Overview
The Arrowsmith Spider Orchid (Caladenia crebra) is an exquisite and rare terrestrial orchid endemic to the wind-swept kwongan sandplains north of Perth, Western Australia. Noted for its luminous lime-green tepals, striking upright posture, and densely crowded labellum calli, this localized spider orchid is a prized jewel of the hyper-diverse Geraldton Sandplains botanical province.
Biology and Physical Characteristics
Caladenia crebra produces an erect, densely pilose stem up to 18 inches tall arising from a single linear-lanceolate basal leaf. In late winter to mid-spring (August to October), the plant bears one or two relatively large, vibrant green to yellow-green flowers. The lateral sepals and petals spread horizontally or arch downward, while the dorsal sepal stands erect. The labellum is broadly cordate, edged with prominent forward-pointing teeth, and covered in a thick carpet of dark glossy calli.
Behavior and Ecology
Caladenia crebra relies on pseudocopulation for cross-pollination. Specialized glandular structures at the tips of the sepals release deceptive chemical compounds that attract male thynnine wasps. Upon landing on the movable, hinged labellum, the insect’s weight tilts it into contact with the reproductive column. The species flowers most profusely in open sandy patches following mild winter rains.
Conservation and Human Impact
Categorized as Vulnerable, the Arrowsmith Spider Orchid occupies a narrow geographical distribution that has been severely fragmented by historical agricultural expansion and coastal infrastructure. Extant populations are safeguarded within Drovers Cave National Park and Beekeepers Nature Reserve. Fire management and control of invasive veldt grass are vital for seedling recruitment.
Interesting Factoid
The specific epithet ‘crebra’ comes from the Latin word for ‘crowded’ or ‘thickly packed,’ describing the dense, velvety carpet of dark calli that entirely coats the center of the labellum.
Last updated: October 2026