|Abstract||The methylated form of mercury in the environment bioaccumulates in the tissues of fish and aquatic insects and is extremely toxic. Methyl Hg is formed by aquatic microbes and has been thought of as an aquatic pollution issue. However, high levels of MeHg have been found in terrestrial birds and spiders. There are two hypotheses concerning how aquatic mercury is entering terrestrial systems. The floodplain hypothesis predicts that MeHg transported from aquatic systems to surrounding terrestrial habitats can become incorporated in the terrestrial food chain. The emergent insect hypothesis predicts that larval insects that develop in aquatic systems become contaminated with Hg and when they emerge as adults transport Hg to terrestrial food chains. The purpose of this study is to distinguish between these hypotheses and allow us to determine if spiders are appropriate biosentinels of aquatic MeHg pollution. We sampled aquatic emergent and terrestrial insects in the summers of 2011 and 2012 using floating emergence traps and sweep nets, respectively. We found elevated concentrations of total inorganic Hg0 and methylated Hg+ in spiders and aquatic insects compared to terrestrial insects, indicating that the MeHg in spiders likely originates from aquatic food chains. In addition, there was a positive correlation between MeHg levels in emergent insects and spiders for 2011. Between the summer of 2011 and 2012 changes in nutrient levels in the ponds suppressed emergent insect biomass and reduced their MeHg concentration but had little effect on the MeHg in adult spiders which had overwintered from the previous year. Thus, our study indicates that spiders could be used as a proxy for aquatic MeHg contamination, but MeHg levels present in adult spiders may be more indicative of conditions during the previous year due to overwintering of adult spiders.