Spiracles are openings on the surface of some animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...

s that usually lead to respiratory system
Respiratory system
The respiratory system is the anatomical system of an organism that introduces respiratory gases to the interior and performs gas exchange. In humans and other mammals, the anatomical features of the respiratory system include airways, lungs, and the respiratory muscles...



The spiracle is a small hole behind each eye that opens to the mouth in some fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...

es. In the primitive jawless fish
Agnatha is a superclass of jawless fish in the phylum Chordata, subphylum Vertebrata. The group excludes all vertebrates with jaws, known as gnathostomes....

 the first gill opening immediately behind the mouth is essentially similar to the other gill opening. With the formation of the jaw
The jaw is any opposable articulated structure at the entrance of the mouth, typically used for grasping and manipulating food. The term jaws is also broadly applied to the whole of the structures constituting the vault of the mouth and serving to open and close it and is part of the body plan of...

 in the early jawed vertebrates, this gill slit was "caught" between the forward gill-rod (now functioning as the jaw) and the next rod, the hyomandibular bone
The hyomandibula, commonly referred to as hyomandibular [bone] , and Latin: mandibula, "jawbone") is a set of bones that is found in the hyoid region in most fishes. It usually plays a role in suspending the jaws and/or operculum...

, supporting the jaw hinge and anchoring the jaw to the skull
The skull is a bony structure in the head of many animals that supports the structures of the face and forms a cavity for the brain.The skull is composed of two parts: the cranium and the mandible. A skull without a mandible is only a cranium. Animals that have skulls are called craniates...

 proper. The gill opening was closed off from below, the remaining opening was small and hole-like, and is termed a "spiracle". The spiracle is still found in all cartilaginous fish
Chondrichthyes or cartilaginous fishes are jawed fish with paired fins, paired nares, scales, two-chambered hearts, and skeletons made of cartilage rather than bone...

 except chimaera
Chimaeras are cartilaginous fish in the order Chimaeriformes, known informally as ghost sharks, ratfish , spookfish , or rabbitfishes...

s, and in bony fishes
Osteichthyes , also called bony fish, are a taxonomic group of fish that have bony, as opposed to cartilaginous, skeletons. The vast majority of fish are osteichthyes, which is an extremely diverse and abundant group consisting of over 29,000 species...

 in the coelacanth
Coelacanths are members of an order of fish that includes the oldest living lineage of Sarcopterygii known to date....

, sturgeon
Sturgeon is the common name used for some 26 species of fish in the family Acipenseridae, including the genera Acipenser, Huso, Scaphirhynchus and Pseudoscaphirhynchus. The term includes over 20 species commonly referred to as sturgeon and several closely related species that have distinct common...

, paddlefish
Acipenseriformes are an order of primitive ray-finned fishes that includes the sturgeons and paddlefishes, as well as some extinct families.Notable characteristics of Acipenseriformes include:* Cartilaginous endoskeleton* Lack of vertebral centrum...

 and bichir
The bichirs are a family, Polypteridae, of archaic-looking ray-finned fishes, the sole family in the order Polypteriformes.All species occur in freshwater habitats in tropical Africa and the Nile River system, mainly swampy, shallow floodplains and estuaries.-Anatomy and appearance:Bichirs are...

s. It is also seen as an otic notch in the skull of the extinct labyrinthodonts
Labyrinthodontia is an older term for any member of the extinct subclass of amphibians, which constituted some of the dominant animals of Late Paleozoic and Early Mesozoic times . The group is ancestral to all extant landliving vertebrates, and as such constitutes an evolutionary grade rather...

, and is thought to be associated with the ear
The ear is the organ that detects sound. It not only receives sound, but also aids in balance and body position. The ear is part of the auditory system....

 opening in amniote
The amniotes are a group of tetrapods that have a terrestrially adapted egg. They include synapsids and sauropsids , as well as their fossil ancestors. Amniote embryos, whether laid as eggs or carried by the female, are protected and aided by several extensive membranes...

s and frog
Frogs are amphibians in the order Anura , formerly referred to as Salientia . Most frogs are characterized by a short body, webbed digits , protruding eyes and the absence of a tail...



Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...

s and some more advanced spider
Spiders are air-breathing arthropods that have eight legs, and chelicerae with fangs that inject venom. They are the largest order of arachnids and rank seventh in total species diversity among all other groups of organisms...

s have spiracles on their exoskeleton
An exoskeleton is the external skeleton that supports and protects an animal's body, in contrast to the internal skeleton of, for example, a human. In popular usage, some of the larger kinds of exoskeletons are known as "shells". Examples of exoskeleton animals include insects such as grasshoppers...

s to allow air to enter the trachea
Invertebrate trachea
The invertebrate trachea refers to the open respiratory system composed of spiracles, tracheae, and tracheoles that terrestrial arthropods have to transport metabolic gases to and from tissues....

. In the respiratory system of insects
Respiratory system of insects
The respiratory system of insects is the means by which insects obtain oxygen from the air.-The Respiratory System:Air enters the respiratory systems of most insects through a series of external openings called spiracles. These external openings, which act as muscular valves in some insects, lead...

, the tracheal tubes primarily deliver oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 directly into the animals' tissue
Biological tissue
Tissue is a cellular organizational level intermediate between cells and a complete organism. A tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function. These are called tissues because of their identical functioning...

s. The spiracles can be opened and closed in an efficient manner to reduce water loss. This is done by contracting closer muscles surrounding the spiracle. In order to open, the muscle relaxes. The closer muscle is controlled by the central nervous system
Central nervous system
The central nervous system is the part of the nervous system that integrates the information that it receives from, and coordinates the activity of, all parts of the bodies of bilaterian animals—that is, all multicellular animals except sponges and radially symmetric animals such as jellyfish...

but can also react to localized chemical stimuli. Several aquatic insects have similar or alternative closing methods to prevent water from entering the trachea. Spiracles may also be surrounded by hairs to minimize bulk air movement around the opening, and thus minimize water loss.
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