Comparative study of orb web hygroscopicity and adhesive spiral composition in three araneid spiders

Orb web hygroscopicity was examined in three species of araneid spiders, Argiope aurantia, Argiope trifasciata, and Araneus cavaticus. Both whole webs and fractions generated from them (water-soluble and -insoluble fractions; ethanol-soluble and -insoluble subfractions [Etsol and -insol]) were examined at various relative humidities. In general, the water-soluble fraction, consisting of low molecular weight components (LMW) of the adhesive spiral cover, was more hygroscopic than the water-insoluble (protein) fraction. As the Et-sol of the water extract was typically more hygroscopic than the Et-insol, it appears that organic LMW are more responsible for water adsorption by the adhesive spiral cover than inorgnic LMW. Hygroscopicity measurements made on individual synthetic or commercially obtained LMW compounds known to be in the web (both organic and inorganic) agree with this interpretation. As a first attempt toward correlating web hygroscopicity with composition, the water-soluble web fractions from the three species were examined by proton nuclear magnetic resonance (1H-NMR) spectroscopy and the molar percentages of the organic LMW were estimated. To accomplish this, it was necessary to identify the principal organic LMW in the web of A. trifasciata (since, unlike the situation in other araneids which have been studied, GABamide is not the principal organic LMW in A. trifasciata). Using 1H-NMR and thin layer chromatography this compound was identified as N-acetylputrescine. While considerable interspecific variation in the molar percentages of GABamide, N-acetylputrescine, isethionic acid, and N-acetyltaurine was observed, interspecific differences in Et-sol hygroscopicities were not apparent