Idiosoma corrugatum 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 : 27-30

Idiosoma corrugatum Rix & Harvey sp. n. Figs 101-110, 111-113, 114-122, 377

Type material.

Holotype male. Eyre Peninsula, 54 miles SW. of Kimba (IBRA_EYB), South Australia, Australia, ca. 33°42'S, 136°18'E [NB. precise locality unknown], 2 December 1959, B.Y. Main (SAM NN29858).

Paratypes. 1 ♀, same data as holotype (WAM T144751); 1 ♀, same data (WAM T144752); 1 juvenile, same data (WAM T144753); 1 ♀, same data (WAM T144754); 1 ♀, same data (WAM T144755); 1 ♀, same data (WAM T144748); 1 ♀, same data (WAM T144747); 1 ♀, same data (WAM T144749); 1 ♀, same data (WAM T144750); 1 juvenile, same data (SAM NN29859); 1 ♀, same data as holotype except ca. 11 miles NW. of Cleve (NB. precise locality unknown) (WAM T144636); 1 ♀, same data (SAM NN29860); 1 juvenile, same data (WAM T144630); 1 ♀, same data (WAM T144635); 1 ♀, same data (WAM T144629); 1 ♀, same data (WAM T144746); 1 ♀, same data (WAM T144631); 1 ♀, same data (WAM T144632); 1 ♀, same data (WAM T144634).

Etymology.

The specific epithet is derived from the Latin corrugatus (adjective: ‘ridged’; see Brown 1956), in reference to the corrugate abdominal morphology of this species. This name was first coined informally by Barbara Main, and we reproduce it here in recognition of her discovery of this species.

Diagnosis.

Males and females of Idiosoma corrugatum can be distinguished from all other species of Idiosoma in the nigrum-group (i.e., I. arenaceum , I. clypeatum , I. dandaragan , I. formosum , I. gardneri , I. gutharuka , I. incomptum , I. intermedium , I. jarrah , I. kopejtkaorum , I. kwongan , I. mcclementsorum , I. mcnamarai , I. nigrum , I. schoknechtorum and I. sigillatum ) by the shape of the eye group, which is compact and not broadly trapezoidal (Figs 104, 117). Males can also be distinguished by the shape of the prolateral clasping spurs on tibia I, which are oriented dorso-ventrally (Fig. 109).

Description (male holotype).

Total length 13.7. Carapace 5.5 long, 4.3 wide. Abdomen 6.7 long, 4.5 wide. Carapace (Fig. 101) pale (faded) tan, with darker ocular region; lateral margins with uniformly-spaced fringe of porrect black setae; fovea procurved. Eye group (Fig. 104) trapezoidal (anterior eye row strongly procurved), 0.8 × as long as wide, PLE–PLE/ALE–ALE ratio 1.3; ALE separated by approximately half their own diameter; 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 and labium without cuspules. Abdomen (Figs 102, 107) oval, pale (faded) tan in dorsal view with dorso-lateral corrugations and scattered dorsal sclerotic spots. Dorsal surface of abdomen (Fig. 102) with assortment of stiff, porrect black setae, each with slightly raised sclerotic base. Posterior abdomen sigillate (Figs 102, 107); SP2 sclerites comma-shaped spots; SP3 sclerites large and circular; SP4 sclerites oval, each surrounded by chevron-like pad of unsclerotised cuticle laterally; SP5 obscured. Legs (Figs 108-110) variable shades of pale (faded) tan, with light scopulae on tarsi I–II; distal tibia I with pair of large prolateral clasping spurs oriented dorso-ventrally. Leg I: femur 5.2; patella 2.6; tibia 3.9; metatarsus 3.3; tarsus 2.0; total 17.0. Leg I femur–tarsus /carapace length ratio 3.1. Pedipalpal tibia (Figs 111-113) 2.2 × longer than wide; RTA burr-like, with conical basal protuberance and field of retroventral spinules; digital process porrect, unmodified. Cymbium (Figs 111-113) setose, with field of spinules disto-dorsally. Embolus (Figs 111-113) twisted and gradually tapering distally; embolic apophysis absent.

Description (female WAM T144634).

Total length 17.0. Carapace 6.4 long, 4.8 wide. Abdomen 8.2 long, 7.9 wide. Carapace (Fig. 114) faded tan, with darker ocular region; fovea procurved. Eye group (Fig. 117) trapezoidal (anterior eye row strongly procurved), 0.8 × as long as wide, PLE–PLE/ALE–ALE ratio 1.5; ALE separated by slightly more than half their own diameter; AME separated by more than their own diameter; PME separated by 3.0 × their own diameter; PME and PLE separated by approximately diameter of PME, PME positioned in line with level of PLE. Maxillae with field of cuspules confined to inner corner (Fig. 119); labium without cuspules. Abdomen (Figs 115, 118) faded tan, corrugate and highly sclerotised posteriorly. Posterior face of abdomen (Fig. 118) with truncate ‘shield-like’ morphology; SP3 sclerites large and circular; SP4 sclerites oval; SP5 obscured by thickened cuticle. Legs (Figs 120-121) variable shades of faded tan; scopulae present on tarsi and metatarsi I–II; tibia I with one stout pro-distal macroseta and row of three longer retroventral macrosetae; metatarsus I with six stout macrosetae; tarsus I with distal cluster of short macrosetae. Leg I: femur 3.7; patella 2.6; tibia 2.1; metatarsus 1.7; tarsus 1.3; total 11.4. Leg I femur–tarsus /carapace length ratio 1.8. Pedipalp faded tan, spinose on tibia and tarsus, with thick tarsal scopula. Genitalia (Fig. 122) with pair of widely spaced and obliquely angled spermathecae, each bearing dense field of glandular vesicles distally.

Distribution and remarks.

Idiosoma corrugatum has a highly restricted distribution in the central-eastern Eyre Peninsula of South Australia, where it is known from only one or possibly two indeterminate sites west of Cleve (both from the late 1950s) (Fig. 377). The type locality ("54 miles SW. of Kimba") and the other known locality ("11 miles NW. of Cleve") likely correspond to roughly the same site (or nearly so), although this is difficult to confirm given the collection data available. No other records of the species are known, and dedicated recent searching in the vicinity of Cleve has so far failed to detect any extant populations.

The highly disjunct distribution of I. corrugatum relative to other species is unique among shield-backed trapdoor spiders, and raises the question of whether it is actually a member of the nigrum-group at all. Convergent evolution of a phragmotic abdominal morphology has already been demonstrated in I. galeosomoides (see Rix et al. 2017b, d), and it remains possible that I. corrugatum has independently evolved a corrugate abdomen with enlarged SP3 and SP4 sclerites (a possibility further supported by the morphology of the eye group, the female spermathecae and the male leg I clasping spurs). Burrow morphology is undescribed, and it is unclear whether the single known male from December 1959 was collected from within its burrow or wandering in search of females.

Conservation assessment.

Due to the known occurrence of this species at only one (or possibly two; see above) sites, we consider it data deficient for the purposes of conservation assessment. However, the absence of recent records (despite extensive collections from the area), the scale of land clearing on the Eyre Peninsula, and the population declines that have occurred among Idiopidae since the 1950s ( Rix et al. 2017c), would suggest that this species may be critically endangered or extinct. Assuming extant populations can eventually be detected, further close assessment under both Criteria A and B is warranted in the future.