Fig. 2. Experimental apparatus for evaluation of thoracic spiracle seals. (A) The thorax of Drosophila is sliced into halves along the sagittal plane and mounted on top of a flow-through respirometric chamber. Flight musculature is removed and the metathoracic spiracle is permanently sealed by epoxy glue. A 0.5 mm hole in the wall of the respirometric chamber permits ambient gas to be pulled through the open mesothoracic spiracle inside the chamber. A bell-shaped gas outlet mounted above the chamber allows alterations in ambient CO₂ concentration by connecting the gas tubing either to pressurized room air or to a CO₂ reservoir using an electric valve. (B) Example of how gas flux through the open mesothoracic spiracle varies while alternately connecting the gas tubing to room air (grey) and CO₂ (blue). Measuring units are given in parts-per-million analysed air (p.p.m.). (C) The mesothoracic spiracle seal completely blocks CO₂ flux into the respirometric chamber. (D) Removing the spiracle seal from the mesothoracic spiracle after testing restores spiracle conductance for carbon dioxide (same thorax half in B–D).