Divergence of streamlines approaching a pectinate insect antenna: consequences for chemoreception.

Pectinate (feathery) antennae have high resistance to air flow, and therefore most of the air approaching an antenna is diverted around it and is not available for chemical sampling by the sensory hairs on that antenna. The small fraction (approximately 10-20%) of approaching air that passes through the air spaces or gaps in the antenna decelerates and the streamlines diverge as the air approaches the antenna. Sampling a small fraction of air that is decelerating and diverging has consequences for chemoreception that are described here for the first time. The behavior of the air is predicted from application of a fluid mechanical law: the principle of continuity. As this small fraction of air decelerates and flows through the air gaps in the antenna, it will be "stretched" in the plane perpendicular to the air flow. Therefore, the air may be sampled by the sensory hairs at a greater spatial resolution than expected from the distribution of the odorant molecules in the air upstream of the antenna. However, the slowing down of odorant-laden air as it passes through an antenna will not change the perceived temporal characteristics of the chemical stimulus (e.g., the rate of odorant filament encounter). This distortion or stretching of the air sample is expected to develop within about one antennal width upstream of the antenna, as verified by examining wakes of simple physical models.