Internally tracheae form branching invaginations of the cuticle deep to the spiracles, the internal diameters of the tracheae growing smaller and smaller at each bifurcation. This system of bifurcations transports oxygen directly to where it is utilized without its passing through the hemolymph. The terminal branches of the tracheae are the tracheoles. The tracheoles end on or close to the cells. For example, up to ten percent of the mass of flight muscle may be air tubes. There is, always a tradeoff between filling a muscle's volume with muscle fibers or airways, as space on board is at a premium. Some of the smallest terminals of the tracheoles may even indent cell membranes, reducing the distances for diffusion to the mitochondria. Diffusion through these smallest air tubes is continuous with diffusion of oxygen through the tissues. At rest, tracheoles contain some liquid. During flight, however, this liquid is absorbed, so that now a continuous pathway of air supplies the increased metabolic demand of the mitochondria, as the diffusion of oxygen occurs faster in air than in water. The lengths of the paths for tissue diffusion, however, set an upper limit on how big tissues, organs and ultimately the size of an insect may be.

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