Museum collections contain a wealth of insect remains originating from a wide eographic range, which can be used to investigate their utility as a proxy for environmental isotope ratios. Chitinous remains of insects such as beetles (Coleoptera) are chemically stable and their stable isotope composition is
strongly related to that of environmental water in the period of cuticle formation. We present a dataset of chitin dD and d18O in two genera of water beetles from a museum collection containing 40 locations for Helophorus (water scavenging beetles) and 48 locations for Hydroporus (predaceous diving beetles) that were
selected from latitudes 27–82N in North America. Only two genera were used to minimize inter-sample variation caused by species-specific differences in
metabolic effects, feeding strategy, habitat and life cycle. The isotopic composition of water beetle exoskeletons had a strong latitudinal trend (North–South) from -160 to ?65 % for dD and from 7 to 34 % for d18O, paralleling gradients of isotopes in precipitation. Strong relationships were observed between isotopic composition of beetles and modelled July precipitation (0.71\R 2 \0.82, p\0.001). The relationship between dD and d18O in the beetle samples had a systematic offset from the global meteoric water line, which was likely caused by metabolic effects during chitin formation. The offset between dD values in beetles and in modelled precipitation was 33 %larger, on average, for Hydroporus compared with Helophorus, suggesting fractionation of hydrogen isotopes during passage through the food chain. This trophic level effect was not observed for stable oxygen isotopes. Furthermore, the observed deviations between isotopic composition of water beetles and modelled precipitation at
collection sites were not constant and indicated local hydrological deviations from modelled precipitation. The largest deviations were observed for sites in the
Southern US and the Arctic that are highly evaporative and at sites in the Rocky Mountains and Coastal Mountains that were fed by snow melt. Our results
indicated that the isotopic composition of water beetles from a museum collection was systematically related to dD and d18Ovalues of precipitation at the collection site.