highly hypertrichous posterior to setae st3, with 130–150 setae around genital opening; posterior sternoventral shield with setae along margin; these setae smooth or sparsely barbed, shortest setae 70–80 (posterolateral setae) and longest setae 115–135 (between, and just behind, coxa IV); posterior sternoventral shield ( Fig. 12D ) with two discs of porose cuticle (diameter 130–145), each bearing an anterior and posterior pair of unmodified pair of smooth setae 20–35, and a highly modified pair of setae centrally, length 40–50.

Genital opening ( Fig. 14D ) length 115–120, width 90–110, surrounded by a ring of heavily sclerotised cuticle; this cuticle with 6–10 loosely paired pores laterad and posterolaterad genital opening.

Ventrianal shield ( Fig. 12C ) a curved trapezoid, length 300–365, anterior width 420–450, maximum width 585–660, widest just behind anus; shield reticulate, with 14–16 loosely paired setae. Peritreme and peritrematal shield similar to female.

Gnathosoma ( Figs 15B ). Gnathotectum similar to female, medial length 780–810; subcapitulum similar, except with submedial projections (inner length 20-40). Setae h1 smooth, 200–215; setae h2 smooth, 125–155, setae h3 smooth or with 1–2 barbs, 210–215, palp coxal seta smooth or with few small barbs, 150–160. Corniculi straight, lacking tooth-like processes, length 365–385. Palp with distoventral cusps on trochanter (length 25–30) and femur (length 40–45). Chelicerae same as female.

Legs. Similar to female except distal spurs and spines larger.

Differential diagnosis. This species is most similar to M. zachariei in having females with both setae st4 absent and their dorsal setae of all one form, being thin and sparsely barbed. Both species are also the only Megisthanus found on the genus Aulacocyclus . Female M. southcotti are most easily distinguished from M. zachariei by the length of the sternogynal setae (60–110 versus 20–35 in M. zachariei ), and by the number of setae behind the genital shields within the ventral shield margin (5–11 setae vs 1–2 setae). This reduction in hypertrichy in M. zachariei is also apparent in males ( Fig. 22E ). Female M. southcotti also have short dorsal setae with large barbs (longer dorsal setae with small barbs in M. zachariei ) and a denser setation in the mid-opisthonotal region of the dorsal shield ( Figs 12A , 21A ).

Remarks. This species is usually found on A. fracticornis in north-eastern Queensland. Each beetle tended to carry just one mite each (9 of 11 A. fracticornis ). Two specimens were collected from the unusual passalid species Austropassalus hultgreni . Despite the atypical host, the specimens agree closely with M. southcotti .

The specimens from the South Australian Museum were all marked by Dr Herbert Womersley as “ Megisthanus southcotti n. sp. ”, a name that was never published. This material from the South Australian Museum includes an unusual record of two females from a carabid beetle. Considering the strong preference for passalid hosts, this record is treated with suspicion.

Etymology. This species is named for Dr Ronald Southcott, thus fulfilling Dr Herbert Womersley’s intentions to honour one of Australia’s most significant acarologists with this patronym.

Figures

FIGURE 12. Megisthanus southcotti sp. nov. photomicrographs. (A) female, dorsal shield; (B) female, venter; (C) major male, venter; (D) major male, sternoventral shield. Scale bars: 200 µm (A–C), 100 µm (D).

FIGURE 13. Megisthanus southcotti sp. nov. female, photomicrographs. (A) ventral shield; (B) posterior dorsal shield; (C) ventrianal shield. Scale bars = 100 µm.

FIGURE 14. Megisthanus southcotti sp. nov. (A) female, sternogenital region; (B) internal genitalia; (C) female, femur IV; (D) major male, sternogenital region. Abbreviations: g1 = anterior pair of genital setae; lg = latigynal remnant; mg = mesogynal shield; va = vaginal arms; vh = vaginal heads.

FIGURE 15. Megisthanus southcotti sp. nov. (A) female subcapitulum and palp trochanter–genu (shown detached); (B) major male subcapitulum and palp trochanter–genu. Arrows indicate spurs.

FIGURE 21. Megisthanus zachariei sp. nov. photomicrographs. (A) female dorsal shield; (B) female venter; (C) female posterior dorsal shield; (D) major male ventrianal shield. Scale bars: 200 µm (A–B); 100 µm (C–D).

FIGURE 22. Megisthanus zachariei sp. nov. (A) female, posterior dorsal shield; (B) female, sternogenital region; (C) internal genitalia; (D) female, femur IV; (E) major male, sternogenital region. Abbreviations: g1 = anterior pair of genital setae; lg = latigynal remnant; mg = mesogynal shield; va = vaginal arms; vh = vaginal heads.