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KAWAMOTO, T.H. Tenacity and silk investment in orb web spiders. 2007. 106f. Dissertação (Mestrado) – Instituto de Psicologia, Universidade de São Paulo, São Paulo, 2007. The origin of the most diverse branch of orbweavers, the Araneoidea, coincides with the origin of the viscid silk in the group Orbiculariae. Many hypotheses were proposed to explain the increased diversity in Araneoidea, however none of them explain this phenomena completely. The present work investigates a new hypothesis: the disappearance of the cribellate silk (substituted by the viscid silk) allowed a reduction in tenacity, which in turn allowed the exploration of a major diversity of microhabitats, leading to a major diversification in the evolutionary time (Chapter 1). The literature shows that the decision of stay/leave the microhabitat (tenacity) is associated to the investment in web construction: spiders with costly webs (sheet web spiders) tend to present higher tenacity than spiders with low cost webs (orb web spiders). In order to test if this tendency also occurs among orbweavers with different web costs, we evaluated the response of a viscid (Metazygia rogenhoferi, Araneidae) and a cribellate silk orbweaver (Zosis geniculata, Uloboridae) to a reduction in food supply. We measured web parameters and the latency to abandon the site. The ecribellate spider maintain unaltered its low tenacity levels, even after the reduction in food supply. On other hand, the cribellate spider increases the tenacity and reduces web investment. In the second experiment (Chapter 2) we measured the construction and maintenance costs of the two species in order to verify if the cribellate orb webs have effectively a higher cost than the ecribellate ones. This was made through direct measurement of the spider 02 consumption (closed intermittent respirometry technique) while resting and building the web. These measurements show that Z. geniculata keeps a lower resting metabolism, what suggests a metabolic adaptation to energetic restrictions. While building the web, the consumption of O2 per centimeter of adhesive silk was three times higher in the cribellate than in the ecribellate web. The lower

Tenacidade e investimento em seda em aranhas de teia orbicular