Megisthanus modestus Berlese 1910
Creators: name: Seeman, Owen D.
Publisher: name: Zenodo
Year: 2019
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Arachnida
Mesostigmata
Megisthanidae
Megisthanus
Megisthanus modestus

description: Megisthanus modestus Berlese, 1910
Figures 1–4
Megisthanus modestus Berlese, 1910: 377. Material examined. Australia, Queensland, ex Pharochilus scutellonotus Kuwert, 1898. 1 female, Lamington NP, Box Forest Circuit, 13.ii.1996, O. Seeman, ex beetle #298 (QMS 109449). 1 female, same data except beetle #299 (QMS 109450). 1 female, 1 major male, same data as except beetle #311 (QMS 109451–2). 1 female, 1 minor male, Main Range NP, 11.ii.1996, O. Seeman, ex beetle #177 (log 3, passalid 1) (QMS 109453–4). In QM. Australian Capital Territory, Canberra, ex passalid. 1 female, 21.ix.1941, A. Shepherd, RL105 (J17305). Tasmania, ex passalid. 1 female, Launceston, 15.i.1925, V.V.H. (J17301 –2), in poor condition (originally identified as M. doreianus). New South Wales: ex Pharochilus dilatatus. 1 female, 1 male, Glen Innes, 29°44′S 151°44′E, 9.x.1966, G. Bornemissza (J17380). ex Pharochilus rugiceps (Hope & Westwood, 1845). 3 females, 1 male, University of Western Sydney, A. Katlav, 25.xii.2017 (QMS 109445–8). No host given. 1 female, Upper Williams River, N.S.W., Wilson coll., x.1926 (J17336), in poor condition; 1 female, Coonabarabran, 31°16′S 149°17′E, 20.i.1974, G.F. Gross, skeletal knoll, dry forest (J17369). All J-registration numbers in SAM.
Diagnosis. Female and male. Sternal setae st4 present. Seta pd2 of femur IV on rudimentary projection. Dorsal shield finely punctate with faint reticulation. Ventrianal shield widest at anterior level of anus. Female. Dorsal shield ovate, length 1.80–2.10 mm, maximum width 0.90–1.15 mm, not including secondary sclerotisation. Most dorsal shield setae 250–320 in length, smooth or with few barbs; some marginal setae thicker and longer; podonotum more setose than opisthonotum. Sternogynal shields separate, each bearing 2–6 setae (usually 4–6); internal genitalia with rudimentary latigynal element. Ventrianal shield weakly to moderately reniform. Major male. Broader than female, dorsal shield length 2.40 mm, width 1.40 mm, with a few developed setae (up to 75 in length) anteriorly and posteriorly; setae on soft cuticle laterad dorsal shield short, length 40–80; 30 setae around genital opening; corniculi with small to rudimentary medial inner tooth; setae h1, h2 smooth, h3 and palp coxal seta barbed.
Description (based on specimens ex P. scutellonotus, Queensland). Female. Dorsum (Figs 1 A–B). Idiosomal length 2.30–2.40 mm, width 1.50–2.10 mm, body length (including gnathosoma, to tip of gnathotectum) 2.90–3.20 mm. Dorsal shield ovate, length 1.80–2.10 mm, width 0.90–1.15 mm; secondary sclerotisation present (Fig. 1A), laterally and posteriorly; maximum length of dorsal shield including secondary sclerotisation 2.15–2.60 mm, width 1.25–1.75 mm. Dorsal shield hypertrichous, especially mid-dorsally, setation sparser posteriorly, at margins and off dorsal shield. Most dorsal shield setae smooth or with few small barbs, length 250–320; anterior and lateral setae similar but longer, and several marginal setae may be thicker, more barbed and longer (up to 500) (Figs 1 A–B, 3D). Dorsal shield cuticle finely punctate with faint pattern of reticulation. Setae in soft cuticle with small barbs, on small platelets; setae close to dorsal shield long (300–350), subequal length compared with marginal dorsal setae; some posterolateral setae very long (550–600). Idiosomal pores absent in soft membrane, present on dorsal shield and on small platelets, often associated with setae.
Venter (Figs 1 C–D, 3A–C). Setae st1 sparsely barbed, length 160–180. Sternal shield (Fig. 2A) reticulate, with three pairs of setae, 3–6 pairs of pores opening ventrally, without pores opening laterally, and one pair of lyrifissures; setae st2–3 sparsely barbed, st4 smooth, st2 105–125, st3 50–75, st 4 25–40; shield medial length 80– 85.
Sternogynal shields (Fig. 3A) free posteriorly, genital opening length 285–305, width 325–340, each sternogynal shield length 260–285, weakly lineate-reticulate, each bearing 4–6 setae (asymmetrical) and one lyrifissure; anterior seta similar in form and length to other setae (18–27, smooth). Internal genitalia comprise three obvious elements (mg, va, vh) and rudimentary latigynal (lg) elements, which are represented by small porose area on thickened ventral shield margin, near attachment of vaginal arms with sternogynal shield (Figs 3 B–C).
Ventral shield (Figs 1 C–D) extends 790–850 posteriorly from posterior margin of genital opening; maximum width behind coxa IV 360–400; shield with 11–12 pairs of setae around shield margin (posteriorly sometimes with one of these pairs within shield), no setae within shield close to sternogynal shield; setae often expressed asymmetrically; setae smooth, length 70–140; shield weakly reticulate, mostly appearing smooth.
Ventrianal shield weakly to moderately reniform (Figs 1 C–D), anterior width 450 (400–450), maximum width either level with or anterior to anus, 520–575, length 320–355; shield reticulate; 14–16 loosely paired smooth setae, length 50–90.
Peritrematal shield hypertrichous, post-peritrematal region extends 450–520 behind coxa IV into metapodal region (Figs 1 C–D); setae laterad legs smooth, length 10–50, those directly above legs III–IV shortest; anterior margin of peritrematal shield with three pairs of long barbed setae, length 260–340.
Gnathosoma (Fig. 4A). Gnathotectum reticulate, medial length 680–740. Subcapitulum with setae h1 250– 275, h2 145–165, h3 250–270, pc 165–170, all setae barbed; medial subcapitulum with some fine striae, hypostome smooth. Corniculi length 250–270 with medial tooth. Palp trochanter with ventro-distal spur, length 20–30 (asymmetrical in holotype), palp femur with distal spur, length 10–15. Chelicerae with entire fixed digit length 670–730, movable digit length 380–390. Morphology as per genus description. Fixed digit with 12 teeth; cheliceral seta length 30–35. Movable digit with 8–10 teeth.
Legs (Fig. 1E). Setation as per genus description. Selected setal measurements: genu II setae ad1, pd1 subequal, 100–130; seta pd1 on genu III very long, 520–560, much longer than ad1, 150–155, tarsus III ad3 moderately long, 305–345, femur IV pd1 long, coarsely barbed, length 300–350, genu IV ad1 very long, 700–750, pd1 very long, 740–750, tibia IV pd1 long, 560–590 and tarsus IV ad4 long 400–450, pd3 long, 350–370. Femur IV with rudimentary projections bearing setae pd2, ad2, small projection bearing setae ad3.
Male (major) (n = 1). Dorsum (Fig. 2A). Idiosomal length 2.90 mm, width 2.60 mm, body length (including gnathosoma, to tip of gnathotectum) ca 3.70 mm. Dorsal shield ovate, length 2.40 mm, width 1.40 mm; secondary sclerotisation present, expanding laterally and posteriorly, length of dorsal shield including secondary sclerotisation 2.90 mm, width 2.40 mm. Dorsum hypertrichous. Setae on dorsal shield tiny, smooth, length <5 excepting few anterior and posterior setae up to 75 in length. Setae in soft membrane close to dorsal shield smooth, small, length 40–80, setae becoming more barbed and longer laterally (80–100).
Venter (Figs 2B, 3F). Setae st1 barbed, length 145. Sternoventral shield reticulate, length from posterior margin of genital opening 940. Sternal setae st2 and st3 barbed, length 110 and 105, respectively; setae st4 shorter, 35; sternoventral shield (Fig. 3F) hypertrichous posterior to setae st3, with 30 setae around genital opening; posterior sternoventral shield with setae along margin; these setae smooth or sparsely barbed, shortest setae 75 (posterolateral setae) and longest setae 150 (just behind coxa IV); posterior sternoventral shield (Figs 2B) with two discs of porose cuticle (diameter 90), each bearing an anterior and posterior pair of unmodified smooth setae 55– 60, and a highly modified pair of setae centrally, length 80.
Genital opening (Fig. 3F) length 150, width 125, surrounded by a ring of heavily sclerotised cuticle; this cuticle with 10–12 loosely paired pores laterad and posterolaterad genital opening.
Ventrianal shield (Fig. 2B) reniform, length 330, anterior width 450, maximum width 630, widest at anterior level of anus; shield reticulate, with 15–16 loosely paired setae. Peritreme and peritrematal shield similar to female.
Gnathosoma (Fig. 4B). Gnathotectum similar to female, medial length 800; subcapitulum similar, except with submedial projections (inner length 25). Setae h1 smooth, 240; setae h2 with few barbs, 150, setae h3 barbed, 265, palp coxal seta barbed, 190. Corniculi length 390, with medial tooth-like process present but smaller than female’s process. Palp trochanter with small proximoventral rectangular process bearing proximal seta, and with distoventral cusps on trochanter (length 30) and femur (length 50). Chelicerae same as female.
Legs. Similar to female except spurs and spines larger.
Male (minor) (n =1): Dorsum (Fig. 2C). Slightly smaller specimen compared to major male. Dorsal shield ovate, length 1.95 mm, width 1.10 mm; secondary sclerotisation present, extensive. Dorsum hypertrichous, with numerous long setae, similar to female. Venter similar to major male (Fig. 2D). Porose discs about same size, diameter 90–95, their modified setae slightly shorter, length 60–70. Ventrianal shield smaller, anterior width 420, maximum width 525, length 290. Gnathosoma similar to major male, except corniculi with medial tooth as in female; corniculi much shorter, length 275. Legs similar to major male, spurs and spines of a similar size.
Differential diagnosis. Megisthanus modestus retains setae st4, which are absent in all other Australian species except M. womersleyi, and these species are separated by numerous features (see Remarks for M. womersleyi). The species seems closest to M. zachariei sp. nov. in sharing a podonotum that is considerably more setose than the opisthonotum (Figs 1 A–B, 21A). In addition to the presence of setae st4 only in M. modestus, this species has smaller ventral shields than M. zachariei, even though the species are a similar size, a difference especially apparent in the ventrianal shield (length 320–355, width 520–575 for M. modestus, 380 x 620 for M. zachariei).
Remarks. Berlese (1910) gave a rudimentary description of M. modestus based on specimens from an undetermined passalid and unspecified locality in New South Wales. His description compared the species with M. doreianus, noting that the ventral shield was somewhat narrower, the dorsal setae longer, the body size smaller, and that the post-peritrematal shields scarcely extended past the posterior margin of the ventral shield. Womersley (1937) found this description so inadequate that he seriously considered synonymising these species, but M. doreianus and M. modestus are clearly differentiated herein (see Remarks section for M. womersleyi sp. nov.). Briefly, M. doreianus differs by having a large dorsal tubercle on femur IV that bears seta pd2, eroded medial margins of the sternogynal shield, and females with a dense pelage of short setae (Fig. 16).
I attempted to see photographs of the holotype of M. modestus, but the specimen was not in a suitable condition for diagnostic photography. Of the features noted by Berlese (1910), only the character of the postperitrematal shields is noteworthy. All specimens examined largely agree with Berlese’s description, while other Australian species tend to have much larger post-peritrematal shields, extending to the ventrianal shield. However, this feature is not reliable because distortion of the ventral shields can occur with slide-mounting, and because of this some specimens of other species appear to have the smaller post-peritrematal shields of M. modestus.
My treatment of M. modestus makes it a broadly distributed species found in southern Queensland, New South Wales, Canberra and Tasmania. It is the only Australian species of Megisthanidae recorded from New South Wales—indeed any state other than Queensland—and is therefore also very likely to be the same species described by Berlese (1910). The host beetles are three species of Pharochilus; the host species P. rugiceps is perhaps the most widespread of all Australia’s passalid beetles, being found from southern Queensland to Tasmania (Dibb 1938; Seeman 2002). It is the only species of Passalidae in Tasmania. The specimens of M. modestus from Queensland, New South Wales and Canberra are indistinguishable from each other, despite being separated by up to 700 km and occurring in different habitats (rainforest in Queensland; sclerophyll in New South Wales and Canberra) and on different host beetles (P. dilatatus, P. scutellonotus in Queensland; P. rugiceps elsewhere). However, the single specimen in good condition from Tasmania is tentatively considered M. modestus. Under phase-contrast microscopy, a denser reticulation is visible on the dorsal shield, and the opisthonotal setae are shorter than those of specimens found in mainland Australia (Table 2). However, with just a single good-quality specimen and no males, I consider this specimen M. modestus pending further collection of specimens from Tasmania.
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 5-12, DOI: 10.11646/zootaxa.4563.1.1, http://zenodo.org/record/2600858