Influence of tracheal microstructure on insect respiratory flows

Insects breathe using a network of tracheal tubes that brings respiratory gasses directly to the tissues without using an intermediate oxygen carrier like blood. Mimicking the mechanics of this respiration in microscale fluid manipulations could lead to advances in microfluidic technology. Despite this significance, our current understanding of the underlying principles of insect respiration is limited. In this study, we consider the effect of irregularities in the tracheal wall, which contains local, microscale thickenings called taenidia. These periodic features, which present as 'bumps' on the wall, may have considerable impact in the slip flow regime for smaller tubes within the tracheal system. We carried out three-dimensional finite-volume simulations of air flow in idealized tracheal models and analyzed the effects of the geometric parameters of the taenidia. We performed simulations at a Reynolds number of 0.1 and Knudsen numbers ranging from 0.0005 to 0.1, which span the continuum and slip flow regimes, investigating the effects of rarefaction, taenidial spacing, taenidia-trachea scaling, and taenidia type. We found that the presence of taenidia can accelerate the peak flow, enhancing longitudinal dispersion. We further compared the flow in the idealized trachea to the flow through an anatomically accurate tracheal model from the darkling beetle Zophobas morio and found that small differences from the idealized geometries could lead to significant differences in flow patterns.