Tuesday, March 19, 2013

BIG MEALS FOR BIG SPIDERS




A female Nephila pilipes daintily dines on a dead bat (Rhinolophus cornutus orii) caught in its web in Japan.

Spiders are a fascinating and remarkably successful group of arthropods, with 41,000+ living species and 500 or so new species being discovered and described each year.  They are also a very old lineage, the earliest definite spider fossils date to some 300,000,000 years and indicate that the  history of the group extends back even further than that. Spiders come in a wide range of sizes and shapes and exhibit a remarkable diversity of ecology and lifestyles. 


A small rhinolophoidean bat entangled in the web of Nephila pilipes in Australia.

  All spiders are predators and while they most feed on invertebrates, especially other arthropods, occasionally larger prey is taken.  A recent paper by Martin Nyffeler and Mirjam Knornschild documents the little known fact that spiders sometimes also fed on bats.  In most cases these are large spiders that produce large orb-webs that snare small, insectivorous bats that are actually smaller than the spider. While none of these spiders specializes in capturing and feeding exclusively on bats, this fascinating behavior has been observed on all continents (with the exception of Antarctica) and is not insignificant. 


Map showing reported incidents (red dots) of bat predation by spiders.

Nor is it just bats that are caught in webs. There are a number of published records of small birds taken by spiders as well, although that appears to be much rarer than incidents with bats.

A Nephila feeds on a finch in Australia.

In most cases the bat/bird predator is a large spider of the genus Nephila, the largest orb weaving spiders. Nephila is known as a golden orb weaver, a reference to the color of its spider silk.  Its poison is a neurotoxin similar to that of the famous Black Widow Spider.  While not lethal to humans, the amount of toxin injected by Nephila when biting a bat smaller than itself is relatively huge --- and fatal.


Nephila jurassica, the largest fossil spider. Actual specimen on left, explanatory drawing on right.

Because their exoskeleton is only weakly mineralized, spiders have a relatively poor fossil record.  However, it turns out that the largest known fossil spider is also referable to Nephila. Nephila jurassica is known from a single specimen, a female, and comes from rocks of Middle Jurassic age of China, some 165,000,000 years-old.  The fossil is reasonably complete and the preservation is spectacular, with even fine details of sensory “hairs” clearly visible. 

The remarkable preservation of N. jurassica is evidenced by these fine sensory hairs on the opisthosoma.

Insect fossils are very diverse and common in the rocks in which N. jurassica was found and certainly were the primary food source for the species. However, given the work of Nyffeler and Knornschild on modern spiders and bats, one might naturally wonder if birds and/or bats might have also fallen prey to N. jurassica.

The oldest definite fossil bats, known from well preserved skeletons, date from the Eocene Period about 100,000,000 years younger than N. jurassica.  The oldest known bird, Archaeopteryx, is from the Late Jurassic of Europe, about 15 million years younger than N. jurassica. Furthermore, Archaeopteryx is the size of a raven or crow and much too large to be caught in a spider web.  However, there is another group of Mesozoic flying vertebrates that might have provided a feast, at least occasionally, for Nephila.


The small pterosaur Anurognathus ammoni pursues an insect in a Late Jurassic forest in Europe.

Pterosaurs first took to the skies in the Late Triassic and continued soaring and flapping until the end of the Cretaceous. During those 160,000,000 years they evolved into many different sizes and shapes, and flew in everything from forests to soaring great distances over open oceans. While some pterosaurs were gigantic, others were quite small.  Anurognathus had a body length of just 4 inches and a weight of only 1.5 ounces.  This is small enough be caught in a large web, such as that made by N. jurassica.

The complete, but very small and delicate, skeleton of Anurognathus ammoni is more easily seen under UV light.

Pterosaur predation by spiders is just an inference.  Paleontologists have not found any fossils of pterosaur remains preserved in a spider web and the chances of such a find are small, although not impossible.  However, given what we known about the predatory behavior of large orb weavers and the very small size of some groups of pterosaurs it seems very likely that pterosaurs provide occasional sustenance to Nephilia and its kin.  I’m waiting for one of the many talented paleoartists to do an appropriately tasteful painting of N. jurassica straddling across and feeding on an anuroganthid that is all wrapped up in spider silk. And above the image, it reads: 

 Nephila.  Proudly killing and eating flying vertebrates for 165,000,000 years.

It would be a beautiful thing.

Thanks to Tracy Ford for calling my attention to N. jurassica.


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SOURCES

Kuntner, M. and Coddington, J.A. 2009. Discovery of the largest orb weaving spider species: The evolution of gigantism in Nephila. PLoS ONE 4(10): e7516. doi:10.1371/journal.pone.0007516
( http://www.plosone.org/article/info:doi/10.1371/journal.pone.0007516)

Nyffeler,  M. and Knornschild, M.  2013.  Bat Predation by Spiders. PLoS ONE 8(3): e58120. doi:10.1371/journal.pone.0058120 (http://www.plosone.org/article/info:doi/10.1371/journal.pone.0058120)

Penny, D. and Selden, P.A. 2005. Assembling the Tree of Life—Phylogeny of Spiders: a review
of the strictly fossil spider families. European Arachnology  (Deltshev, C. and Stoev, P., eds). Acta zoologica bulgarica, Suppl. No. 1: pp. 25-39.

Selden, P.A., Shih, C.K., and Ren, D. 2011. A golden orb-weaver spider (Araneae: Nephilidae:
Nephila) from the Middle Jurassic of China. Biology Letters 7: 775–778. (http://rsbl.royalsocietypublishing.org/content/7/5/775)

Witton, M.P. 2008. A new approach to determining pterosaur body mass and its implications for pterosaur flight.  Zitteliania B28: 143-159.


PHOTOS

Nephila pilipes with bats in Japan and Australia and map of incidents: Nyffeler and Knornschild 2013.

Nephila with finch: http://entophile.com/page/5/

Nephila jurassica photos:  Selden, Shih, and Ren 2011

Anurognathus ammoni flesh restoration: http://commons.wikimedia.org/wiki/File:AnurognathusDB.jpg

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