Biological Journal of the Linnean Society (2000) 70: 107–120. With 5 figures

doi:10.1006/bijl.1999.0340, available online at http://www.idealibrary.com on

Capture thread extensibility of orb-weaving

spiders: testing punctuated and associative

explanations of character evolution

BRENT D. OPELL1* AND JASON E. BOND2

1 Department of Biology, Virginia Tech, Blacksburg, VA 24061, U.S.A.

2 Insect Division, Department of Zoology, Field Museum of Natural History,

1400 S. Lake Shore Drive, Chicago, IL 60605, U.S.A.

Received 22 December 1998; accepted for publication 6 July 1999

Spider orb-webs contain sticky prey capture threads and non-sticky support threads. Primitive

orb-weavers of the Deinopoidea produce dry cribellar threads made of thousands of silk

fibrils that surround supporting axial fibres, whereas the viscous threads of modern Araneoidea

orb-weavers produce adhesive threads with an aqueous solution that coalesces as droplets

around the axial fibres. We have previously shown that the greater diversity of the Araneoidea

is phylogenetically significant and attributed this disparity to a number of advantages,

considered key innovations, that adhesive thread has over cribellar thread. An important

putative advantage of adhesive thread demonstrated by Ko¨hler and Vollrath in their 1995

study is its greater extensibility, a feature that better adapts it to absorb the kinetic energy

of a prey strike. However, this conclusion is based on a two-species comparison that does

not take advantage of the modern comparative method that requires hypotheses to be tested

in a phylogenetic context. Using a transformational analysis to examine threads produced

by nine species, our study finds no support for the punctuated explanation that adhesive

thread has a greater extensibility than cribellar thread. Instead, it strongly supports the

associative null hypothesis that capture thread extensibility is tuned to spider mass and to

architectural features of the web, including its capture area, capture spiral spacing, and

capture area per radius.

2000 The Linnean Society of London

ADDITIONAL KEY WORDS:—capture thread – character evolution – orb-web–

spider–transformational analysis.