Helox suggests advective ventilation of the insect tracheal system during discontinuous gas exchange

About a century ago, Krogh theorised that gas exchange by insects between spiracles and tissues occurs by diffusion without an advective (convective) component but physiological data supporting the relative roles of diffusion and advection are unclear, especially for small, flightless insects that lack air sacs. We examined the role of diffusion and advection in gas exchange for the small red flour beetle (Tribolium castaneum; 1.5 mg; no air sacs) and the larger speckled cockroach (Nauphoeta cinerea; 700 mg; air sacs) by comparing spiracular cycle durations during discontinuous gas exchange in air and helox (21% oxygen in helium), which should increase diffusive but not advective exchange. Helox reduced the burst-phase duration of beetles by 13%, suggesting a substantial role for advection and a minor role for diffusion, challenging the theory that small insects rely on diffusion and consistent with evidence for advective gas exchange mediated by tracheal compression. For cockroaches, burst phase duration was unaffected, supporting a primary role of advection and unmeasurable diffusion. We conclude that helox may provide a novel and useful experimental approach to discriminate between tracheal diffusion and advection. A mechanistic understanding of tracheal gas exchange for the red flour beetle could facilitate development of physiologically informed fumigation regimes for pest management.