A recent paper published in the open access journal Zookeys provides a first-time glimpse in the natural history of the enigmatic（神秘的） spider species Progradungula otwayensis. Lurking¹ in the hollows of old myrtle（桃金娘） beech²（山毛榉） trees and thus hard to collect, this extraordinary spider is an endemic species confined strictly³ to the beautiful Great Otway National Park (Victoria, Australia). P. otwayensis belongs to the small spider 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 fibers⁴ which are combed out by the calamistrum, producing silk with a wooly⁵ texture⁶. The fibers are so small in diameter that prey⁷ insects easily become entangled⁸ in them, without any glue needed.

 

To add to its mystery P. otwayensis weaves highly stereotyped⁹ ladder-shaped webs, where they stand facing down after sunset, waiting for preys¹⁰ which will be caught by using the ladder as a trap -- a behavior which was already described in detail by now retired¹¹ 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 catching¹² ladders and supporting webs of juveniles¹³ 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 secluded¹⁴ retreats of the spiders.

 

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.

 

The unusual living habits and high degree of endemicity makes this spider a rare and remarkable¹⁵ species. The new study suggests that this spider is dependent 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 confinement¹⁶ of the species distribution range.

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