Spiracles are external openings found in some animal species, such as insects, spiders and some species of fish and whales. The function of spiracles is linked to respiration, helping oxygen to reach internal respiratory organs, such as lungs in whales and tracheae in insects. In vertebrate species, spiracles are found in the head while in invertebrates these organs are located in the thorax or abdomen.
Spiracles in Insects
Insects have a network of tubes called tracheae or tracheal tubes, which are responsible for their respiration. Oxygen enters these tubes through several spiracles, which are valve-like openings in the exoskeleton. In insects, spiracles are situated laterally on the thorax and abdomen. Small muscles in each spiracle control their opening and air flow. Carbon dioxide is also expelled through the spiracles.
Spiracles in Arachnids
Members of the class Arachnida, spiders and scorpions, also use spiracles to breathe. Spiracles control the air flow in and out of the body of arachnids, which have two types of respiratory organs: tracheae and book lungs. Pseudoscorpions (Pseudoscorpionida), sunspiders or camel spiders (Solifugae), hooded tickspiders or ricinuleids and daddy longlegs have only tracheae. Scorpions, whip scorpions (Thelyphonida) and tailless whip scorpions (Amblypygi) have organs called book lungs connected to their spiracles. Most spider species have both tracheae and book lungs linked to the spiracles. Unlike insects, arachnids only have one or two spiracles.
Spiracles in Fish
Some species of elasmobranch fish, such as sharks, rays and skates, have a pair of spiracles that originated from the gills during their evolution. In fish, the gills are the main respiratory organs: oxygenated water enters the mouth and passes through the gills, where oxygen enters the bloodstream. In addition to increasing the intake of oxygen from the water, spiracles also make possible closed mouth-breathing. Oxygen that enters the spiracles reaches the bloodstream through diffusion and supplies the cells of important organs, such as the brain and eyes. In fish, spiracles are located near the eyes.
Spiracles in Whales
Like their terrestrial counterparts, whales, dolphins and other aquatic mammals have a pair of lungs to breathe. However, instead of nostrils like other mammals, whales and dolphins have a spiracle on the top of their head, from which they inhale and exhale air. These aquatic mammals have a muscular flap in their spiracle that controls their opening and closing, thus avoiding water entering the lungs.