Valve structures

The tracheal orifice, through which air enters the tracheal system, is surrounded by a set of mobile lips and fixed bars. The mobile lips are attached to a sclerotized (=hardened) lever. The lever is operated by a closer muscle. Counteracting to the closer muscle is either an elastic ligament or an opener muscle. Muscles and ligaments can be either attached to sructures of the spiracle or to structures of the body wall.

Valve function

Click on the image to see an animated video (AVI format) of an active spiracle.

The closer muscle is under control of the central nervous system. When the closer muscle gets the signal from the motoneuron, it contracts and pulls the lever. The lever is attached to a set of mobile lips. The lips bend inwards and occlude the tracheal orifice. In some insects the sealing is so perfect, that no air can enter. This is one of the prerequisites for Discontinuous Gas Exchange (inactive link).

When the signal of the central nervous system stops or the closer muscle is anesthetized, the muscle relaxes and the spiracular valve opens again. In some insect groups (Butterflies, Beetles) the spiracles opens passively by the elastic contraction of a ligament. In other insect groups (...) the spiracle is opened actively by an opener muscle.