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Wednesday, Dec. 12, 2007

ANIMAL TRACKER

Wasp spider


* Japanese name: Naka-koganegumo
* Scientific name: Argiope bruennichii
* Description: You wouldn't really mistake this spider for a wasp, but its egg-shaped abdomen is dramatically striped in yellow, black and white, and the legs too are striped, though the overall pattern is perhaps more tigerish than wasplike. In any case, the coloration has the same function of that of a wasp. It says: "I'm dangerous, stay away." This is a fairly large spider, with a fat bulbous abdomen, a white thorax and head and a total body length of 2-2.5 cm. The legs, too, are some 2.5 cm long. There is another obvious identifying feature of this animal, however: on its web, the spider weaves a zig-zag pattern of silk (not seen in this photo), sometimes as an X-shape. This pattern is called a stabilimentum, from the idea that the silk stabilizes the web — but more of this later.
* Where to find them: In paddy fields, gardens, forests and on plains from Honshu to Kyushu and Okinawa. The web is built very early every morning, in a low place in bushes. Each evening the female (the females spin the webs) consumes the silk, and rebuilds the web the next day. If you gently touch the web with a twig, the spider will bounce on it, shaking the structure with her legs. This is thought to help locate a prey item that has been caught.
* Food: Small flying insects, such as aphids and dragonflies, crickets, grasshoppers and moths. The wasp spider will happily eat a wasp that gets trapped in her silk.
* Special features: Wasp spiders make patterns on the web, perhaps to provide structural support, or perhaps in order to lure insects. Another idea is that the three-dimensional additions the female makes to the spiral structure of the web have an antipredator function. The silk she uses is a non-sticky kind used to build support lines, and could form a barrier function to repel animals from the web that are too large to trap. Field surveys have found that spiders with a wider abdomen and a smaller web constructed more barrier webs. This suggests an antipredatory function, because well-fed individuals, with fat abdomens, will put more effort into defense than trapping prey. Males, incidentally, are tiny — only 5-9 mm long — and creep up on females after they have molted, when the female's body is still soft. Males often choose this vulnerable moment to try to copulate with females.

PHOTO COURTESY OF BIO-IMAGE NET



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