Spiracle (arthropods)

In insects, the external spiracular opening commonly leads into an atrium and a valve apparatus that regulates airflow into the tracheal trunks. The rim of the opening (often termed the peritreme) may bear cuticular hairs or filtering structures that reduce entry of dust, pathogens, and liquid water while also limiting convective water loss. Comparative studies highlight substantial diversity in spiracle form across insect orders, including differences in atrial depth, cuticular filter design, and the mechanical arrangement of valve elements. These anatomical features can be interpreted as tradeoffs among protection, resistance to airflow, and the speed and precision of opening and closing. == Control of spiracles ==

In most species, the spiracles are controlled by motor neurons in the central nervous system. It can be opened and closed in an efficient manner to admit air while minimizing associated physiological costs, such as water loss during respiration. Many sensory stimuli can affect the control of spiracles in insects, e.g., chemosensory (carbon dioxide, oxygen, etc.) or mechanosensory (sound, touch, etc.). It has been shown that during metabolically intensive behaviors, such as flight, insects can dynamically modulate the spiracle opening size just so to meet the metabolic demand of flight, while not losing too much water. Control of spiracle opening is done by a wide range of mechanisms, such as elastic closure, and closer muscles surrounding the spiracle or kinking the tube. In some the muscle relaxes to open the spiracle, in others to close it. Several aquatic insects have similar or alternative closing methods to prevent water from entering the trachea. The timing and duration of spiracle closures can affect the respiratory rates of the organism. Spiracles may also be surrounded by hairs to minimize bulk air movement around the opening, and thus minimize water loss. In larger insects, spiracle control is more complex and critical for managing gas exchange due to their higher metabolic demands. Larger insects, such as locusts and some beetles, exhibit active ventilation, where spiracle control works in concert with abdominal movements. These abdominal contractions force air in and out of the tracheal system, and the spiracles open and close in a synchronized manner to maximize oxygen intake and carbon dioxide expulsion. This active process allows these insects to regulate their internal environment more precisely, especially during periods of high activity, such as flight. Research has shown that neural circuits in the insect's central nervous system adjust the spiracle opening in response to carbon dioxide concentration, ensuring efficient gas exchange and preventing hypoxia or hypercapnia. Other body parts, such as the proboscis, might also extend or contract so as to ventilate the insect during various behaviors. == Spiracles in other animals besides insects ==

Most myriapods have paired lateral spiracles similar to those of insects. Scutigeromorph centipedes are an exception, having unpaired, non-closable spiracles at the posterior edges of tergites. Harvestmen, camel spiders, ricinuleids, mites, and pseudoscorpions all breathe through a tracheal system and lack book lungs. == Literature ==

• Chapman, R.F. (1998): The Insects, Cambridge University Press ==References==