Microrheology of hemolymph plasma of the bumblebee Bombus terrestris.

Viscosity, which impacts the rate of hemolymph circulation and heat transfer, is one of the transport properties that affects the performance of an insect . Measuring the viscosity of insect fluids is challenging due to the small amount available per specimen. Using particle tracking microrheology, which is well suited to characterise the rheology of the fluid part of the hemolymph, we study the plasma viscosity in the bumblebee Bombus terrestris. In a sealed geometry, the viscosity exhibits an Arrhenius dependence with temperature, with an activation energy compared to that of hornworm larvae estimated in Kenny et al., 2018. In an open to air geometry, it increases by 4 to 5 orders of magnitude during evaporation. Evaporation times are temperature dependent and longer than typical insect hemolymph coagulation times. Unlike standard bulk rheology, microrheology can be applied to even smaller insects, paving the way to characterise biological fluids such as pheromones, pad secretions or cuticular layers.