Antipredator Defense in the Spiny Orb-Weaving Spider, Micrathena gracilis

Antipredator defenses are diverse, and so demonstrate predation’s impact on evolution. Many traits hypothesized to have antipredator functions, however, have never been directly tested. Spiny orb-weavers are web-building spiders with diverse coloration and spination which, along with their thickened abdominal cuticle, are hypothesized to confer defense. Female Micrathena gracilis have large, spined abdomens with variable, white and black/brown coloration, while males have smooth, monomorphic brown abdomens. I synthesize a holistic perspective on antipredator defense in M. gracilis and its relatives. I first tested if a single potential antipredator trait – stridulating to produce vibrations when attacked – confers defense (Chapter 1). I then used visual modeling to predict the conspicuousness of M. gracilis to predators. I tested these predictions with laboratory experiments on the responses of jumping spiders to visual stimuli from M. gracilis (Chapter 2). Next, I explored if jumping spiders could successfully attack, kill, and consume female Micrathena through laboratory predator-prey interactions (Chapter 3). Finally, I investigated if spider-specialist mud-dauber wasps encounter and prey on M. gracilis by comparing the representation of M. gracilis in nature to spider prey in wasp nests (Chapter 4). I propose that female M. gracilis possess effective physical defenses against multiple (but not all) predator types, and that they signal these defenses to predators through conspicuous visual traits. The interactive effects of these traits could explain how and why M. gracilis are abundant across a wide geographic range. Meanwhile, since male M. gracilis lack spines and thickened abdomens, but can stridulate and have conspicuous coloration, the degree of predation risk they face is an open question. My research provides a foundation for studying how the diversity of coloration and morphology in spiny orb-weavers is shaped through selection from predators. Investigating physically-defended conspicuous prey species such as female Micrathena can help us more broadly study how suites of antipredator defense traits evolve within species. Future research on these spiders will continue to advance our understanding of how a ubiquitous source of selection – predation – impacts prey trait evolution and the composition of ecological communities