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Swan Coastal Plain Shield-backed Trapdoor Spider (Idiosoma sigillatum)

The Swan Coastal Plain Shield-backed Trapdoor Spider is a burrowing spider recorded from south-west Western Australia, including the Swan Coastal Plain and Jarrah Forest bioregions. Its burrows can be difficult to detect and may be affected by clearing, so retain and check suitable sandy or sandy-gravelly habitat before works. It is listed as DBCA Priority 3, meaning it is poorly known, known from several locations and not under imminent threat. No national listing is recorded.

Conservation status

Western Australia, WA Threatened and Priority Fauna List: Priority 3.

Seasonal planning

Seasonal observations are broad rather than tied to precise monthly dates. Use the windows below as planning guidance, not as a substitute for site surveys.

Male movement: Males leave burrows to search for female nests in autumn and early winter.

Young: Spiderlings hatch in early summer and remain in the mother's burrow until the following autumn.

Survey planning: Burrow signs may be present outside the reported male movement period; seasonal detection rates are not known.

Month bands are indicative only. Exact dates for mating or egg laying are not known.

  • Male movement and nest searching: Autumn and early winter are reported for males leaving burrows to search for female nests.
  • Young in the maternal burrow: Spiderlings are reported to hatch in early summer and remain in the mother's burrow until the following autumn. Monthly boundaries are approximate.
  • Precautionary works planning: Planning window only. Check for burrows before ground disturbance in any month; this is a precaution, not a reported seasonal activity pattern.

Identification: Appearance and size

Look for the spider's distinctive corrugated abdomen and, where visible, its burrow entrance with attached plant fragments. Field encounters may be with burrows rather than spiders.

Large females are stout-bodied and about 2.5 to 3 cm long including the jaws. The abdomen has dark reddish-brown ridges with tooth-like spines and long bristles, yellowish grooves, and two pairs of large button-like bare patches on its hind part.

Males are slightly smaller, with longer, thinner legs. The eight eyes are visible to the naked eye and arranged in three rows, with two large eyes at the front edge of the head.

Identification: Similar species

No field comparison with similar species or reliable burrow-based method for separating them is documented. Where identification is uncertain, photograph the spider and burrow without disturbing them and seek an experienced arachnologist.

Identification: Field signs

The burrow is deep and straight, with a vertical, cup-like entrance. A half-circle of radiating twigs and leaves is attached to the rim, and the door is wafer-like and made of silk-bound litter fragments.

Distribution: Known range

The species is recorded in south-west Western Australia. Historical range descriptions and recent records do not confirm that it remains at every named site.

The reported range includes the Swan Coastal Plain and Jarrah Forest bioregions, including the Darling Scarp and Rottnest Island. Older accounts also describe it from the Perth metropolitan area, Garden Island and northern jarrah forest.

Atlas of Living Australia records show 290 Western Australian records at 47 distinct 0.1-degree locations, between latitude -33.5 and -31.1 and longitude 115.4 and 116. Records outside Western Australia were excluded as likely errors.

Distribution: WA regions and site context

The DBCA list flags the Swan and South West regions. The species has been described as becoming rarer in the metropolitan area as bush habitat was lost, and occasional specimens were reported from older built-up suburbs where native trees remained.

Habitat: Recorded habitat

Prioritise inspection of remaining native vegetation on suitable soils, including small or isolated patches within developed landscapes.

Favoured habitat is sandy and sandy-gravelly soil in sheoak, wattle and banksia patches. The species is also described from open forest in the Swan Coastal Plain and Jarrah Forest bioregions.

Habitat: Habitat on works sites

Use these habitat descriptions to guide searches, but recognise they do not form a complete habitat model. Habitat requirements for every life stage are not known, and not all suitable-looking patches are necessarily occupied.

Foraging habitat: Prey and capture

The spider is an insect predator that captures prey at its burrow entrance.

Insects are reported as prey. The burrow's radiating twig and leaf lines are described as feeling lines that pick up movement, allowing the spider to respond to prey near the entrance.

Foraging habitat: Search implications

Inspect the ground and plant litter around likely burrow entrances without pulling at attached material. Particular insect prey and a species-specific feeding schedule have not been documented.

Breeding habitat: Eggs and young

Breeding takes place in the female's burrow. Exact egg-laying dates and clutch size are not known.

Several months after mating, the female lays eggs in a silk cocoon attached to the burrow wall. Spiderlings hatch in early summer and stay in the mother's burrow until the following autumn, when they leave and dig individual burrows.

Breeding habitat: Precaution

Treat occupied or potentially occupied burrows as breeding shelter, even when no spider is visible. Do not excavate, open or block a burrow to check for eggs or young.

Sheltering habitat: Burrow structure

The burrow is the spider's long-term shelter and is central to field detection and disturbance planning.

The species digs a deep, straight burrow with a vertical, cup-like entrance. A wafer-like door made of silk-bound litter fragments is associated with the entrance, which has a half-circle of radiating twigs and leaves.

Sheltering habitat: Burrow use

Each spider spends its whole life in the same burrow, enlarging or repairing it over time. A spider may not be visible at the surface even where a burrow is present.

Sheltering habitat: Disturbance response

Disturbed spiders may be aggressive and can bite, but are not known to be dangerously venomous. Avoid handling them and do not put hands into burrows.

Threats: Recorded pressures

The DBCA Priority 3 category indicates that the species is poorly known, known from several locations and not under imminent threat. Habitat loss and disturbance are also concerns.

Urban sprawl, industrial development, horticulture and other human activities have been associated with declining frequency. Bush habitat has been lost in the metropolitan area.

Threats: Works-related risks

Excavation, road making, landscaping and gardening can disturb females in their burrows. Clearing or ground disturbance in suitable habitat may also damage burrows and surrounding vegetation.

Threats: Listing

The DBCA list gives this species Priority 3. This is a DBCA priority category, not a listing under the Biodiversity Conservation Act 2016. No national listing is recorded.

Survey methodology: 1. Desktop assessment

Surveys should focus on burrow signs in the sandy and sandy-gravelly native vegetation described for the species. No validated survey protocol or detection rate has been established.

Review available fauna records, vegetation mapping, soil information and previous survey reports for the site and nearby areas. Check whether the site lies within the known south-west WA range, while recognising that records do not confirm current occupancy.

Survey methodology: 2. Ground searches

Search suitable sheoak, wattle and banksia patches on sandy or sandy-gravelly soils. Carefully scan ground surfaces and litter for the described cup-like entrance, door and attached radiating plant fragments.

As general trapdoor-spider survey practice, work slowly and avoid probing, opening or damaging suspected burrows. Record the position, photographs, entrance features, surrounding vegetation and soil context.

Survey methodology: 3. Timing and identification

Males are reported to leave burrows in autumn and early winter in search of female nests, so surface searches in that period may encounter wandering males. Their detectability during this period is not known.

Do not rely on surface sightings alone. If species-level identification is uncertain, retain photographs and records for expert review and follow applicable fauna handling and permit requirements.

Vegetation associations: Eucalypt forest

Source-checked associations and automatically indexed candidates are separated below. An association does not establish site presence, preference or dependence.

The species is also described from open forest in the Swan Coastal Plain and Jarrah Forest bioregions.

Automatically indexed from existing profile text. Not a verified preference, mapped occurrence or habitat requirement. Check the original source and regional qualifiers before use.

Evidence: Unreviewed profile-derived candidate

Confidence: Not independently verified

Region: Scope as described in the quoted profile text; not independently verified. Profile states: WA.

Habitat use: general

Attribution gap: No sentence-level reference is attached to the original profile block. Review the profile bibliography; this candidate is not source-verified.

Vegetation associations: Casuarina or sheoak

Favoured habitat is sandy and sandy-gravelly soil in sheoak, wattle and banksia patches.

Automatically indexed from existing profile text. Not a verified preference, mapped occurrence or habitat requirement. Check the original source and regional qualifiers before use.

Evidence: Unreviewed profile-derived candidate

Confidence: Not independently verified

Region: Scope as described in the quoted profile text; not independently verified. Profile states: WA.

Habitat use: general

Attribution gap: No sentence-level reference is attached to the original profile block. Review the profile bibliography; this candidate is not source-verified.

Ecosystem associations

Source-checked associations and automatically indexed candidates are separated below. An association does not establish site presence, preference or dependence.

No species-specific ecosystem association has been indexed or source-checked in this evidence library. This is an evidence gap, not evidence of no association.

Soil and substrate associations

Source-checked associations and automatically indexed candidates are separated below. An association does not establish site presence, preference or dependence.

No species-specific soil association has been indexed or source-checked in this evidence library. This is an evidence gap, not evidence of no association.

Water and wetland associations

Source-checked associations and automatically indexed candidates are separated below. An association does not establish site presence, preference or dependence.

No species-specific water association has been indexed or source-checked in this evidence library. This is an evidence gap, not evidence of no association.

Fauna records and mapped habitat: Not yet analysed

Record locations can be overlaid with mapping, but record frequency alone does not measure habitat preference. No species is excluded from desktop assessments by this analysis.

No quality-controlled record-to-map analysis has been completed for this species. The existing generalised distribution dots and indicative range polygons have not been treated as precise occurrence coordinates.

A defensible run needs dated, independently attributable records, coordinate uncertainty, duplicate and captive-record checks, layer versions and regional scope. Preference and absence require additional survey-effort and detection evidence.

Habitat limitations and contrary evidence: No supported exclusion recorded

Not recorded is not the same as cannot occur. Low suitability, resident habitat, breeding habitat and occasional movement are different conclusions.

This library has not established a vegetation, ecosystem or soil-type exclusion for this species. This does not mean all habitat is suitable; the evidence needed to justify an exclusion has not been assembled.

Habitat limitations and contrary evidence: Before using a negative association

  • Confirm the taxon, geographic region, life stage and habitat function covered by the evidence.
  • Check survey methods, season, repeated effort and detectability before interpreting non-detection.
  • Investigate credible contradictory records and microhabitats absent from broad mapping.
  • Do not convert a missing map class, low record count or failed data request into species absence.

Before habitat disturbance: 1. Map habitat and records

Plan to find and avoid burrows before clearing or ground disturbance begins.

Map native vegetation patches, especially sheoak, wattle and banksia on sandy or sandy-gravelly soils. Review relevant known records and clearly mark potential habitat for on-ground inspection.

Before habitat disturbance: 2. Survey and mark

Arrange a careful pre-clearance search by suitably experienced personnel. Mark detected burrows and their associated vegetation so they can be avoided, and communicate locations and no-go areas to the clearing crew.

Before habitat disturbance: 3. Set controls

Where burrows or suitable habitat are present, define avoidance areas and practical access routes before machinery arrives. Brief workers that burrows should not be opened, filled, driven over or handled.

During habitat disturbance: 1. Work to boundaries

Keep disturbance within approved limits and stop work if a likely burrow or spider is found.

Keep vehicles, stockpiles and ground disturbance out of marked avoidance areas. Use the agreed access routes and maintain communication between the fauna spotter catcher and equipment operators.

During habitat disturbance: 2. Respond to a find

Pause work around the find, protect the burrow from people and machinery, and notify the site environmental lead. Record its position and condition without touching or opening it.

During habitat disturbance: 3. Keep people safe

Do not handle the spider or put hands near or into a burrow. Disturbed spiders may bite.

After habitat disturbance: 1. Inspect and document

After works, check that avoidance measures held and that recorded burrows and habitat were not inadvertently damaged.

Inspect the boundaries of disturbed areas and marked avoidance areas for accidental encroachment or damage. Update site records with locations, photographs, survey observations and any incidents.

After habitat disturbance: 2. Escalate concerns

If a burrow has been damaged or a spider encountered, stop nearby work and seek direction from the site environmental lead and relevant fauna specialists. Follow applicable approval and reporting requirements.

Rehabilitation actions: 1. Protect retained patches

Rehabilitation should protect retained habitat and avoid further disturbance to burrows.

Keep retained native vegetation and marked burrows clear of rehabilitation traffic and material storage. Avoid grading or reshaping ground around burrow entrances.

Rehabilitation actions: 2. Restore suitable vegetation

Where required by the project, use locally appropriate native vegetation to restore disturbed areas and maintain connections with retained habitat. A species-specific rehabilitation method or establishment targets have not been established.

Sources