Air flow through pectinate insect antennae
and odorant interception rates

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 odorant molecule interception. We used hot wire anemometry, high-speed videography and flow visualization to study air flow near the pectinate antennae of male silkworm moths (Bombyx mori). When exposed to conspecific female sex pheromone, male B. mori flap their wings through a stroke angle of 90 to 110° at about 40 Hz without flying. This behavior generates an unsteady flow of air (mean speed 0.3 to 0. 4 m/s) 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 <1, hence pulses of faster air (at 40 Hz) should move between sensory hairs. Calculation of flow through arrays of cylinders and random walk simulations suggest that this wing fanning can increase the rate of pheromone interception 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 about fifteen times faster than that generated by walking at top speed (0.023 m/s), air flow through the gaps between the sensory hairs is about 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. The pressure drop across single antennae was measured for both male and female antennae mounted over orifices such that air was forced to pass through the antennae. Resistance to flow of both male and female antennae (Pa?s/m) was approximately independent of air speed and was higher for male antennae. Drag forces were measured on both male and female antennae using a novel terminal velocity assay. By combining the measured resistance and the drag forces, the proportion of approaching air that passes through the antenna was estimated as about 15% for both male and female antennae. This approximate "bluff body" behavior is likely to be exhibited by pectinate antennae from a variety of insect taxa in ambient air speeds of 1 m/s or less.

Index terms: biomechanics, diffusion, chemosensory, Bombyx mori.

*For more information on this topic and to listen to a radio interview by "Quirks and Quarks", please see C. Loudon's home page (http://www.ukans.edu/~entomol/faculty/loudon.html).

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