Darwin's bark spider

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Darwin's bark spider
Caerostris darwini web.png
A web of Darwin's bark spider
Scientific classification
Kingdom: Animalia
Phylum: Arthropoda
Class: Arachnida
Order: Araneae
Family: Araneidae
Genus: Caerostris
Species: C. darwini
Binomial name
Caerostris darwini
Kuntner & Agnarsson, 2010

Darwin's bark spider (Caerostris darwini) is an orb-weaver spider that produces one of the largest known orb webs, web size ranged from 900–28000 square centimeters,[1] with anchor lines spanning up to 25 metres (82 ft). The spider was discovered in Madagascar in the Andasibe-Mantadia National Park in 2009.[2] The species was named in honour of the naturalist Charles Darwin, with the description being prepared precisely 150 years after the publication of The Origin of Species, on 24 November 2009.[1]

Several webs of C. darwini spanning a river, demonstrating their extreme length
Bark spider showing how exoskeleton looks like bark
Bark spider showing eyes

Its silk is the toughest biological material ever studied, over ten times tougher than a similarly-sized piece of Kevlar.[3] The average toughness of the fibres is 350 MJ/m3, and some are up to 520 MJ/m3, making the silk twice as tough as any other spider silk known.[4]

The web of Darwin's bark spider is remarkable in that it is not only the longest spanning web ever observed, but is among the largest orb webs ever seen, at an area of up to 2.8 square metres (30 sq ft).[1] Nephila komaci, discovered in 2009, and some other Nephila species also make webs that can exceed 1 m (3 ft 3 in) across.[2]

According to professor Ingi Agnarsson, director of the Museum of Zoology at the University of Puerto Rico, the spider's web occupies a unique biological niche: "They build their web with the orb suspended directly above a river or the water body of a lake, a habitat that no other spider can use".[2] This position allows the spiders to catch prey flying over the water, with webs observed containing up to 32 mayflies at a time.[1] The strong silk and large web are thought to have coevolved at the same time, as the spider adapted to the habitat.[4] Caerostris darwini uses a unique set of behaviors, some unknown in other spiders, to construct its enormous webs. First, the spiders release unusually large amounts of bridging silk into the air, which is then carried downwind, across the water body, establishing bridge lines. Second, the spiders perform almost no web site exploration. Third, they construct the orb capture area below the initial bridge line. In contrast to all known orb-weavers, the web hub is therefore not part of the initial bridge line but is instead built de novo ("from the beginning"). Fourth, the orb contains two types of radial threads, with those in the upper half of the web doubled. These unique behaviors result in a giant, yet rather simplified web. There is building evidence for the coevolution of behavioral (web building), ecological (web microhabitat) and biomaterial (silk biomechanics) traits that combined allow C. darwini to occupy a unique niche among spiders."[2][5]

Like other spiders of the genus, Darwin's bark spider displays extreme sexual dimorphism, with large females and small males, although neither have been measured larger than 2 cm.[1]

The spider was described along with a previously undescribed species of fly, which appeared to have a kleptoparasitic relationship with it. The flies often feed on the spider's catches before the spider wraps them. Occasionally, spiders have been observed to chase away the flies when they land on something that the spider is eating.[1]

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