Megisthanus womersleyi, Seeman, 2019

Seeman, Owen D., 2019, The megisthanid mites (Mesostigmata: Megisthanidae) of Australia, Zootaxa 4563 (1), pp. 1-40 : 27-33

Megisthanus womersleyi sp. nov.

Figures 16–18

Type material examined. Australia, Queensland , ex Protomocoelus australis (Boisduval, 1835) . Holotype female, Iron Range NP, 20.vii.1999, J. Skevington ( QMS 109499 ) . Paratype: 1 major male, same data as holotype ( QMS 109500 ) . Holotype and paratype deposited in QM .

Other material examined. Papua New Guinea. 1 female, Misima Is., Milne Bay Province, Louisiade Archipelago, Rev. H.K. Bartlett ( J17296 ). 2 females, same data (ex J17373 , slide mounted from spirit specimens) . 2 females, 2 major males, Misima Is., Rev. Bartlett ( SAM J17295 , J17297 –9, J17364 ), in very poor condition. All in SAM .

Diagnosis. Female and male. Sternal setae st4 present. Seta pd2 of femur IV on large projection. Dorsal shield finely punctate. Ventrianal shield widest posterior to anus. Female. Dorsal shield ovate, length 1.80–1.95 mm, maximum width 0.95–1.05 mm, not including secondary sclerotisation. Most dorsal shield setae 50–110 in length, with small barbs; some marginal setae thicker and longer; podonotum and opisthonotum densely setose. Sternogynal shields separate, each bearing 2 setae; internal genitalia with rudimentary latigynal element. Ventrianal shield moderately reniform. Major male. Broader than female, dorsal shield length 2.90 mm, width 1.75 mm, without well-developed setae; setae on soft cuticle laterad dorsal shield short, length 25–70; 19 setae around genital opening; corniculi without medial inner tooth; setae h1–3 smooth, palp coxal seta barbed.

Description (Australian specimens). Female. Dorsum ( Figs 16A, C , 17C ). Idiosomal length ca. 2.60 mm, width ca. 1.90 mm, body length (including gnathosoma , to tip of gnathotectum) 3.20 mm. Dorsal shield ovate, length 2.50 mm, width 1.40 mm; secondary sclerotisation weakly formed but present laterally and posteriorly ( Figs 16A, C ); maximum length of dorsal shield including secondary sclerotisation 2.55 mm, width 1.45 mm. Dorsal shield hypertrichous, densely setose excepting narrow band of bare cuticle posteriorly just within margin. Dorsal shield setae with small barbs, moderately short, length 50–110 ( Figs 16C , 17C ), excepting few anterolateral and posterolateral setae (up to 275); posterior margin with large setal sockets suggesting very long setae are present but broken off. Dorsal shield cuticle appearing smooth, with inconspicuous fine punctation. Setae in soft cuticle barbed, on small platelets, length 50–100, excepting some posterolateral setae close to dorsal shield long (up to 720).

Venter ( Figs 16B , 17A, B ). Setae st1 broken. Sternal shield ( Fig. 17A ) reticulate, with 3 pairs of setae, 1–2 pairs of pores opening ventrally, 1–3 pores opening laterally, and one pair of lyrifissures; setae st2–3 sparsely barbed, st4 smooth, st2 ca. 100, st3 100, st4 55; shield medial length 85.

Sternogynal shields ( Figs 17A ) free posteriorly, genital opening length 260, width 320, each sternogynal shield length 240, weakly lineate-reticulate, each bearing 2 setae and one lyrifissure; anterior seta similar in form and length to other setae (20–25), smooth. Internal genitalia comprises three obvious elements (mg, va, vh) and rudimentary latigynal (lg) elements ( Fig. 17B ); rudimentary latigynal elements fused with thickened margin of sternogynal shield, with small porose area near attachment of va with sternogynal shield.

Ventral shield ( Fig. 16B ) extends 1040 posteriorly from posterior margin of genital opening; maximum width behind coxa IV 430; shield with 13–15 pairs of setae around shield margin, one pair posteriorly within shield, and without setae off shield margin close to sternogynal shield; some setae expressed asymmetrically; setae smooth, length 40–100; shield reticulate posteriorly, otherwise lineate-reticulate.

Ventrianal shield trapezoidal ( Fig. 16B ), with anus in anterior half of shield; anterior width ca. 400, maximum width posterior to anus, 600, length 400; shield reticulate anteriorly, reticulation evanescent posteriorly; 16 loosely paired, smooth setae, length 75–120.

Peritrematal shield hypertrichous, post-peritrematal region extends 760 behind coxa IV into metapodal region ( Fig. 16B ); setae laterad legs smooth, length 40–60, those directly above legs III–IV not much shorter than other setae; three pairs of long setae on anterior margin of peritrematal shield broken.

Gnathosoma ( Fig. 18A ). Gnathotectum reticulate, medial length 700. Subcapitulum with setae h1 285, h2 180, h3 broken, pc 175, all setae sparsely barbed; medial subcapitulum with some fine striae, hypostome smooth. Corniculi length 265 with medial tooth. Palp trochanter with small ventro-distal spur, length 10–15, palp femur without distal spur. Chelicerae ( Fig. 16D ) with entire fixed digit length 720, movable digit length 405. Morphology as per genus description. Fixed digit with 13 teeth; cheliceral seta length 30. Movable digit with 12 teeth.

Legs ( Fig. 17D ). Setation as per genus description. Selected setal measurements: genu II setae ad1, pd1 subequal, length 140–150; seta pd1 on genu III long, 425, much longer than ad1, 185, tarsus III ad3 moderately long, 250, femur IV pd1 long,> 360, genu IV ad1 very long, ca. 600, pd1 broken, tibia IV pd1 long, 570 and tarsus IV ad4 long, 360, pd3 long, 370. Femur IV with large projections bearing setae pd2, ad3, small projection bearing setae ad2.

Male (major) (n = 1). Dorsum. Idiosomal length ca. 3.00 mm, width ca. 2.10 mm, body length (including gnathosoma , to tip of gnathotectum) ca. 3.70 mm. Dorsal shield ovate, length 2.90 mm, width 1.75 mm; secondary sclerotisation present but weakly formed. Dorsum hypertrichous. Setae on dorsal shield tiny, smooth, length <5. Setae in soft membrane close to dorsal shield smooth, becoming more barbed and longer laterally, length 25–70; some setae near posterolateral and posterior margin of shield also long (to 120).

Venter ( Figs 17 E–F). Setae st1 barbed, length 150. Sternoventral shield reticulate, length from posterior margin of genital opening 1190. Sternal setae st2 and st3 barbed, length 130; setae st4 indistinguishable among other sternoventral setae; sternoventral shield ( Fig. 17E ) hypertrichous posterior to setae st3, with 19 setae around genital opening; posterior sternoventral shield with setae along margin; these setae smooth or sparsely barbed, shortest setae 70 (mesad coxa IV) and longest setae 140 (anterolaterad genital opening); posterior sternoventral shield ( Fig. 17F ) with two large discs of porose cuticle (diameter 190), each bearing an anterior and posterior pair of unmodified pair of smooth setae 25–30, and a highly modified pair of setae centrally, length 50.

Genital opening ( Fig. 17E ) length 125, width 115, surrounded by a ring of heavily sclerotised cuticle; this cuticle with 11–12 loosely paired pores laterad genital opening.

Ventrianal shield a curved trapezoid, length 500, anterior width ca. 430, maximum width 660, widest posterolaterally, well behind anus; shield reticulate, with 12 loosely paired setae. Peritreme and peritrematal shield similar to female.

Gnathosoma ( Fig. 18B ). Much larger than female gnathosoma . Gnathotectum similar to female, medial length 910; subcapitulum similar, except with large submedial projections (inner length 75). Setae h1 smooth, blade-like, 250; setae h2 smooth, 140, setae h3 smooth, 215, palp coxal seta barbed, 170. Corniculi without tooth-like processes, length 410. Palp trochanter with distoventral cusps on trochanter (length 30) and femur (length 15–20). Chelicerae similar to female except fixed digit with 17 teeth and movable digit with 13 teeth.

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

Differential diagnosis. Megisthanus womersleyi is most similar to five species of Megisthanus from New Guinea, as discussed below. Megisthanus womersleyi can be most easily distinguished from these species by the density of its dorsal setation, which is far greater than all other species from Australia and New Guinea.

Remarks. With the description of M. womersleyi sp. nov., six species are now known from New Guinea, the others being: Megisthanus doreianus Thorell, 1882 , M. hatamensis Thorell, 1882 , M. moaifensus Oudemans, 1905 , M. orientalis Oudemans, 1905 , M. papuanus Womersley, 1937 and M. postsetosus Karg, 1997 . All these species belong to a species group, previously found only in New Guinea, which is defined by a large dorsal tubercle on leg IV that carries seta pd2. Unlike all other Australian species, these species also have eroded medial margins of the sternogynal shield, so that they appear to form C-shaped shields.

Womersley (1937) originally identified the specimens from Misima Island as M. doreianus . To determine if Womersley’s original decision was correct, I examined the holotypes of M. doreianus , M. hatamensis , M. papuanus , photographs of the holotype of M. postsetosus , and the drawings of Oudemans’ species M. moaifensus and M. orientalis . Megisthanus papuanus is clearly different from all other species of Megisthanus , as explained under the separate treatment for that species. However, all the other species are similar to those of Womersley’s specimens identified as M. doreianus (i.e. M. womersleyi sp. nov.), yet his specimens differ in one important aspect: the density of dorsal setation. Womersley’s specimens have a dense pelage of setae ( Fig. 16A ), while all other species have considerably sparser covering setae ( Figs 19–20 ; Oudemans Mite Collection, Wikimedia Commons).

The specimens from Iron Range match those of the New Guinea specimens closely. The host beetle, Protomocoelus australis , is found in the isolated pockets of rainforest in northern Cape York (such as Iron Range), as well as New Guinea, the Solomon Islands, New Britain, Seram, Waigeo and the Aru Islands ( Gravely 1914). This biogeographic pattern is matched by other genera of Indonesian and New Guinean Passalinae , such as Gonatas and Labienus , which are also found only in Cape York in Australia. Southern regions are instead occupied mostly by species of Pharochilus and Mastachilus , as well as the widespread genus Aulacocyclus , which occurs throughout southeast Asia, northern Australia and along its east coast.

The other species of Megisthanus from New Guinea, excepting M. papuanus , are more difficult to separate from each other. The species described by Oudemans, M. moaifensus and M. orientalis , are both depicted with long setae, while the dorsal setae of M. doreianus , M. hatamensis and M. postsetosus are considerably shorter ( Figs 19– 20 ). These species may also differ in the form of the posterior setae on the dorsal shield, which comprise a row of short stiff setae in M. moaifensus and M. orientalis , as well as several very long setae, which are also present in M. postsetosus ; these setae are either absent or broken in M. doreianus and M. hatamensis . The shape of the anal shield may also help separate species, with M. doreianus , M. moaifensus and M. postsetosus having a broad shield approximately twice as wide as long, and M. hatamensis and M. orientalis having a squarer shield approximately 1.5 x as wide as long. The species of New Guinea require revision based on new material so that intraspecific variation, host beetles and male morphology become known.

Etymology. This species is named in honour of Australian acarologist Dr Herbert Womersley, who treated the specimens from Misima Island under M. doreianus in his revision of the Megisthanidae ( Womersley 1937) .

Figures

FIGURE 16. Megisthanus womersleyi sp. nov. photomicrographs (A) female, dorsum; (B) female, venter; (C) female, posterior dorsal shield; arrows indicate sockets of broken large setae along posterior margin; (D) major male, chelicera, showing dendritic excrescences characteristic for Megisthanidae. Scale bars: 200 µm (A–B); 100 µm (C–D).

FIGURE 17. Megisthanus womersleyi sp. nov. female. (A) sternogenital region; (B) internal genitalia; (C) female, posterior dorsal shield, large setae broken (large sockets along margin remain), note submarginal porose band; (D) female, femur IV; (E) major male, sternogenital region; (F) major male, posterior margin of ventral shield. Abbreviations: g1 = anterior pair of genital setae; lg = latigynal remnant; mg = mesogynal shield; va = vaginal arms; vh = vaginal heads.

FIGURE 18. Megisthanus womersleyi sp. nov. (A) female, subcapitulum and palp trochanter–femur; (B) major male, subcapitulum and palp trochanter–femur. Arrows indicate spurs.

FIGURE 19. Photographs of the holotypes of Megisthanus doreianus Thorell, 1882, (A) dorsum; (B) venter; and Megisthanus hatamanensis Thorell, 1882, (C) dorsum; (D) venter. Photographs by Geoff Thompson, Queensland Museum.

FIGURE 20. The holotype of Megisthanus postsetosus Karg, 1997 (ZMB 45476). (A) photograph; (B) detail of posterior idiosoma, drawn from photograph. Photograph by Anja Friederichs from the Museum für Naturkunde Berlin.