For rapid diffusion to take place, there has to be a steep diffusion gradient maintained in one direction.As oxygen is delivered to the cells from the tracheoles it diffuses across the cell membranes into respiring cells and is used up in respiration, so a steep concentration gradient is maintained between the cells and the tracheoles.Carbon dioxide continually produced by respiring cells creates a diffusion gradient from the cells to the tracheoles, so carbon dioxide diffuses out of the cells down a concentration gradient. As the insect is relatively small and diffusion in air is rapid, gases are exchanged at a sufficient rate.