Surviving in the mountains: temperature and elevation have contrasting physiological effects and no effect on morphology of the hoverfly Eristalis tenax in the Himalayas

Insect populations are experiencing a global decline due to a variety of human-linked environmental changes. Among these changes, how insects’ physiology might be affected by predicted upslope migration due to climate change is unknown. Being ectotherms, insect physiology is impacted by abiotic factors like ambient temperature that change with elevation. Here, we performed in situ experiments to assess the sensory and cardiac physiology of an important generalist pollinating hoverfly Eristalis tenax (Diptera: Syrphidae), across different elevations in the eco-sensitive and biodiverse Himalayan mountains. We built a portable physiology setup and measured hoverfly antennal responses towards common floral volatiles at 3600 masl and 4200 masl. We also recorded their heart rate at 3000 masl, 3500 masl and 4000 masl. We further performed tissue morphometric measurements on wild-caught E. tenax using micro-CT. To our knowledge, we report the first in situ physiology experiments performed in high-altitude mountains and the first measurements of internal structures of wild-caught E. tenax. Our results show a contrasting impact of elevation and temperature on the sensory and cardiac physiology of hoverflies, where antennal olfactory sensitivity decreased with increasing elevation, while average heart rate increased with temperature, independent of elevation. However, elevation did not show impact on morphometric parameters like body size and flight musculature. With upslope migration and climate warming, consequent sensory mismatches and potential cardiac stress could have deleterious effects on the health of both hoverflies and the vulnerable Himalayan ecosystem.