A day in the life of the mysterious odd-clawed spider Progradungula otwayensis

September 25, 2013
This image shows a Habitat with supporting web (blue arrow) and sturdy threat (red arrows) connecting with the retreat in the hollow of a N. cunninghamii tree. Credit: Peter Michalik

A recent paper published in the open access journal Zookeys provides a first-time glimpse in the natural history of the enigmatic spider species Progradungula otwayensis. Lurking in the hollows of old myrtle beech trees and thus hard to collect, this extraordinary spider is an endemic species confined strictly to the beautiful Great Otway National Park (Victoria, Australia).

P. otwayensis belongs to the small family Gradungulidae which consists of seven genera with a total of 16 described species found exclusively in eastern Australia and New Zealand. The genus Progradungula to which the species studied here belongs is among the few cribellate ones in the family. This term refers to the cribellum, a web producing organ which, unlike normal spinnerets, produces extremely fine fibers which are combed out by the calamistrum, producing silk with a wooly texture. The fibers are so small in diameter that prey insects easily become entangled in them, without any glue needed.

To add to its mystery P. otwayensis weaves highly stereotyped ladder-shaped webs, where they stand facing down after sunset, waiting for preys which will be caught by using the ladder as a trap - a behavior which was already described in detail by now retired arachnologist Mike Gray (Australian Museum) for the only known other species of this genus, P. carraensis. A single thick and shiny silk thread is then used by the spiders to provide a zip-line like connection between the external webs and the security of the enigmatic retreat in the hollows of ancient myrtle beech and mountain ash tree.

"On one occasion, we had access to a large hollow mountain ash tree and found catching ladders and supporting webs of juveniles inside of it,"comments the lead author of the study Peter Michalik, Zoological Institute and Museum of the University of Greifswald (Germany), about the unusual bit of luck to have a glimpse into the secluded retreats of the spiders.

This is an image of an immature male of P. otwayensis holding the catching ladder. Credit: Martin J. Ramirez

Martín Ramírez, from the Argentinian Museum of Natural History and co-author of the study, commented on the habitat specificity of this species, "confined to the oldest and extensively hollow myrtle beech trees in the humid forests in the western part of the Great Otway National Park, or in mountain ash trees, upon which the myrtle trees depend," thus making an important case for the conservation of such habitats.

This image shows a catching ladder in front of a hollow in a large tree fern. Credit: Peter Michalik

The unusual living habits and high degree of endemicity makes this spider a rare and remarkable species. The new study suggests that this spider is dependant on the microclimate in the hollows of old myrtle beech trees since other hollow trees were very much less inhabited and to some extent explains the confinement of the distribution range.

Explore further: Joshua Tree spider species named for U2's Bono

More information: Michalik P, Piacentini L, Lipke E, Ramírez MJ (2013) The enigmatic Otway odd-clawed spider (Progradungula otwayensis Milledge, 1997, Gradungulidae, Araneae): Natural history, first description of the female and micro-computed tomography of the male palpal organ. ZooKeys 335: 101-112. DOI: 10.3897/zookeys.335.6030

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