Overview
The Walpole Spider Orchid (Caladenia interjacens) is an elegant, large-flowered terrestrial orchid endemic to the high-rainfall southern coastal belt of Western Australia. Recognized for its softly drooping perianth segments in muted tones of pale yellow, lime green, and burgundy, this striking species bridges morphological traits between northern and southern spider orchid complexes. Flowering during mid to late spring, it thrives in coastal heathland pockets and peppermint (Agonis flexuosa) scrub sheltered behind rugged granite headlands.
Biology and Physical Characteristics
Caladenia interjacens emerges annually from an underground, spherical, starch-storing tuber encased in a fibrous tunic. It produces a single erect, hairy, basal leaf up to 25 centimeters long and 15 millimeters wide, often blotched with red-purple near the base. The robust flowering scape stands up to 60 centimeters high, bearing one to three large, showy flowers. The sepals and petals are extended and arching downward, creamy-yellow with dark reddish-maroon central lines and tapering to glandular osmophores. The broad, heart-shaped labellum is pale yellowish-white with reddish-pink marginal teeth and four dense rows of clubbed calli.
Behavior and Ecology
Caladenia interjacens achieves cross-pollination through pseudocopulation, chemically and visually mimicking receptive female thynnid wasps. Glandular clubs on the sepal tips synthesize volatile kairomones that disperse downwind through coastal breezes. Attracted male wasps patrol along coastal dune corridors, attempting to copulate with the hinged, receptive labellum, in the process tipping backward against the orchid column to pick up or deposit pollinia. The species enters complete summer dormancy when hot dry conditions desiccate the aboveground foliage.
Conservation and Human Impact
Caladenia interjacens is officially listed as Vulnerable due to its restricted coastal geographic range, which exposes populations to localized disturbances including inappropriate fire regimes, coastal recreation, and weed invasion. Extensive fuel-reduction burning during winter and early spring when plants are actively growing poses significant risks to flowering success and tuber replenishment. Conservation agencies focus on fine-scale spatial mapping, habitat protection in national parks, and ecological fire management.
Interesting Factoid
The specific epithet ‘interjacens’ is Latin for ‘lying between,’ commemorating the orchid’s unique taxonomic position intermediate between the Carousel Spider Orchid and the Giant Spider Orchid.
Last updated: October 2026