Idiosoma arenaceum Rix & Harvey,

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-, ZooKeys 756, pp. 1-121: 20-23

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Idiosoma arenaceum Rix & Harvey

sp. n.

Idiosoma arenaceum Rix & Harvey  sp. n. Figs 11, 12, 19, 20, 25, 57-66, 67-69, 70-78, 374

Idiosoma  ‘nigrum’ Main, 1957b: 440 (in part; cited specimens from Canna, N. of Galena and Woolaga Creek).

Type material.

Holotype male. Zuytdorp, site ZU1 (IBRA_GES), Western Australia, Australia, 27°15'42"S, 114°01'30"E, wet pitfall trap, 19 May– 17 August 1995, N. Hall, WAM-CALM Carnarvon Survey (WAM T139527).

Paratype. 1 ♂, same data as holotype (WAM T41787).

Other material examined.

AUSTRALIA: Western Australia: 2 ♂, Zuytdorp, site ZU3 (IBRA_GES), 27°15'34"S, 114°04'03"E, wet pitfall trap, 18 May– 16 August 1995, N. Hall, WAM-CALM Carnarvon Survey (WAM T41364DNA_Voucher_NCB_017); 1 ♂, Zuytdorp Nature Reserve, site ZU2 (IBRA_GES), 27°15'41"S, 114°01'48"E, wet pitfall trap, 18 May– 17 August 1995, N. Hall, WAM-CALM Carnarvon Survey (WAM T41788); 1 ♀, creek just N. of Canna (Irwin River system) (IBRA_AVW), 28°54'S, 115°52'E, 5 October 1953, B.Y. Main (WAM T144780); 1 ♀, 2.3 miles S. of Ebano Creek in Woolaga Creek (IBRA_AVW), 29°12'S, 115°39'E, 29 May 1954, B.Y. Main (WAM T144783); 1 juvenile, same data (WAM T144784); 1 juvenile, Galena, 2 miles N. of Murchison River on North-west Coastal Highway (IBRA_GES), 27°48'S, 114°42'E, 9 July 1954, B.Y. Main (WAM T144788); 1 ♂, Geraldton (IBRA_GES), 28°47'S, 114°37'E, 1943, E.G. Osborne ( AMS KS17388); 1 ♀, same data ( AMS KS6393); 1 ♀, same locality data except 28°46'S, 114°37'E, 16 February 1983, A. Mollan (WAM T27120); 1 ♂, Geraldton, Minnenooka (IBRA_GES), 28°49'S, 114°54'E, 19 August 1971, J. Bryo (WAM T27119); 1 ♀, Kalbarri National Park, George Grey Drive, 18 km (by road) S. of Kalbarri (IBRA_GES), 27°51'22"S, 114°09'02"E, dug from burrow, 17 August 2016, M.S. Harvey, M.E. Blosfelds (WAM T141118DNA_Voucher_NCB_006); 1 ♀, 27 km S. of Kalbarri (IBRA_GES), 27°57'S, 114°10'E, 23 September 1989, D. Mead-Hunter (WAM T21810); 1 ♀, 18 miles S. of Mullewa on Mingenew Road (IBRA_GES), 28°48'S, 115°31'E, 15 July 1955, B.Y. Main (WAM T144835); 1 ♀, same data (WAM T144836); 1 ♀, N. side of Murchison River (IBRA_GES), 27°50'S, 114°41'E, 9 July 1954, B.Y. Main (WAM T144817); 1 ♀, same data (WAM T144789); 1 ♂, Northampton (IBRA_GES), 28°21'S, 114°38'E, hand collected under sheet of iron, 1 January 1982, Dr Allan (WAM T27123); 1 ♀, Sunset Beach (IBRA_GES), 28°43'S, 114°37'E, 9 September 1997, T. Burrows (WAM T44355).


The specific epithet is derived from the Latin arenaceus (adjective: 'of sand’, ‘sandy’; see Brown 1956), in reference to the near-coastal sandy habitats occupied by this species in the Geraldton Sandplains bioregion.


Idiosoma arenaceum  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 and females of I. arenaceum  can be further distinguished from those of all other known 'sigillate complex’ species (i.e., I. clypeatum  , I. dandaragan  , I. kopejtkaorum  , I. kwongan  , I. nigrum  and I. schoknechtorum  ) by the shape of the SP4 sclerites, which are longitudinally elongate-oval (Figs 63, 74, Key panes 7.1, 21.1) [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. 65; cf. Fig. 109), and by the shape of the female eye group, which is broadly trapezoidal (Fig. 73; cf. Fig. 117).

Description (male holotype).

Total length 19.7. Carapace 8.0 long, 5.6 wide. Abdomen 8.4 long, 6.4 wide. Carapace (Fig. 57) dark chocolate-brown, with darker ocular region; lateral margins with uniformly-spaced fringe of porrect black setae; fovea procurved. Eye group (Fig. 60) trapezoidal (anterior eye row strongly procurved), 0.8 × 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 2.8 × their own diameter; PME and PLE separated by slightly more than diameter of PME, PME positioned slightly posterior to level of PLE. Maxillae with field of small cuspules confined to inner corner; labium without cuspules. Abdomen (Figs 58, 63) broadly oval, light beige-brown in dorsal view with lateral sclerotic strips, dorso-lateral corrugations and scattered dorsal sclerotic spots. Dorsal surface of abdomen (Fig. 58) more heavily setose anteriorly, with assortment of stiff, porrect black setae, each with slightly raised, dark brown sclerotic base. Posterior abdomen strongly sigillate (Figs 58, 63); SP2 sclerites small oval spots; SP3 sclerites very large and circular; SP4 sclerites longitudinally elongate-oval; SP5 obscured. Legs (Figs 64-66) variable shades of dark brown, with light scopulae on tarsi I–II; distal tibia I with pair of large prolateral clasping spurs oriented longitudinally. Leg I: femur 7.2; patella 3.5; tibia 4.7; metatarsus 5.0; tarsus 3.4; total 23.8. Leg I femur–tarsus /carapace length ratio 3.0. Pedipalpal tibia (Figs 67-69) 2.5 × longer than wide; RTA burr-like, with conical basal protuberance and field of retroventral spinules; digital process porrect, unmodified. Cymbium (Figs 67-69) setose, with field of spinules disto-dorsally. Embolus (Figs 67-69) broadly twisted and sharply tapering distally, with prominent longitudinal flange and triangular (sub-distal) embolic apophysis.

Description (female AMS KS6393).

Total length 24.8. Carapace 10.9 long, 8.1 wide. Abdomen 11.6 long, 10.8 wide. Carapace (Fig. 70) dark tan and chocolate- brown, with darker ocular region; fovea procurved. Eye group (Fig. 73) trapezoidal (anterior eye row strongly procurved), 0.6 × as long as wide, PLE–PLE/ALE–ALE ratio 2.6; ALE almost contiguous; AME separated by approximately their own diameter; PME separated by 5.0 × 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. 75); labium without cuspules. Abdomen (Figs 71, 74) dark maroon-brown, corrugate and highly sclerotised, with leathery appearance typical of those species in the 'sigillate complex’ (see Fig. 25). Posterior face of abdomen (Fig. 74, Key pane 21.1) with truncate ‘shield-like’ morphology; SP3 sclerites very large and circular; SP4 sclerites longitudinally elongate-oval; SP5 obscured by thickened cuticle. Legs (Figs 76-77) variable shades of dark tan; scopulae present on tarsi and metatarsi I–II; tibia I with one stout pro-distal macroseta and row of five longer retroventral macrosetae; metatarsus I with six stout macrosetae (at least one broken off at base); tarsus I with distal cluster of short macrosetae. Leg I: femur 6.8; patella 4.3; tibia 3.9; metatarsus 3.3; tarsus 2.3; total 20.7. Leg I femur–tarsus /carapace length ratio 1.9. Pedipalp dark tan, spinose on tibia and tarsus, with thick tarsal scopula. Genitalia (Fig. 78) with pair of short, subtriangular spermathecae, each bearing dense field of glandular vesicles distally and more sparsely distributed glandular field sub-distally.

Distribution and remarks.

Idiosoma arenaceum  (formerly known by WAM identification code ‘MYG478’) (Figs 11-12) has a moderately widespread distribution in the Geraldton Sandplains and far northern Wheatbelt bioregions of south-western Western Australia, from near Yandanooka, Canna, and Geraldton north to Zuytdorp (Fig. 374). It is closely related to the three other 'sigillate complex’ species in the northern clypeatum-clade: I. clypeatum  , I. kopejtkaorum  , and I. kwongan  (Fig. 25). Burrows are adorned with a ‘moustache-like’ arrangement of twig-lines, sometimes under Casuarina  (Figs 19, 20), and male specimens have been collected wandering in search of females in late autumn and winter, with an outlying record from January.

Conservation assessment.

Idiosoma arenaceum  has a known extent of occurrence (EOO) of nearly 12,000 km2 [11,939 km2; with coastline as western margin], although this value is possibly an underestimate due to fairly limited survey effort. The area of occupancy within that range is similarly difficult to estimate, although is likely to be small as a proportion of EOO due to the scale of land clearing throughout most of its range. We do not currently consider this species to be of conservation concern, although further close assessment under both Criteria A and B is warranted in the future.