Idiosoma nigrum Main, 1952

Rix, Michael G., Huey, Joel A., Cooper, Steven J. B., Austin, Andrew D. & Harvey, Mark S., 2018, Conservation systematics of the shield-backed trapdoor spiders of the nigrum-group (Mygalomorphae, Idiopidae, Idiosoma): integrative taxonomy reveals a diverse and threatened fauna from south-western Australia, ZooKeys 756, pp. 1-121: 16-19

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Idiosoma nigrum Main, 1952


Idiosoma nigrum Main, 1952   Figs 1-3, 13, 25, 26-35, 36-38, 39-47, 48-56, 374

Idiosoma nigrum   Main, 1952: 133, pl. 1, figs 2-5, fig. 2C (in part; cited specimens from Wongan Hills, Bolgart and Goomalling). Main 1957b: 439, figs 2 G–H, 12D, 14B, 24B (in part; cited specimens from Wongan Hills, Bindi Bindi, Bolgart, Botherling, N. of Bungulla   , Calcarra, Goomalling, Koorda, Minnivale, W. of Nittymarra Hill, E. of Walebing and S. of Wyalkatchem). Main 1985: 13, figs 23, 27, 215. Rix et al. 2017d: 601, fig. 108.

Type material.

Holotype female. Wongan Hills (IBRA_AVW), Western Australia, Australia, 30°54'S, 116°43'E, 19 June 1952, B.Y. Main (WAM T3960; examined).

Paratype. 1 ♀, same data as holotype except 20 June 1952 ( AMS KS6392).

Other material examined.

AUSTRALIA: Western Australia: 1 ♀, Wongan Hills, Mount Matilda (IBRA_AVW), 30°49'14"S, 116°38'13"E, 5 November 2011, B.Y. Main (WAM T122017DNA_Voucher_149); 1 ♀, same data except 30°49'15"S, 116°38'12"E (WAM T122018DNA_Voucher_150); 1 juvenile, same data except 1 km NW. of Wongan Hills, 30°52'35"S, 116°42'46"E (WAM T122019DNA_Voucher_151); 1 ♀, Wongan Hills, water catchment area NW. of the town (IBRA_AVW), 30°53'S, 116°43'E, 5 November 2011, B.Y. Main, Ecologia staff (WAM T143998); 1 ♀, NW. of Wongan Hills, W. side of Wongan Hills about half way along range (IBRA_AVW), 30°52'S, 116°42'E, 19 June 1952, B.Y. Main (WAM T144765); 1 juvenile, same data (WAM T144766); 1 ♀, same data (WAM T144767); 1 juvenile, same data (WAM T144768); 1 ♀, same data (WAM T144769); 1 ♀, same data (WAM T144806); 1 ♀, same data (WAM T144807); 1 juvenile, same data (WAM T144808); 1 ♀, same data except 20 June 1952 (WAM T144770); 1 ♀, about 1 mile N. of Bindi Bindi (IBRA_AVW), 30°33'S, 116°21'E, 4 August 1955, B.Y. Main (WAM T144793); 1 ♀, same data (WAM T144794); 1 juvenile, same data except 20 May 1954 (WAM T144815); 1 ♀, 1 mile S. of Bolgart on main road (IBRA_AVW), 31°17'S, 116°31'E, 18 June 1952, B.Y. Main (WAM T144805); 1 juvenile, 7 miles W. of Bolgart (IBRA_AVW), 31°16'S, 116°28'E, 4 June 1953, B.Y. Main (WAM T144811); 1 ♀, Botherling (IBRA_AVW), 31°07'S, 116°48'E, 30 September 1952, B.Y. Main (WAM T144773); 1 ♀, first creek W. of Calcarra (IBRA_AVW), 31°08'S, 116°27'E, 4 May 1953, B.Y. Main (WAM T144810); 1 ♂, Durokoppin Nature Reserve, NW. tip, Transect E (IBRA_AVW), 31°24'S, 117°45'E, pitfall trap, 3 May– 25 June 1988, B.Y. Main (WAM T139511); 1 ♀, East Yorkrakine Nature Reserve (IBRA_AVW), 31°23'S, 117°40'E, 8 December 2001, B.Y. Main (WAM T144622); 1 ♀, ‘Fairfields’, 9 miles N. of Bungulla   (IBRA_AVW), 31°30'S, 117°35'E, 18 May 1956, B.Y. Main (WAM T144624); 1 juvenile, same data (WAM T144837); 1 ♀, same data except 10 March 1957 (WAM T144627); 1 ♀, same data (WAM T144838); 1 ♀, same data (WAM T144839); 3 ♀, same data (WAM T144840); 1 juvenile, same data (WAM T144842); 1 juvenile, same data except 1 September 1955 (WAM T144790); 1 juvenile, same data (WAM T144791); 1 juvenile, same data (WAM T144792); 1 ♀, Goomalling Reserve, 1 mile N. of town, near trotting course (IBRA_AVW), 31°17'S, 116°50'E, 18 June 1952, B.Y. Main (WAM T144764); 1 ♀, about half way between Kalguddering and Calingiri (W. of Nittymarra Hill) (IBRA_AVW), 31°02'S, 116°38'E, 4 May 1953, B.Y. Main (WAM T144779); 1 ♀, Minnivale, scrub on S. side of townsite (IBRA_AVW), 31°08'S, 117°11'E, 20 July 1954, B.Y. Main (WAM T144787); 1 ♀, Minnivale Nature Reserve, 19 km WNW. of Wyalkatchem (IBRA_AVW), 31°07'55"S, 117°11'39"E, hand collected from open mallee woodland, 21 April 2014, M.G. Rix, M.S. Harvey (WAM T132737DNA_Voucher_62); 1 juvenile, Minnivale Fauna Reserve (IBRA_AVW), 31°08'S, 117°11'E, 4 April 1984, B.Y. Main (WAM T144843); 1 ♀, same data except 18 May 1985 (WAM T144844); 1 ♀, 37 km N. of Minnivale (IBRA_AVW), 30°48'S, 117°13'E, 20 October 1984, B.Y. Main (WAM T144626); 1 ♀, 20.9 km E. of New Norcia (IBRA_AVW), 31°00'49"S, 116°25'37"E, dug from burrow, 15 September 2012, T. Sachse (WAM T127020DNA_Voucher_NCB_020); 1 ♂, North Bungulla   , Lind Road, W. of Reserve (IBRA_AVW), 31°31'S, 117°35'E, pitfall trap, 23 June– 4 August 1987, B.Y. Main (WAM T139514); 1 ♀, 5 miles E. of Walebing on Great Northern Highway (IBRA_AVW), 30°39'S, 116°17'E, 24 April 1955, B.Y. Main (WAM T144799); 1 juvenile, about 7 miles E. of Walebing (IBRA_AVW), 30°42'S, 116°20'E, 5 June 1953, B.Y. Main (WAM T144812); 1 juvenile, same data (WAM T144813); 1 juvenile, same data (WAM T144814); 1 ♂ ‘allotype’ [NB. not a paratype as subsequently designated by Main, 1957b: 439], Walk Walkin, via Koorda (IBRA_AVW), Western Australia, Australia, 30°50'S, 117°29'E, hand collected, 13 May 1940, G.F. Best (WAM T3301); 1 ♂, Walk Walkin Nature Reserve, site WH12 (IBRA_AVW), 30°48'08"S , 117°19'19"E, wet pitfall trap, 15 September 1998-25 October 1999, B. Durrant, CALM Survey (WAM T139510); 1 ♂, Wroth Road Nature Reserve, site JB6 (IBRA_AVW), 31°19'16"S, 116°33'38"E, wet pitfall trap, 15 September 1998-4 November 1999, N. Guthrie, CALM Survey (WAM T139515); 1 ♀, 1 mile S. of Wyalkatchem (IBRA_AVW), 31°12'S, 117°24'E, 20 July 1954, B.Y. Main (WAM T144785); 1 ♀, same data (WAM T144786).


Idiosoma nigrum   is one of seven highly autapomorphic species in the polyphyletic 'sigillate complex’ (Fig. 25); members of this complex can be distinguished from all other species in the nigrum-group from south-western Australia (i.e., I. formosum   , I. gardneri   , I. gutharuka   , I. incomptum   , I. intermedium   , I. jarrah   , I. mcclementsorum   , I. mcnamarai   and I. sigillatum   ) by the presence of well-defined lateral sclerotic strips on the male abdomen (e.g., Figs 32, 63, 256), and by the very heavily sclerotised, leathery, ‘shield-like’ morphology of the female abdomen (e.g., Figs 1-3, 9-12, 52, 74, 96). Males of I. nigrum   can be further distinguished from those of I. arenaceum   by the shape of the SP4 sclerites, which are not elongate-oval (Fig. 32; cf. Fig. 63); from I. kwongan   by the absence of semi-circular lateral indentations adjacent to the SP4 sclerites (Fig. 32; cf. Fig. 278, Key pane 13.1); from I. clypeatum   and I. kopejtkaorum   by the presence of a prominent sub-distal embolic apophysis (Key pane 14.1; cf. Key panes 14.2, 14.3); and from I. dandaragan   and I. schoknechtorum   by the largely asetose dorsal abdomen (Fig. 27; cf. Figs 124, 330), and by the shape of the SP4 sclerites, which are large and subquadrate (Fig. 32; cf. Figs 129, 335).

Females can be distinguished from those of I. arenaceum   by the shape of the SP4 sclerites, which are not elongate-oval (Figs 43, 52, Key pane 21.3; cf. Fig. 74, Key pane 21.1); from I. clypeatum   and I. kopejtkaorum   by the size of the SP4 sclerites, which are greater than half the size of the SP3 sclerites (Figs 43, 52, Key pane 22.3; cf. Figs 96, 267, Key panes 22.1, 22.2); and from I. dandaragan   and I. schoknechtorum   by the shape of the SP4 sclerites, which are subquadrate (Figs 43, 52, Key pane 21.3; cf. Figs 140, 346, Key panes 23.2, 23.3), and by the absence of well-defined SP5 sclerites (Figs 43, 52, Key pane 21.3; cf. Figs 140, 346, Key panes 23.2, 23.3) [NB. females of I. kwongan   are unknown].

This species can also be distinguished from I. corrugatum   (from the Eyre Peninsula of South Australia) by the shape of the prolateral clasping spurs on the male tibia I, which are oriented longitudinally (Fig. 34; cf. Fig. 109), and by the shape of the female eye group, which is broadly trapezoidal (Figs 42, 51; cf. Fig. 117).

Description (male WAM T3301).

Total length 16.1. Carapace 7.0 long, 4.8 wide. Abdomen 7.3 long, 4.4 wide. Carapace (Fig. 26) tan, with darker ocular region; lateral margins with uniformly-spaced fringe of porrect black setae; fovea procurved. Eye group (Fig. 29) trapezoidal (anterior eye row strongly procurved), 0.7 × as long as wide, PLE–PLE/ALE–ALE ratio 2.0; ALE almost contiguous; AME separated by less than their own diameter; PME separated by 3.4 × their own diameter; PME and PLE separated by slightly more than diameter of PME, PME positioned in line with level of PLE. Maxillae and labium without cuspules. Abdomen (Figs 27, 32) oval, beige-brown in dorsal view with lateral sclerotic strips, dorso-lateral corrugations and scattered dorsal sclerotic spots. Dorsal surface of abdomen (Fig. 27) largely asetose, with only a sparse assortment of stiff, porrect black setae, each with slightly raised, dark brown sclerotic base. Posterior abdomen strongly sigillate (Figs 27, 32); SP2 sclerites irregular, comma-shaped spots; SP3 sclerites very large and circular; SP4 sclerites subquadrate; SP5 obscured. Legs (Figs 33-35) variable shades of tan, with light scopulae on tarsi I–II; distal tibia I with pair of large prolateral clasping spurs oriented longitudinally; proximal-most clasping spur with missing macroseta. Leg I: femur 6.0; patella 3.0; tibia 4.4; metatarsus 4.6; tarsus 2.8; total 20.7. Leg I femur–tarsus /carapace length ratio 3.0. Pedipalpal tibia (Figs 36-38) 2.3 × longer than wide; RTA burr-like, with conical basal protuberance and field of retroventral spinules; digital process porrect, unmodified. Cymbium (Figs 36-38) setose, with field of spinules disto-dorsally. Embolus (Figs 36-38) broadly twisted and sharply tapering distally, with prominent longitudinal flange and triangular (sub-distal) embolic apophysis.

Description (female WAM T132737).

Total length 19.6. Carapace 8.3 long, 6.6 wide. Abdomen 8.8 long, 10.2 wide. Carapace (Fig. 48) dark tan (dark brown-black in life; Fig. 1), with darker ocular region; fovea slightly procurved. Eye group (Fig. 51) trapezoidal (anterior eye row strongly procurved), 0.6 × as long as wide, PLE–PLE/ALE–ALE ratio 2.4; ALE almost contiguous; AME separated by approximately their own diameter; PME separated by 2.4 × their own diameter; PME and PLE separated by more than diameter of PME, PME positioned in line with level of PLE. Maxillae with field of cuspules confined to inner corner (Fig. 53); labium without cuspules. Abdomen (Figs 49, 52) dark brown-black (shiny black in life; Figs 1-3), corrugate and highly sclerotised, with leathery appearance typical of those species in the 'sigillate complex’ (see Fig. 25). Posterior face of abdomen (Fig. 52, Key pane 23.1) with truncate ‘shield-like’ morphology; SP3 sclerites very large and circular; SP4 sclerites subquadrate; SP5 obscured by thickened cuticle. Legs (Figs 54-55) variable shades of dark tan (darker brown-black in life; Fig. 1); scopulae present on tarsi and metatarsi I–II; tibia I with two stout pro-distal macrosetae and row of five longer retroventral macrosetae; metatarsus I with eight stout macrosetae; tarsus I with distal cluster of short macrosetae. Leg I: femur 5.2; patella 3.4; tibia 3.3; metatarsus 2.6; tarsus 2.1; total 16.6. Leg I femur–tarsus /carapace length ratio 2.0. Pedipalp dark tan, spinose on tibia and tarsus, with thick tarsal scopula. Genitalia (Fig. 56) with pair of short, obliquely angled spermathecae, each bearing dense field of glandular vesicles distally, and more sparsely distributed glandular field sub-distally.

Distribution and remarks.

Idiosoma nigrum   (Figs 1-3), a 'sigillate complex’ member of the diverse sigillatum-clade (Fig. 25), has a restricted distribution in the central and central-western Wheatbelt bioregion of south-western Australia (Fig. 374). Its range roughly corresponds to the polygon demarcated by Bolgart, New Norcia, Walebing, and Bindi Bindi along its western margin, east to Koorda along its northern margin, south to Durokoppin and Kellerberrin along its eastern margin, and from Kellerberrin to Bolgart along its southern margin. While additional populations may occur slightly outside of this area, this range appears to roughly correspond to the area south and west of the 300 mm annual rainfall isohyet, bounded to the south by the Mortlock River and Avon River catchments, and to the west by the Lake Ninan and Lake Hinds catchments along the western edge of the Avon Wheatbelt bioregion. The distribution of I. nigrum   closely abuts those of three other species in the 'sigillate complex’ (Figs 25, 374): at its northern extent it nears the southern limit of the range of the distantly related I. kopejtkaorum   ; along its southern margin (near Northam and Meckering) it approaches the range of the closely related sister species I. schoknechtorum   ; and at its north-western extent (near New Norcia) it abuts the range of the closely related I. dandaragan   . Unsurprisingly, all three species have in the past been confused with I. nigrum   (e.g., Main 1957b), although morphology and/or molecular data can now be used to convincingly separate these species. Burrows are adorned with a ‘moustache-like’ arrangement of twig-lines (Fig. 13), and male specimens have been collected wandering in search of females in late autumn and winter.

Conservation assessment.

Idiosoma nigrum   is the only spider in Australia to be afforded threatened species status under both State and Commonwealth legislation. In Western Australia in 2017, it was elevated from Vulnerable to Endangered (B1ab[iii] + B2ab[iii]) under the Western Australian Wildlife Conservation Act 1950 (approved 16 January 2018; see W. A. Government Gazette 2018); this assessment incorporated the latest taxonomic, geographic and genetic data summarised in the current study (with a number of additional records also identified subsequently). At a Commonwealth level, under the Environmental Protection and Biodiversity Act 1999 (EPBC), I. nigrum   is listed as Vulnerable, although this EPBC assessment will need to be revised in light of current knowledge. The threats to the species are manifold, and its long-term persistence in the severely fragmented central Wheatbelt is tenuous ( Main 2003). Its extent of occurrence is nearly 8,500 km2 [8,410 km2], and its area of occupancy within that range is < 500 km2. The current strongholds of I. nigrum   are the larger nature reserves (NR) within its range, namely Durokoppin NR, East Yorkrakine NR, Minnivale NR, North Bungulla   NR and Wongan Hills NR. However, multiple lines of evidence suggest populations are suffering severe contemporary declines ( Main 2003; Rix et al. 2017c). Indeed, at Yorkrakine Rock NR it is now very difficult to find any idiopid trapdoor spiders, despite the seemingly healthy populations that once occurred at this site in the 1950s (see Main 1967; cf. Rix et al. 2017c). Similarly, at Durokoppin NR, only a single male was collected in pitfall traps set in 1988, and no males were collected during a long-term pitfall trapping survey run at four sites in the reserve from 1997-1998 (see Keighery 2004). Main (2003) further documented the decline of the East Yorkrakine Nature Reserve population during a long-term demographic survey run from 1989-1999, and Main (1987) highlighted a similar decline on a private property ( ‘Fairfields’) near North Bungulla   Nature Reserve between 1952 and 1980. If remaining I. nigrum   populations continue to decline at the current rate throughout the species’ range, a Critically Endangered assessment may need to be considered in the future.