Sniffing by a silkworm moth: wing fanning enhances air penetration through and pheromone interception by antennae

Many organisms increase the air or water flow adjacent to olfactory surfaces when exposed to appropriate chemical stimuli; such 'sniffing' samples fluid from a specific region and can increase the rate of interception of odorant molecules. We used hot-wire anemometry, high-speed videography and flow visualization to study air flow near the feathery olfactory antennae of male silkworm moths (Bombyx mori L.). When exposed to conspecific female sex pheromone, male B. mori flap their wings through a stroke angle of 90-110 degrees at approximately 40 Hz without flying. This behavior generates an unsteady flow of air (mean speed 0.3-0.4 m s(-1)) towards the antennae from the front of the male. A pulse of peak air speed occurs at each wing upstroke. The Womersley number (characterizing the damping of pulsatile flow through the gaps between the sensory hairs on the antennae) is less than 1; hence, pulses of faster air (at 40 Hz) should move between sensory hairs. Calculation of flow through arrays of cylinders suggest that this wing fanning can increase the rate of interception of pheromone by the sensory hairs on the antennae by at least an order of magnitude beyond that in still air. Although wing fanning produces air flow relative to the antennae that is approximately 15 times faster than that generated by walking at top speed (0.023 m s(-1)), air flow through the gaps between the sensory hairs is approximately 560 times faster because a dramatic increase in the leakiness of the feathery antennae to air flow occurs at the air velocities produced by fanning.