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Giant Webs

Between reality TV and the 24-hour news cycle, it's hard to be surprised anymore by anything much humans do. Spiders, on the other hand, get up to all sorts of things that I doubt most of us would believe without documentary evidence. A case in point: In the newest issue of the always-interesting Journal of Arachnology (published by the American Arachnological Society), arachnologists Matjaz Kuntner and Ingi Agnarsson provide the first description of a remarkable species of spider from Madagascar. Even if you're not up to reading a whole paper like this, make sure to take a look at the extraordinary photographs included.

Kuntner and Agnarsson's newly described spider belongs to the little-studied genus Caerostris, commonly known as bark spiders because they often look like lumpy pieces of tree bark. They've named it C. darwini, or Darwin's bark spider, in honor of the 150th anniversary of the publication of On the Origin of Species. The Darwin connection isn't casual, because C. darwini poses a bunch of interesting evolution questions. Kuntner and Agnarsson present some tantalizing observations about this previously unknown (at least to us vazaha) spider: It can build orb webs up to 2.8 square meters in area suspended from anchor lines 25 meters long (for us nonmetric Americans, that's 82 feet--whoah!). It engages in "enigmatic sexual behaviors such as mate guarding, male-male aggressiveness, genital mutilation, mate plugging, and self castration." What's more, in a related paper, Agnarsson, Kuntner, and their colleague Todd Blackledge (who contributed a beautiful gum-footed web photo to Spider Silk) describe how they tested the spider's dragline silk and found it to be tougher than any spider silk yet tested, in fact 10 times tougher than Kevlar.

So far, only 11 Caerostris species are known, 6 in Madagascar, but Kuntner and Agnarsson are sure there are many more. Think "giant orb web," and another genus, Nephila, usually comes to mind. Why would giant webs evolve in different genera? What, if any, advantage do giant webs give spiders? They may yield more prey, but they also cost more energy. Kuntner and Agnarsson think being able to sling up such huge webs allowed C. darwini to gain advantage by allowing it to exploit previously unexploited air space above the middle of wide streams; they are exploring how to test this hypothesis. What advantage do male C. darwini gain from their rather rough sexual practices and how might these practices have evolved? What is the molecular sequence of C. darwini's dragline silk protein and how does it differ from the sequence of Nephila dragline silk protein? Will those differences reveal more secrets about how protein structure affects protein function? Can environmental stresses, such as those encountered by a web strung across so wide a stream, affect the evolution of silk properties? How? More or less than the influence of insect evolution? And can experiments be designed to test any hypotheses arising from these questions?

The discovery of such huge webs and tough silk is awfully exciting, and Caerostris darwini clearly has a lot to teach us. Unfortunately, like many unusual animals native to Madagascar, its future is in doubt because it lives in what Kuntner and Agnarsson accurately describe as that giant island nation's "rapidly diminishing forests." The connections between poverty and environmental degradation in Madagascar are glaring--for a heartbreaking introduction, read Robert Draper's article in the September 2010 issue of National Geographic. C. darwini gives me one more reason to support Spider Silk co-author Cay Craig's CPALI project.
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