Tracheolen

Tracheolen, in English commonly called tracheoles, are the smallest components of the insect tracheal system. They arise from larger tracheal tubes and branch throughout tissues, ending near individual cells to deliver air directly where it is needed for respiration.

Structure and placement: Tracheolen are extremely fine tubes, roughly 0.2–0.5 micrometers in diameter, with thin walls that allow rapid gas exchange. They typically lack the rigid reinforcing taenidia found in larger tracheae, enabling them to extend into the spaces between cells. The tips of tracheolen are often in close proximity to mitochondria, creating short diffusion distances for oxygen and carbon dioxide. In some resting states, tracheolen can contain a thin film of fluid that can be displaced during activity to aid diffusion.

Function and physiology: Gas exchange occurs mainly by diffusion between the air within the tracheoles and surrounding tissues. The extensive branching increases surface area and minimizes diffusion distances, supporting high metabolic demands without a circulating blood system for oxygen transport. Ventilation of the tracheal system is achieved through movements that compress or expand air sacs and through rhythmic body movements, which help move air through the network during activity.

Ecology and variation: The tracheolar system is a key adaptation allowing insects to sustain a wide range of sizes and lifestyles on land. Some aquatic or semi-aquatic species use alternative respiration methods, such as plastron respiration or cutaneous exchange, in certain environments, reducing their reliance on tracheolar air delivery.