The spiracles are clearly visible as black dots on the side of each body segment of this beetle larva.
Insects take in Oxygen and expel Carbon Dioxide using a series of internal air tubes, the tracheae. These pass fine branches, the tracheoles, to all parts of the body. The cuticle lines the tracheae. These linings are shed with the rest of the cuticle when the insect moults. There is a main pair of lateral tracheae running the length of the body, one on each side. Some insects show two such pairs, one dorsal, the other ventral.
From the main trunks, tracheoles pass throughout the body. The tracheae open to the air at the paired spiracles. There is one pair per segment in the thorax and also in the first nine abdominal segments. This arrangement does vary with different species. In most species the spiracles on the first segment of the thorax are missing. The pair on the ninth abdominal segment is also missing.
Diagram of Part of the Tracheal System of a Cockroach
The tracheoles end within the body cells. Gases move by diffusion within the tracheal system. When the insect is less active the ends of the tracheoles contain fluid. It is where the fluid and gas meet (= the fluid/gas interface), that exchange of gases occurs (oxygen is taken up, carbon dioxide is given off). As activity increases, the fluid is removed from the tracheoles. This means that the exchange of gases occurs nearer the cells. In the extreme case of fatigued flight muscle, the exchange interface lies within the muscle cells.
Gas/Water interface in the tracheole
Diffusion alone does not meet the respiratory demands of the animal. Gases are moved, to a large extent, by pumping actions of the body segments.
The respiratory system of an insect is very efficient for small organisms. As body size increases, the efficiency decreases. When body diameter exceeds about 3cm, the respiratory needs cannot be met. Hence it is the respiratory system of insects which restricts their body size. Note that no transport system, such as blood, is involved in moving oxygen or carbon dioxide around the body.