Editing Trachea (section)

==Other animals==
Allowing for variations in the length of the neck, the trachea in other [[mammal]]s is, in general, similar to that in humans. Generally, it is also similar to the [[reptile|reptilian]] trachea.<ref name=VB>{{cite book | vauthors = Romer AS, Parsons TS |year=1977 |title=The Vertebrate Body |publisher=Holt-Saunders International |location= Philadelphia, PA|pages= 336–337|isbn= 978-0-03-910284-5}}</ref>

===Vertebrates===
In [[bird]]s, the trachea runs from the [[pharynx]] to the [[syrinx (biology)|syrinx]], from which the primary bronchi diverge. [[Swan]]s have an unusually elongated trachea, part of which is coiled beneath the sternum; this may act as a resonator to amplify sound. In some birds, the tracheal rings are complete, and may even be [[ossification|ossified]].<ref name=VB/>

In [[amphibian]]s, the trachea is normally extremely short, and leads directly into the lungs, without clear primary bronchi. A longer trachea is, however, found in some long-necked [[salamander]]s, and in [[caecilian]]s. While there are irregular cartilagenous nodules on the amphibian trachea, these do not form the rings found in [[amniote]]s.<ref name=VB/>

The only [[vertebrate]]s to have lungs, but no trachea, are the [[lungfish]] and the ''[[Polypterus]]'', in which the lungs arise directly from the pharynx.<ref name=VB/>

===Invertebrates===
[[File:Tracheal system of dissected cockroach.tif|thumb|left|Tracheal system of dissected [[cockroach]].  The largest tracheae run across the width of the body of the cockroach and are horizontal in this image. Scale bar, 2 mm.]]
[[File:Cockroach tracheae supplying crop.tiff|thumb|The tracheal system branches into progressively smaller tubes, here supplying the [[Crop (anatomy)|crop]] of the cockroach.  Scale bar, 2 mm.]]

The word ''trachea'' is used to define a very different organ in [[invertebrate]]s than in vertebrates. Insects have an open [[respiratory system of insects|respiratory system]] made up of spiracles, tracheae, and [[tracheole]]s to transport [[metabolism|metabolic]] gases to and from tissues.<ref>Wasserthal, Lutz T. (1998). Chapter 25: The Open Hemolymph System of Holometabola and Its Relation to the Tracheal Space. In "Microscopic Anatomy of Invertebrates". Wiley-Liss, Inc. {{ISBN|0-471-15955-7}}.</ref> The distribution of spiracles can vary greatly among the many [[order (biology)|orders]] of [[insect]]s, but in general each segment of the body can have only one pair of spiracles, each of which connects to an atrium and has a relatively large tracheal tube behind it. The tracheae are invaginations of the cuticular [[exoskeleton]] that branch ([[anastomosis|anastomose]]) throughout the body with diameters from only a few micrometres up to 0.8&nbsp;mm. [[Diffusion]] of oxygen and [[carbon dioxide]] takes place across the walls of the smallest tubes, called tracheoles, which penetrate tissues and even indent individual cells.<ref name=":0">{{Cite book|title=Borror and DeLong's introduction to the study of insects| vauthors = Triplehorn C, Johnson NF, Borror DJ |publisher=Thompson Brooks/Cole |year=2005|isbn=978-0030968358|edition= 7th |location=Belmont, CA|pages=28–29|oclc=55793895}}</ref> Gas may be conducted through the respiratory system by means of active [[Ventilation (physiology)|ventilation]] or passive diffusion. Unlike vertebrates, insects do not generally carry oxygen in their [[hemolymph]].<ref>{{cite journal | vauthors = Westneat MW, Betz O, Blob RW, Fezzaa K, Cooper WJ, Lee WK | title = Tracheal respiration in insects visualized with synchrotron x-ray imaging | journal = Science | volume = 299 | issue = 5606 | pages = 558–560 | date = January 2003 | pmid = 12543973 | doi = 10.1126/science.1078008 | s2cid = 43634044 | bibcode = 2003Sci...299..558W }}</ref>
This is one of the factors that may limit their size.

A tracheal tube may contain ridge-like circumferential rings of [[taenidia]] in various [[geometry|geometries]] such as loops or [[helix|helices]]. Taenidia provide strength and flexibility to the trachea. In the head, thorax, or [[abdomen]], tracheae may also be connected to air sacs. Many insects, such as [[grasshopper]]s and [[bee]]s, which actively pump the air sacs in their abdomen, are able to control the flow of air through their body. In some aquatic insects, the tracheae exchange gas through the body wall directly, in the form of a [[gill]], or function essentially as normal, via a [[Gill#Plastron|plastron]]. Note that despite being internal, the tracheae of arthropods are lined with cuticular tissue and are shed during moulting ([[ecdysis]]).<ref name=":0" />