Idiosoma formosum 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-western Australia, ZooKeys 756, pp. 1-121 : 32-36

Idiosoma formosum Rix & Harvey sp. n. Figs 25, 145-154, 155-157, 158-166, 375

Type material.

Holotype male. Mount Gibson Station, 93 km NE. of Wubin (IBRA_YAL), Western Australia, Australia, 29°41'57"S, 117°24'28"E, pitfall trap, 21-29 August 2001, K. Ottewell, R. Leys (WAM T139470DNA_Voucher_NCB_012).

Paratypes. 1 ♀, 25 juveniles, Mummaloo, ca. 75 km NE. of Wubin (IBRA_AVW), Western Australia, Australia, 29°39'33"S, 117°13'52"E, hand collected from under Eucalyptus tree, 1 May 2012, M.K. Curran, G.B. Pearson (WAM T125751DNA_Voucher_159); 1 ♀, same data except 29°40'16"S, 117°13'41"E, 2 May 2012 (WAM T125754DNA_Voucher_NCB_013).

Other material examined.

AUSTRALIA: Western Australia: 1 ♂, Dajoing Rock (IBRA_AVW), 30°26'S, 118°04'E, pitfall trap, 28 June– 26 July 1985, B.Y. Main (WAM T139495); 1 juvenile, Mummaloo, ca. 75 km NE. of Wubin (IBRA_AVW), 29°39'33"S, 117°13'52"E, 1 May 2012, M.K. Curran, G.B. Pearson (WAM T125750); 1 juvenile, same data (WAM T125752); 1 juvenile, same data except 3 July 2012, M.K. Curran, S.R. Bennett (WAM T125755); 2 ♂, Mungarri Nature Reserve, north, site BE12 (IBRA_AVW), 30°19'51"S, 117°45'12"E, wet pitfall traps, 15 September 1998-25 October 1999, P. Van Heurck, CALM Survey (WAM T139516); 1 ♀, Mt Churchman (IBRA_COO), 29°55'S, 117°54'E, 27 July 1985, B.Y. Main (WAM T144852); 1 ♀, 16 km N. of Rothsay (IBRA_YAL), 29°09'12"S, 116°49'12"E, 6 July 2014, M. Bamford (WAM T136187).

Etymology.

The specific epithet is derived from the Latin formosus (adjective: ‘beautiful’; see Brown 1956), in reference to the ornate abdominal colouration of this species.

Diagnosis.

Idiosoma formosum is one of nine south-western Australian species in the intermedium- and sigillatum-clades which does not belong to the distinctive 'sigillate complex’ (Fig. 25); these nine species can be distinguished from those 'sigillate complex’ taxa (i.e., I. arenaceum , I. clypeatum , I. dandaragan , I. kopejtkaorum , I. kwongan , I. nigrum and I. schoknechtorum ) by the absence of well-defined lateral sclerotic strips on the male abdomen (e.g., Figs 151, 212, 234), and by the significantly less sclerotised morphology of the female abdomen (which may be strongly corrugate but never leathery and ‘shield-like’) (e.g., Figs 4, 7, 8, 159, 220, 242). Males of I. formosum can be further distinguished from those of I. gutharuka and I. incomptum by the presence of enlarged (i.e., clearly visible) SP4 sclerites (Fig. 151; cf. Figs 186, 199); from I. jarrah and I. mcclementsorum by the colour of the legs, which do not have strongly contrasting bright yellow or orange-yellow femora (Fig. 152; cf. Figs 235, 292); from I. gardneri and I. sigillatum by the absence of well-defined dorso-lateral abdominal corrugations or striations (Figs 146, 151; cf. Figs 168, 173, 352, 357, Key pane 9.1); and from I. intermedium and I. mcnamarai by the shape of tibia I, which is short and stout (with the prolateral clasping spurs occupying most of the distal half of the segment) (Fig. 152; cf. Figs 174, 213, 314), and by the colour of the abdomen, which is ornately bi-coloured dorsally and postero-dorsally (Figs 146, 151; cf. Figs 168, 173, 207, 212, 308, 313).

Females can be distinguished from those of I. mcclementsorum and I. sigillatum by the absence of reinforced, sclerotised ridges on the abdomen (Figs 159, 162; cf. Figs 299, 302, 365, 368); from I. intermedium and I. jarrah by the size of the SP4 sclerites, which are significantly larger than the SP2 sclerites (Fig. 162; cf. Figs 223, 245); and from I. mcnamarai by the colour of the abdomen, which is ornately bi-coloured dorsally and postero-dorsally (Figs 159, 162; cf. Figs 321, 324) [NB. females of I. gardneri , I. gutharuka and I. incomptum 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. 153; cf. Fig. 109), and by the shape of the female eye group, which is broadly trapezoidal (Fig. 161; cf. Fig. 117).

Description (male holotype).

Total length 18.1. Carapace 7.8 long, 5.9 wide. Abdomen 8.2 long, 5.6 wide. Carapace (Fig. 145) tan, with darker ocular region; lateral margins with uniformly-spaced fringe of porrect black setae; fovea slightly procurved. Eye group (Fig. 148) 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 2.9 × their own diameter; PME and PLE separated by approximately diameter of PME, PME positioned in line with level of PLE. Maxillae with field of small cuspules confined to inner corner; labium without cuspules. Abdomen (Figs 146, 151) oval (slightly indented anteriorly), beige-brown in dorsal view with darker grey pattern medially and posteriorly, and assortment of stiff, porrect black setae, each with slightly raised, dark brown sclerotic base. Posterior abdomen moderately sigillate (Figs 146, 151); SP2 sclerites irregular, comma-shaped spots; SP3 sclerites subcircular with irregular margins, each with unsclerotised triangular ‘corner’ laterally; SP4 sclerites circular, each surrounded by chevron-like pad of unsclerotised cuticle laterally; SP5 obscured. Legs (Figs 152-154) variable shades of tan, with light scopulae on tarsi I–II; distal tibia I with pair of large prolateral clasping spurs oriented longitudinally. Leg I: femur 6.6; patella 3.5; tibia 4.2; metatarsus 4.7; tarsus 2.8; total 21.8. Leg I femur–tarsus /carapace length ratio 2.8. Pedipalpal tibia (Figs 155-157) 2.2 × longer than wide; RTA burr-like, with conical basal protuberance and field of retroventral spinules; digital process porrect, unmodified. Cymbium (Figs 155-157) setose, with field of spinules disto-dorsally. Embolus (Figs 155-157) broadly twisted and sharply tapering distally (broken at tip), with prominent longitudinal flange and triangular (sub-distal) embolic apophysis.

Description (female WAM T125751).

Total length 24.3. Carapace 9.6 long, 9.8 wide. Abdomen 10.7 long, 9.8 wide. Carapace (Fig. 158) tan, with darker ocular region; fovea procurved. Eye group (Fig. 161) trapezoidal (anterior eye row strongly procurved), 0.7 × as long as wide, PLE–PLE/ALE–ALE ratio 2.4; ALE almost contiguous; AME separated by approximately their own diameter; PME separated by 3.5 × 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. 163); labium without cuspules. Abdomen (Figs 159, 162) broadly oval and somewhat truncate posteriorly, beige-tan in dorsal view with darker brown pattern medially and posteriorly, with numerous stout setae on sclerotic bases and scattered sclerotic spots; longest stout setae clustered along median cardiac region. Posterior abdomen moderately sigillate (Figs 159, 162); SP2 sclerites irregular, comma-shaped spots; SP3 sclerites subcircular with irregular margins, each surrounded by pad of unsclerotised cuticle; SP4 sclerites subcircular with irregular margins, each surrounded by chevron-like pad of unsclerotised cuticle laterally; SP5 obscured; posterior margin of abdomen weakly corrugate, with rows of modified stout setae. Legs (Figs 164-165) variable shades of tan; 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 5.9; patella 3.8; tibia 3.8; metatarsus 3.0; tarsus 1.9; total 18.6. Leg I femur–tarsus /carapace length ratio 1.9. Pedipalp tan, spinose on tibia and tarsus, with thick tarsal scopula. Genitalia (Fig. 166) with pair of obliquely angled spermathecae on broad ‘stems’, each bearing dense field of glandular vesicles distally, and more sparsely distributed glandular field sub-distally.

Distribution and remarks.

Idiosoma formosum (formerly known by WAM identification code ‘MYG262’), a member of the diverse sigillatum-clade (Fig. 25), is restricted to the Lake Moore catchment of south-western Australia, in a relatively small area near the junction of the Wheatbelt, Yalgoo, and Coolgardie bioregions (Fig. 375). It extends from near Rothsay in the north, south-east to Mungarri Nature Reserve and Dajoing Rock, both in the Cleary-Beacon-Wialki region. This distribution seems to be strongly correlated with annual rainfall of 250-300 mm, and red clay soils to the west and south-east of Lake Moore. North of Cleary and Beacon, the range of I. formosum overlaps the northern extent of the range of the closely related sister species I. mcnamarai .

Although first collected by Barbara Main at Dajoing Rock in the mid-1980s, and subsequently collected during the 'Salinity Action Plan Survey’ of the late 1990s ( Keighery 2004), this species came to prominence during environmental impact assessment surveys conducted in the resource-rich Mount Gibson and Mummaloo regions in the years 2001-2012. Morphological and molecular data were concordant in evidencing two nigrum-group species in this area, and I. formosum was designated the working codes ‘MYG262’ (WAM) and 'sp. B01' (Bennelongia Environmental Consultants) to distinguish it from what is now known to be I. kopejtkaorum (the latter formerly confused with I. nigrum ). Under this ‘MYG262’ code name, I. formosum was also formally assessed in 2017 for threatened species listing under the Western Australian Wildlife Conservation Act 1950 (see below). Burrows of this species are adorned with a ‘moustache-like’ arrangement of twig-lines, and males have been collected wandering in search of females in winter.

Conservation assessment.

In 2017, Idiosoma formosum was formally assessed and listed as 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). In the heavily cleared north-eastern Wheatbelt, the threats to the species are manifold (as they are for I. nigrum ; see above), and in the Mummaloo/Mount Gibson region to the west of Lake Moore, the species is (and will continue to be) at risk from mining and minerals resource development. It has a known extent of occurrence of nearly 4,000 km2 [3,780 km2], and an area of occupancy within that range of <500 km2. Further close assessment under both Criteria A and B will be crucial to the continued survival of this species.