Sunday, 24 November 2013

Silver spiders


The garden is full of orb-weaving spiders. Not the big, fat Nephila that spin golden webs metres wide, which they anchor with threads made of unbreakable monofilament coated in superglue, but their smaller cousins, Leucauge. Leucauge webs are also coated in superglue, but their anchor threads are easier to break. I speak from experience.

Two species of Leucauge (family Tetragnathidae) live in close proximity in one of the flower beds. They both build small, horizontal wheel webs that are so fine, they are almost invisible. The spiders usually sit at the hub, on the underside of the web, so their ventral surfaces are visible.

The silver sheen (which is seen most clearly in the previous blog post) is caused by guanine, a substance also found in fish scales (and consequently in seabird guano). 

Leucauge decorata

Leucauge decorata (underside)
Sorry about the graininess

 
Leucauge granulata

Leucauge granulata (underside)


Charles Darwin collected a specimen on what is now known as L. venusta in May 1832 while in Rio de Janeiro. He recognised its similarity to Epeira (a name applied to a variety of taxa, but subsequently synonymised with Araneus) and proposed the name Leucauge . This was the only spider genus named by Darwin. When Adam White formally described the spider in 1841, he adopted Leucauge as a subgenus of Linyphia, dubbing the spider Linyphia (Leucauge) argyrobapta. White included the following from Darwin's notes:

Web very regular, nearly horizontal, with concentric circles; beneath, but sometimes above, the concentric web, there is, irregular or thin tissue of network; the animal rests in the centre, on the inferior surface: abdomen brilliant; the red colour like a ruby with a bright light behind.

Unfortunately, this specimen — the holotype of Linyphia (Leucauge) argyrobapta — went missing. Dimitar Dimitrov and Gustavo Hormiga performed some nifty taxonomic detective work to identify the species. Using the original descriptions by Darwin and White and the collections of the Museo Nacional do Rio de Janeiro, they confirmed that the spider was Leucauge venusta, described earlier by Charles Walckenaer as Epeira venusta.

Currently, about 115 species are assigned to Leucauge. The genus is in need of revision. Even the two species in my garden L. decorata and L. granulata come with their own taxonomic baggage. Although well known in Asia, Leucauge decorata was only formally recognised as an Australian species in 2012. Leucauge granulata is also known as L. dromedaria, but that name might be a synonym or it might refer to a completely different species. While that's being sorted out by people with more arachnological nous than me, I'll just admire the shiny spiders and take photos of them.


References

Dimitrov, D. & Hormiga, G. (2010) Mr. Darwin’s mysterious spider: on the type species of the genus Leucauge White, 1841 (Tetragnathidae, Araneae) Zootaxa 2396: 19–36.

White, A. 1841. Descriptions of new or little known Arachnida. Annals and Magazine of Natural History 7: 471–477


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