Editing Arachnid (section)

==Physiology==
{{See also|Hemolymph}}
There are characteristics that are particularly important for the terrestrial lifestyle of arachnids, such as internal respiratory surfaces in the form of [[Invertebrate trachea|trachea]]e, or modification of the [[book gill]] into a [[book lung]], an internal series of [[Blood vessel|vascular]] [[lamella (zoology)|lamella]]e used for [[gas exchange]] with the air.<ref name=Garwood>{{cite journal |last1=Garwood |first1=Russell J. |last2=Edgecombe |first2=Gregory D. |date=September 2011 |title=Early Terrestrial Animals, Evolution, and Uncertainty |journal=Evolution: Education and Outreach |volume=4 |issue=3 |pages=489–501 |doi=10.1007/s12052-011-0357-y |name-list-style=amp|doi-access=free }}</ref> While the tracheae are often individual systems of tubes, similar to those in insects, ricinuleids, pseudoscorpions, and some spiders possess sieve tracheae, in which several tubes arise in a bundle from a small chamber connected to the [[Spiracle (arthropods)|spiracle]]. This type of tracheal system has almost certainly evolved from the book lungs, and indicates that the tracheae of arachnids are not [[Homology (biology)|homologous]] with those of insects.<ref name=IZ/>

Further adaptations to terrestrial life are [[appendage]]s modified for more efficient locomotion on land, [[internal fertilisation]], special sensory organs, and water conservation enhanced by efficient [[excretory]] structures as well as a waxy layer covering the cuticle.{{citation needed|date=March 2025}}

The excretory glands of arachnids include up to four pairs of [[coxal gland]]s along the side of the prosoma, and one or two pairs of [[Malpighian tubule]]s, emptying into the gut. Many arachnids have only one or the other type of excretory gland, although several do have both. The primary nitrogenous waste product in arachnids is [[guanine]].<ref name=IZ/>

Arachnid blood is variable in composition, depending on the mode of respiration. Arachnids with an efficient tracheal system do not need to transport oxygen in the blood, and may have a reduced circulatory system. In scorpions and some spiders, however, the blood contains [[haemocyanin]], a copper-based pigment with a similar function to [[haemoglobin]] in vertebrates. The [[heart]] is located in the forward part of the abdomen, and may or may not be segmented. Some mites have no heart at all.<ref name=IZ/>