One is an air tube transport phase and the other is a tissue transport phase. In the early days of insect physiology, it was not at all clear from a simple consideration of the tracheal system that diffusion of gasses could account for the movement of oxygen and carbon dioxide to and from the tissues. This was the argument that led to the general acceptance of the diffusion theory in tracheal transport of oxygen. Recall that there are two diffusion phases, an air tube phase and a tissue phase. Krogh also calculated permeability constants for oxygen.
Permeability constants are a linear function of diffusion coefficients.
We can infer that oxygen diffuse faster in air than in tissue. To be more precise, oxygen diffuses 785,714.28571 times faster in air than in tissue. Consider a concentration gradient along a tracheal tube of 3.0 cm length. If the gradient were sufficient for the diffusion of oxygen over the 3 cm of air phase diffusion, that same gradient would be adequate for a diffusion distance of about 0.000003818 cm in the tissue phase. This is about 0.04 microns.