Book lung

<h2 id="structure-and-function">Structure and function</h2>
<p class="note">Further information: <a href="/facts/Spider_anatomy/zk901PKI">Spider anatomy</a></p>
<p>Book lungs are not related to the <a href="/facts/Lungs/gTWXI6Fj">lungs</a> of modern land-dwelling <a href="/facts/Vertebrate/tT54FSTT">vertebrates</a>. Their name instead describes their structure and purpose as a case of <a href="/facts/Convergent_evolution/dd0L8z4R">convergent evolution</a>. Stacks of alternating air pockets and tissue filled with <a href="/facts/Hemolymph/6tHgrY6c">hemolymph</a><a class="footnote-ref" id="fnref:1" href="#fn:1"><sup>1</sup></a> give them an appearance similar to a "folded" book.<a class="footnote-ref" id="fnref:2" href="#fn:2"><sup>2</sup></a>
</p><p>Their number varies from just one pair in most spiders to four pairs in scorpions. The unfolded "pages" (plates) of the book lung are filled with hemolymph. The folds maximize the surface exposed to <a href="/facts/Air/hQ95Jznz">air</a>, and thereby maximize the amount of <a href="/facts/Gas/hVVAoodV">gas</a> exchanged with the environment. In most species, no motion of the plates is needed to facilitate this kind of respiration.
</p>
<h3>Occasionally absent</h3>
<p>Many <a href="/facts/Arachnid/xPJbsLga">arachnids</a>, such as <a href="/facts/Mite/M52qFjlE">mites</a> and <a href="/facts/Opiliones/uhNdARTD">harvestmen</a>, have no traces of book lungs and breathe through their body-surfaces only or through <a href="/facts/Invertebrate_trachea/zMSEwjAy">tracheae</a>. Gas exchange is performed by the thin walls inside the cavity instead, or with their surface area increased by branching into the body as thin tubes, the tracheae. These tracheae may possibly have evolved directly from book lungs because the tracheae in some spiders have a small number of greatly elongated chambers.
</p>
<h2 id="arachnid-taxonomy">Arachnid taxonomy</h2>
<p>The absence or presence of book lungs divides the Arachnida into two main groups:
</p>
The pulmonate arachnids
book lungs present; <a href="/facts/Tetrapulmonata/E2D06yKY">Tetrapulmonata</a> (<a href="/facts/Whip_scorpion/9pBlKGm2">whip scorpions</a>, <a href="/facts/Schizomida/R1VS0FSc">Schizomida</a>, <a href="/facts/Amblypygi/57IJC9zI">Amblypygi</a>, and <a href="/facts/Spider/aqpU7eg3">spiders</a>) and <a href="/facts/Scorpion/clJKHDdY">scorpions</a>
The a-pulmonate arachnids
book lungs absent; <a href="/facts/Microwhip_scorpion/74Qfarof">microwhip scorpions</a>, <a href="/facts/Opiliones/uhNdARTD">harvestmen</a>, <a href="/facts/Acarina/XCs0r7gN">Acarina</a>, <a href="/facts/Pseudoscorpion/tMl9v40m">pseudoscorpions</a>, <a href="/facts/Ricinulei/QodJckDz">Ricinulei</a>, and <a href="/facts/Solifugae/mJ4r0p6y">sunspiders</a>
<p>Tetrapulmonata have two pairs of book lungs found on the second and third abdominal segments (Schizomida have lost a pair, and most advanced spiders have replaced at least one of the pairs with trachea). Scorpions have four pairs of book lungs, found on abdominal segments number three, four, five, and six.<a class="footnote-ref" id="fnref:3" href="#fn:3"><sup>3</sup></a>
</p><p>The pulmonate arachnids also appears to be the only members of Arachnida where the respiratory pigment <a href="/facts/Hemocyanin/kCxIlQW9">hemocyanin</a> is present in their blood.<a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a>
</p><p>One of the long-running controversies in arachnid evolution is whether the book lung evolved from book gills just once in a common arachnid ancestor,<a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a> or whether book lungs evolved separately in several groups of arachnids as they came onto land. While the third abdominal segment in Tetrapulmonata have book lungs, the scorpions have a pair of sensory organs called pectines instead.
</p><p>The oldest book lungs have been recovered from extinct <a href="/facts/Trigonotarbid/RhQL3eYk">trigonotarbid</a> arachnids preserved in the 410 million-year-old <a href="/facts/Rhynie_chert/7ywl5WMG">Rhynie chert</a> of Scotland. These <a href="/facts/Devonian/nWgakqzk">Devonian</a> fossil lungs are almost indistinguishable from the lungs of modern arachnids, fully adapted to a terrestrial existence.<a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a>
</p>
<h2 id="book-gills">Book gills</h2>

<p>Book lungs are thought to have evolved from book <a href="/facts/Gill/FWgM3Jp8">gills</a>, water-breathing structures among marine <a href="/facts/Chelicerates/9YNa15kz">chelicerates</a>. Although they have a similar book-like structure, book gills are external, while book lungs are internal.<a class="footnote-ref" id="fnref:7" href="#fn:7"><sup>7</sup></a> Both are considered appendages rather than conventional internal organs, as they develop from limb buds before the buds flatten into segmented <a href="/facts/Lamella_(surface_anatomy)/1VNAwSq6">lamellae</a>. <a class="footnote-ref" id="fnref:8" href="#fn:8"><sup>8</sup></a>
</p><p>Book gills are still present in the marine arthropod <i><a href="/facts/Limulus/QT4e4wkd">Limulus</a></i> (<a href="/facts/Horseshoe_crab/IuaBGa9T">horseshoe crabs</a>) which have five pairs of them, the flap in front of them being the genital operculum which lacks gills. Book gills are flap-like appendages that effect gas exchange within water and seem to have their origin as modified legs. On the inside of each appendage, over 100 thin page-like membranes, <i>lamellae</i>,  appearing as pages in a book, are where gas exchange takes place. These appendages move rhythmically to drive blood in and out of the lamellae and to circulate water over them. Respiration being their main purpose, they can also be used for swimming in young individuals. If they are kept moist, the horseshoe crab can live on land for many hours.
</p>
<h2 id="footnotes">Footnotes</h2>

<h2 id="references">References</h2>

<ol>
<li id="fn:1"><p>Hemolymph is the arthropod equivalent of blood. <a href="/wiki/Hemolymph" target="_blank">/wiki/Hemolymph</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li>
<li id="fn:2"><p>Foelix, Rainer F. (1996). Biology of Spiders. Oxford University Press US. pp. 61–64. ISBN 0-19-509594-4. book lung. <a href="0-19-509594-4" target="_blank">0-19-509594-4</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li>
<li id="fn:3"><p>The origins of tetrapulmonate book lungs and their significance for chelicerate phylogeny <a href="https://www.european-arachnology.org/esa/wp-content/uploads/2015/08/009-016_Dunlop.pdf" target="_blank">https://www.european-arachnology.org/esa/wp-content/uploads/2015/08/009-016_Dunlop.pdf</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li>
<li id="fn:4"><p>The diversity and evolution of chelicerate hemocyanins <a href="https://bmcecolevol.biomedcentral.com/articles/10.1186/1471-2148-12-19" target="_blank">https://bmcecolevol.biomedcentral.com/articles/10.1186/1471-2148-12-19</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li>
<li id="fn:5"><p>Scholtz, Gerhard; Kamenz, Carsten (2006). "The book lungs of Scorpiones and Tetrapulmonata (Chelicerata, Arachnida): Evidence for homology and a single terrestrialisation event of a common arachnid ancestor". Zoology. 109 (1): 2–13. doi:10.1016/j.zool.2005.06.003. PMID 16386884. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li>
<li id="fn:6"><p>Kamenz, Carsten; Dunlop, Jason A.; Scholtz, Gerhard; Kerp, Hans; Hass, Hagen (2008). "Microanatomy of early Devonian book lungs". Biology Letters. 4 (2). London, UK: Royal Society: 212–215. doi:10.1098/rsbl.2007.0597. PMC 2429929. PMID 18198139. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2429929" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2429929</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li>
<li id="fn:7"><p>Bhamrah, H. S.; Juneja, Kavita (2002). An Introduction to Arthropoda. Anmol Publications Pvt. Ltd. ISBN 81-261-0673-5. <a href="81-261-0673-5" target="_blank">81-261-0673-5</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></p></li>
<li id="fn:8"><p>Pechmann, Matthias; Khadjeh, Sara; Sprenger, Frederik; Prpic, Nikola-Michael (November 2010). "Patterning mechanisms and morphological diversity of spider appendages and their importance for spider evolution". Arthropod Structure & Development. 39 (6): 453–67. doi:10.1016/j.asd.2010.07.007. PMID 20696272. Retrieved 20 August 2020. <a href="https://www.sciencedirect.com/science/article/pii/S1467803910000551" target="_blank">https://www.sciencedirect.com/science/article/pii/S1467803910000551</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></p></li>
</ol>

Book lung open-in-new

Book lung