Explain how the tracheal system of insects allows gas exchange to occur without the need for a circulatory system to transport respiratory gases.

The tracheal system consists of a network of tubes that branch from the spiracles into tracheae and then into very fine tracheoles that reach individual cells or tissues directly. Oxygen therefore diffuses straight from the tracheoles into the cells, and carbon dioxide diffuses out along the same short pathway. A concentration gradient is maintained because the cells continuously consume O₂ and produce CO₂, so the gradient keeps the gases moving. Because the tracheae bring air close to the cells, the circulatory system does not have to transport respiratory gases; haemolymph carries nutrients and hormones but not O₂ or CO₂.