Summary

1. The tracheal system of insect wing muscle is so dense that between 10^-1 and 10^-3 of any cut area is occupied by air tubes. In most cases, air tube diffusion of O₂ and CO₂ through the muscle is therefore several thousand times quicker than diffusion in the liquid phase.

2. In large insects the primary tracheal supply must be strongly ventilated while diffusion is sufficient in the remaining part of the air tubes, even at the highest metabolic rates encountered in any insect.

3. The tracheoles represent the main site of exchange between the gaseous and the liquid phase while the tracheae are of little significance in this respect. The fibres cannot exceed about 20 µ in diameter unless the tracheoles indent the surface and become ‘internal’.

4. Muscular pumping of air and blood due to shortening is of little importance for the exchange of gases but of major importance for the supply with fuel for combustion. However, the large fibre diameters and the tidal nature of the pumping necessitates a very high concentration of fuel in the haemolymph. The high concentration of trehalose in insect blood is considered to be an essential adaptation to flapping flight.

5. The transport by diffusion of O₂ and CO₂ was followed in detail in a number of concrete examples in the gaseous as well as in the liquid phase. Within a safety factor of 2-3, the rate of transport was always found to be adequate. There is no reason to suggest other mechanisms than a simple, normal diffusion.