Darwin's bark spider

Article By:

Agnarsson, Ingi Department of Biology, Faculty of Natural Sciences, University of Puerto Rico, Puerto Rico.

Kuntner, Matjaa Slovenian Academy of Arts and Sciences, Ljubljana, Slovenia.

Blackledge, Todd A. Department of Biology, University of Akron, Akron, Ohio.

Last reviewed:2012

DOI:https://doi.org/10.1036/1097-8542.YB120265

Spiders are exceptionally diverse and abundant, being the primary predators of insects and other arthropods in many terrestrial ecosystems. Many spiders use silk traps to catch insects; in these cases, the familiar wagon wheel–shaped webs, or orb webs, are classical examples (Fig. 1). Spider orb webs are highly efficient and specialized traps that are thought to account for the success of web spiders. In the short term, orb webs allow spiders to catch flying insects that are not readily caught by many other kinds of predators, which may explain the ecological abundance of orb spiders. In the long term, the evolutionary origin of orb webs can explain a major radiation of spiders, resulting in the many thousands of orb spiders that are alive today. This diversity of orb spiders includes spiders that build webs of varying sizes: from webs as small as 1–2 cm (0.4–0.8 in.) in diameter, which are aimed at small flies such as fruit flies and mosquitoes, to webs that are more than 1 m (3.3 ft) in diameter, which can catch large insects and even small vertebrates. Among orb webs, however, none is larger than that of Darwin's bark spider (Caerostris darwini; a new species discovered in Madagascar), which can span up to 2 m (6.6 ft) across (Fig. 1). These webs are suspended along rivers and lakes, often crossing the water on bridge-lines that can span more than 20 m (66 ft). These large webs built over water allow access to prey that are not caught frequently by more typical terrestrial orbs. The prey include insects and possibly small vertebrates that use the rivers as passageways, as well as those that live part of their life in water, such as mayflies.

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