Sniffing by a silkworm moth: Wing fanning enhances air penetration throughand pheromone interception by antennae

Many organisms increase the air or water flow adjacent to olfactory surface s when exposed to appropriate chemical stimuli; such 'sniffing' samples flu id from a specific region and can increase the rate of interception of odor ant molecules. We used hot-wire anemometry, high-speed videography and flow visualization to study air flow near the feathery olfactory antennae of ma le 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 d egrees at approximately 40 Hz without flying. This behavior generates an un steady flow of air (mean speed 0.3-0.4 m s(-1)) towards the antennae from t he front of the male. A pulse of peak air speed occurs at each wing upstrok e. The Womersley number (characterizing the damping of pulsatile flow throu gh the gaps between the sensory hairs on the antennae) is less than 1; henc e, pulses of faster air (at 40 Hz) should move between sensory hairs. Calcu lation of flow through arrays of cylinders suggest that this wing fanning c an increase the rate of interception of pheromone by the sensory hairs on t he antennae by at least an order of magnitude beyond that in still air. Alt hough wing fanning produces air flow relative to the antennae that is appro ximately 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 approxima tely 560 times faster because a dramatic increase in the leakiness of the f eathery antennae to air flow occurs at the air velocities produced by fanni ng.