Megisthanus zachariei, Seeman, 2019

Seeman, Owen D., 2019, The megisthanid mites (Mesostigmata: Megisthanidae) of Australia, Zootaxa 4563 (1), pp. 1-40 : 33-37

Megisthanus zachariei sp. nov.

Figures 21–23

Material examined. Australia, Queensland, ex Aulacocyclus kaupii Macleay, 1871 . Holotype female, Goomburra State Forest , 27°58′S 152°19′E, 18.iv.2012, O. Seeman, H. Urbina & J. Bartlett ( QMS 109500 ) . Paratype: 1 major male, same data as holotype ( QMS 109501 ) . Holotype and paratype deposited in QM .

Diagnosis. Female and male. Sternal setae st4 absent. Seta pd2 of femur IV on rudimentary projection. Dorsal shield finely reticulate. Ventrianal shield widest at mid-level of anus. Female. Dorsal shield ovate, length 2.20 mm, maximum width 1.15 mm, not including secondary sclerotisation. Dorsal setae long (160–380), smooth anteriorly, sparsely barbed posteriorly; podonotum more setose than opisthonotum. Sternogynal shields separate, each bearing 3–4 setae; internal genitalia with rudimentary latigynal element, small mesogynal element. Ventrianal shield weakly reniform. Major male. Larger than female, dorsal shield length 2.45 mm, width 1.70 mm; setae on soft cuticle laterad dorsal shield short to moderate length, 40–100; 25 setae around genital opening; corniculi with small medial inner tooth; setae h1–3 smooth, palp coxal seta barbed.

Description. Female (n = 1). Lightly sclerotised specimen. Dorsum ( Figs 21A, C , 22A ). Idiosomal length ca. 2.35 mm, width 1.60 mm, body length (including gnathosoma , to tip of gnathotectum) 3.05 mm. Dorsal shield ovate, length 2.20 mm, width 1.15 mm; secondary sclerotisation absent. Dorsal shield hypertrichous, densely setose anteromedially; all setae long (160–380), longest laterally, setae smooth anteriorly and sparsely barbed posteriorly. Dorsal shield cuticle finely reticulated. Setae in soft cuticle barbed, on small platelets, length 180–550, longer posterolaterally.

Venter ( Figs 21B , 22 B–C). Setae st1 135, sparsely barbed. Sternal shield ( Fig. 22B ) reticulate, with two pairs of setae, 3–4 pairs of pores opening ventrally and one pair of lyrifissures; setae st2–3 sparsely barbed, st2 100, st3 80, st4 absent; shield medial length 100.

Sternogynal shields ( Fig. 22B ) free posteriorly, genital opening length 305, width 340, each sternogynal shield length 270, lineate-reticulate, each bearing 3–4 smooth setae and one lyrifissure; anterior setae (35) slightly thicker and longer than posterior setae (20–30). Internal genitalia comprises three obvious elements (mg, va, vh) and rudimentary latigynal (lg) elements ( Fig. 22C ); rudimentary latigynal elements fused with thickened margin of sternogynal shield, with small porose area near attachment of va with sternogynal shield.

Ventral shield ( Fig. 21B ) extends 880 posteriorly from posterior margin of genital opening; maximum width behind coxa IV 420; shield with 12–13 pairs of setae around shield margin, 1 pair posteriorly within shield (holotype, just within margin), and with 1–2 pairs of setae off shield margin close to sternogynal shield; setae smooth, length 45–90; shield rugose due to incomplete sclerotisation.

Ventrianal shield slightly reniform ( Fig. 21B ), with anus in centre of shield; anterior width ca. 440, maximum width at mid-level of anus, 620, length 380; shield reticulate; 14–16 loosely paired, smooth setae, length 55–80.

Peritrematal shield hypertrichous, post-peritrematal region extends 530 behind coxa IV into metapodal region ( Fig. 21B ); setae laterad legs smooth to sparsely barbed, length 30–55; three pairs of long setae on anterior margin of peritrematal shield, length to 250.

Gnathosoma ( Figs 23 A–B). Gnathotectum reticulate, medial length 690. Subcapitulum with setae h1 260, h2 145 (detached), h3 240 (detached), pc 185, all setae sparsely barbed; subcapitulum poorly sclerotised, with some fine striae, subcapitular gutter with five folds where rows of denticles should be. Corniculi length 250 with medial tooth. Palp ( Fig. 23B ) trochanter with ventro-distal spur, length 30, palp femur with distal spur, length 15. Chelicerae with entire fixed digit length 660, movable digit length 380. Morphology as per genus description. Fixed digit with 13 teeth; cheliceral seta length 30. Movable digit with 10 teeth; excrescences not apparent due to poor sclerotisation.

Legs ( Fig. 22D ). Setation as per genus description. Selected setal measurements: genu II setae ad1, pd1 subequal, length 90–110; seta pd1 on genu III long (length 525), much longer than ad1 (length 135), tarsus III ad3 moderately long, length 250, femur IV pd1 long, length 420, genu IV ad1 very long, length ca. 500, pd1 ca. 500, tibia IV pd1 long, length ca. 500 and tarsus IV ad4 long, length ca. 350, pd3 long, length ca. 300. Femur IV with rudimentary projections bearing setae pd2, ad2, larger projection bearing seta ad3.

Male (major) (n = 1). Lightly sclerotised specimen. Dorsum. Idiosomal length ca. 2.50 mm, width ca. 1.70 mm, body length (including gnathosoma , to tip of gnathotectum) ca. 3.35 mm. Dorsal shield ovate, length 2.45 mm, width 1.45 mm; secondary sclerotisation absent. Dorsum hypertrichous. Most setae on dorsal shield tiny, smooth, length <5; some marginal setae longer (30–80), especially anteriorly and posteriorly. Setae in soft membrane close to dorsal shield sparsely barbed, short to moderate length (40–100), becoming more barbed and longer laterally; platelets and pores in soft cuticle not apparent, probably due to poor sclerotisation.

Venter ( Figs 21D , 22E ). Setae st1 barbed, length 170. Sternoventral shield reticulate (poorly expressed due to weak sclerotisation), length from posterior margin of genital opening 940. Sternal setae st2 and st3 barbed, length 125–135; sternoventral shield ( Fig. 22E ) hypertrichous posterior to setae st3, with 25 setae around genital opening; posterior sternoventral shield with setae along margin; these setae smooth to sparsely barbed, shortest setae 80 (near genital opening) and longest setae 145 (posterior to coxa IV); posterior sternoventral shield ( Fig. 21D ) with two large discs of porose cuticle (diameter 90), each bearing an anterior (55) and posterior pair (35) of unmodified smooth setae, and a highly modified pair of setae centrally, shrivelled due to poor sclerotisation.

Genital opening ( Fig. 22E ) length 135, width 115, surrounded by a ring of sclerotised cuticle; this cuticle with 12–15 pairs of loosely paired pores laterad genital opening.

Ventrianal shield ( Fig. 21D ) a curved trapezoid, length 355, anterior width ca. 525, maximum width 600, widest at mid-level of anus; shield reticulate, with 13 loosely paired setae. Peritreme and peritrematal shield similar to female.

Gnathosoma ( Figs 23 C–E). Gnathotectum similar to female, medial length 840; subcapitulum similar, except with submedial projections (inner length 40). Setae h1 smooth, blade-like, 200; setae h2 smooth, 180, setae h3 smooth, 180, palp coxal seta barbed, 180. Corniculi with small medial tooth-like process, corniculi length 195 (similar to female in length). Palp ( Figs 23 D–E) trochanter with two distoventral cusps on trochanter (length 25, 35) and one large cusp on femur (length 70). Chelicerae similar to female.

Legs. Similar to female, distal spurs and spines similar in size.

Etymology. It is with great pleasure that I name this species for my nephew Zacharie West.

Differential diagnosis. Megisthanus zachariei is most similar to M. southcotti and their diagnostic differences are explained under the latter species. The species is also similar to M. modestus , from which it differs by lacking setae st4.

Remarks. Both specimens of M. zachariei are lightly sclerotised, so several aspects are difficult to see, but the setation of the dorsal and ventral shields distinguish the two species of Megisthanus found on the genus Aulacocyclus , M. southcotti and M. zachariei . The host beetle of M. zachariei , A. kaupii , is very common in southeast Queensland where it tends to be found on the verges of rainforests and into drier habitats where other genera, such as Pharochilus and Mastachilus , are rare. Despite this beetle species being collected frequently (n> 100), these two specimens are the only megisthanids collected from this host. Therefore it is a rare species on this host, at least in southeast Queensland, where my collecting effort was concentrated ( Seeman 2002).

Figures

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.

FIGURE 23. Megisthanus zachariei sp. nov. (A) female anterior subcapitulum; (B) female palp, trochanter–genu; (C) major male anterior subcapitulum; (D) major male palp, trochanter–genu; (E) palp trochantal spur, aspect from right palp. Arrows indicate spurs.