Female Caerostris darwini erect their giant webs across streams, rivers, and lakes, suspending the orb directly above the water on anchor threads attached to vegetation on each side of the river. These anchor lines can span up to 25 meters (M. Gregoric pers. comm., cited in Kuntner and Agnarsson 2010). Bridgelines are reinforced daily and the orb is apparently renewed for many days. Web size ranges from 900 to 28,000 cm2, with the largest reported web measuring about 2.8 m2, apparently the largest orb ever measured. With anchor lines capable of bridging over 25 m, Darwin's Bark Spider also builds the longest webs among all spiders. How the spiders establish lines across the river, allowing the building of the web, remains uncertain but is being investigated (M. Gregoric pers. comm., cited in Kuntner and Agnarsson 2010). Agnarsson et al. (2010) report that bridgeline length reflects the habitat: across a mid-sized river in Ranamofana National Park, most webs had bridgelines between 10 and 14 meters long; across smaller rivers in Perinet, bridgelines were much shorter (averaging 3.5 meters), whereas bridgelines across a lake in Perinet reached 25 meters (Agnarsson et al. 2010; M. Gregoric pers. comm., cited in Agnarsson et al. 2010 2010). Silk from C. darwini is extremely tough: it is able to absorb about ten times more kinetic energy before fracturing than does Kevlar (Agnarsson et al. 2010).
Although Kuntner and Agnarsson (2010) observed numerous Caerostris darwini webs over a long period of time, prey items were rarely observed. They did report a mass capture of ephemeropteran (mayfly) prey in these webs. Webs contained up to 32 mayflies that were subsequently wrapped en masse, with the spider wrapping together several prey items before feeding on them. Most wrapped prey packages were heavily kleptoparasitized by flies (apparently undescribed and representing several species and at least two families). Up to 10 flies were observed on a single package being consumed by the host spider, and numerous flies were constantly hovering around the spider and its prey. Flies were also found on prey items that had not been wrapped by the spider. (Kuntner and Agnarsson 2010)
Caerostris darwini exhibit extreme sexual size dimorphism, with large females and small males (Kuntner and Agnarsson 2010).
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