Why Do Some Cicadas Appear Only Every 17 Years?

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Please refer to the appropriate style manual or other sources if you have any questions. </div> <div class="label mt-20 mb-10">Select Citation Style</div> <select class="js-citation-format-select form-select"> <option selected value="mla">MLA</option> <option value="apa">APA</option> <option value="chicago">Chicago Manual of Style</option> </select> <div class="citation font-serif border rounded p-15 mt-20" data-authors="Matthias, Meg" data-authors-initial="Matthias, M." data-title="Why Do Some Cicadas Appear Only Every 17 Years?" data-published-date="13 Apr. 2021" data-url="https://www.britannica.com/story/why-do-some-cicadas-appear-only-every-17-years" > <div class="citation-text"></div> </div> <button class="js-copy-citation-button mt-20 btn btn-xs btn-outline-blue border shadow-sm pr-10" > <em class="material-icons md-icon ml-n10 my-n5 mr-5" data-icon="file_copy"></em> <span class="js-citation-status-text">Copy Citation</span> </button> </div> </div> </div> <div> <button class="js-share-modal-button js-modal gtm-topic-tool btn btn-xs btn-outline-blue border shadow-sm pr-10" data-modal=".md-share-modal"> <em class="material-icons md-icon ml-n10 my-n5 mr-5" data-icon="share"></em> Share </button> <div class="md-share-modal size-lg d-none qa-share-modal"> <div class="md-modal-body"> <div class="h2 font-serif d-flex align-items-center pb-15 border-bottom"> <em class="material-icons text-blue mr-10" data-icon="share"></em> Share </div> <div class="label my-20">Share to social media</div> <div class="md-social-toolbar-circle d-flex align-items-start inverted" data-value="share" title="Why Do Some Cicadas Appear Only Every 17 Years?" data-url="https://www.britannica.com/story/why-do-some-cicadas-appear-only-every-17-years" > <a class="social-icon facebook justify-content-center d-flex align-items-center align-self-center" data-provider="facebook" href="https://www.facebook.com/BRITANNICA/" target="_blank" rel="noopener"><span>Facebook</span></a> <a class="social-icon x justify-content-center d-flex align-items-center align-self-center" data-provider="x" href="https://x.com/britannica" target="_blank" rel="noopener"><span>X</span></a> </div> <div class="label pt-20 mt-20 mb-5 border-top">URL</div> <a class="font-serif text-truncate d-inline-block" href="https://www.britannica.com/story/why-do-some-cicadas-appear-only-every-17-years">https://www.britannica.com/story/why-do-some-cicadas-appear-only-every-17-years</a> </div> </div> </div> </div> <div class="md-byline module-spacing font-12"> <div class="font-serif font-12"> <span class="written-by text-gray-700"> Written by </span> <div class="editor-popover popover p-0"> <a class="d-block p-20 qa-editor-popup gtm-byline font-12 byline-contributor" href="/editor/Meg-Matthias/12593612" > <div class="editor-title font-16 font-weight-bold">Meg Matthias</div> <div class="editor-description font-12 font-serif mt-5 clamp-description text-black">Meg Matthias is Senior Video Production Manager at Encyclopædia Britannica.</div> </a> <div data-popper-arrow></div> </div> <span class="btn btn-link editor-link p-0 qa-byline-link gtm-byline font-12 byline-contributor text-decoration-underline"> Meg Matthias</span></div> <div class="font-serif font-12 text-gray-700"> <span class="qa-fact-checked-by">Fact-checked by</span> <div class="editor-popover popover p-0"> <a class="d-block p-20 qa-editor-popup font-12" href="/editor/The-Editors-of-Encyclopaedia-Britannica/4419" > <div class="editor-title font-16 font-weight-bold">The Editors of Encyclopaedia Britannica</div> <div class="editor-description font-12 font-serif mt-5 text-black">Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree. They write new content and verify and edit content received from contributors.</div> </a> <div data-popper-arrow></div> </div> <span class="btn btn-link editor-link p-0 qa-byline-link font-12 "> The Editors of Encyclopaedia Britannica</span></div></div> </div> <article class="topic-content content mt-20"><figure class="md-assembly md-feature-assembly print-false" data-type="1" data-descid="1"><div class="md-assembly-wrapper card-media" data-type="image"><a href="https://cdn.britannica.com/54/222554-050-09076121/periodical-cicada.jpg" target="_blank" class="gtm-assembly-link position-relative d-flex align-items-center justify-content-center " data-href="*"><img src="https://cdn.britannica.com/54/222554-050-09076121/periodical-cicada.jpg?w=600&q=60" alt="Periodical Cicada, Adult, Magicicada spp. Requires 17 years to complete development. Nymph splits its skin, and transforms into an adult. Feeds on sap of tree roots. Northern Illinois Brood. This brood is the largest emergence of cicadas anywhere" data-width="1600" data-height="1053" loading="eager"><button class="magnifying-glass btn btn-circle position-absolute shadow btn-white top-10 right-10" aria-label="Zoom in"><em class="material-icons link-blue" data-icon="zoom_in"></em></button></a></div><figcaption class=""><div class="md-assembly-caption text-muted font-14 font-serif line-clamp"><span><button class="js-more-btn d-none btn btn-unstyled font-12 bg-white js-content" aria-label="Toggle more/less fact data"><span class="link-blue">(more)</span></button></span></div><cite class="credit d-block mt-5">© Ed Reschke-Stone/Getty Images</cite></figcaption></figure> <p class="topic-paragraph">You already know <a href="https://www.britannica.com/story/why-are-cicadas-so-noisy" class="md-crosslink " data-show-preview="true">why cicadas are so unbelievably noisy</a>. But why do some of them appear aboveground only every 17 years?</p><span class="MOD1"></span> <p class="topic-paragraph">The 17-year <a href="https://www.britannica.com/animal/cicada" class="md-crosslink " data-show-preview="true">cicadas</a> are species of <a href="https://www.britannica.com/animal/homopteran/Natural-history#ref134271" class="md-crosslink " data-show-preview="true">periodical cicadas</a>, a group of <a href="https://www.britannica.com/animal/homopteran" class="md-crosslink " data-show-preview="true">homopterans</a> with the longest known insect life cycle. The largest brood makes its appearance every 17 years, like clockwork, in the northeastern quarter of the United States. Shortly after a 17-year cicada <a href="https://www.britannica.com/science/nymph-entomology" class="md-crosslink " data-show-preview="true">nymph</a> hatches from its egg, it burrows into the ground, where it spends—as its name suggests—the first 17 years of its life. When it emerges from the ground, it lives only four to six more weeks—just long enough to mate, fertilize or lay eggs, and start the cycle all over again.</p><span class="MOD2"></span> <p class="topic-paragraph">Contrary to popular misconception, periodical cicadas don’t spend their years underground in <a href="https://www.britannica.com/science/hibernation" class="md-crosslink " data-show-preview="true">hibernation</a>. Rather, they are conscious and active in their wingless nymph forms, excavating tunnels and feeding on the sap from tree roots.</p><span class="MOD3"></span> <figure class="md-assembly md-feature-assembly print-false" data-type="1" data-descid="5"><div class="md-assembly-wrapper card-media" data-type="image"><a href="https://cdn.britannica.com/32/222432-050-F01F54BF/map-emergence-years-cicada-broods-United-States.jpg" target="_blank" class="gtm-assembly-link position-relative d-flex align-items-center justify-content-center " data-href="*"><img src="https://cdn.britannica.com/32/222432-050-F01F54BF/map-emergence-years-cicada-broods-United-States.jpg" alt="Map of the emergence years for cicada broods in the Eastern United States." data-width="2000" data-height="1806" loading="eager"><button class="magnifying-glass btn btn-circle position-absolute shadow btn-white top-10 right-10" aria-label="Zoom in"><em class="material-icons link-blue" data-icon="zoom_in"></em></button></a></div><figcaption class=""><div class="md-assembly-caption text-muted font-14 font-serif line-clamp"><span><button class="js-more-btn d-none btn btn-unstyled font-12 bg-white js-content" aria-label="Toggle more/less fact data"><span class="link-blue">(more)</span></button></span></div><cite class="credit d-block mt-5">Encyclopædia Britannica, Inc./Kenny Chmielewski</cite></figcaption></figure> <p class="topic-paragraph">The cicada nymphs’ emergence from their underground homes is tied primarily to temperature. After their 17 years are up, the cicadas wait for the perfect temperature before making the journey to the surface—when the soil about 8 inches (20 cm) underground reaches 64 °F, or 18 °C. This means that cicadas in different parts of the United States appear at different times: the soil in Virginia might reach that ideal temperature before the soil in Illinois. But once that temperature <em>is</em> reached, all the cicadas in the area will feel it and make the journey to the surface together.</p><span class="MOD4"></span> <p class="topic-paragraph">But how do the cicadas know when 17 years have passed? Though no one theory has been proven, many scientists speculate that periodical cicadas possess an internal <a href="https://www.britannica.com/science/molecular-clock" class="md-crosslink " data-show-preview="true">molecular clock</a> that notes the passage of years through environmental cues. As trees go through their seasonal cycles, shedding and growing leaves, the composition of their sap changes. And when cicada nymphs feed on that sap, they likely pick up clues about the passage of time. The 17th iteration of the trees’ seasonal cycle gives the nymphs their final cue: it’s time to emerge.</p><span class="MOD5"></span> <figure class="md-assembly md-feature-assembly print-false" data-type="3" data-descid="7"><div class="md-assembly-wrapper card-media" data-type="video" video-id="254588"><a data-id="254588" class="gtm-assembly-link d-flex justify-content-center" style="--aspect-ratio: 16/9" href="/video/demystified-cicadas-life-cycles-periodical-vs-annual-17-year-and-13-year-cycles-brood/-"><img src="https://cdn.britannica.com/88/254588-138-68EEDA64/demystified-cicadas-life-cycles-periodical-vs-annual-17-year-and-13-year-cycles-brood.jpg?w=800&h=450&c=crop" alt="Demystified: Why do some cicadas only appear every 17 years? Periodical vs. annual cicadas, 17-year and 13-year cicadas, prey satiation, broods, brood." loading="lazy"><script type="application/json"> { "sources": [ { "file" : "//content.jwplatform.com/manifests/8UDXtuVs.m3u8" } ], "image": "https://cdn.britannica.com/88/254588-138-68EEDA64/demystified-cicadas-life-cycles-periodical-vs-annual-17-year-and-13-year-cycles-brood.jpg" ,"tracks": [ { "file" : "//assets-jpcust.jwpsrv.com/tracks/1hWfq0dB.vtt", "label": "English" } ] ,"adfile": "//content.jwplatform.com/manifests/iYO2U68a.m3u8" } </script><div class="btn btn-xl btn-white btn-circle position-absolute shadow" style="top: 50%; transform: translateY(-50%)"><em class="material-icons" data-icon="play_arrow"></em></div></a></div><figcaption class=""><div class="md-assembly-caption text-muted font-14 font-serif line-clamp"><span><button class="js-more-btn d-none btn btn-unstyled font-12 bg-white js-content" aria-label="Toggle more/less fact data"><span class="link-blue">(more)</span></button></span></div><cite class="credit d-block mt-5">Encyclopædia Britannica, Inc.</cite></figcaption></figure> <p class="topic-paragraph">When they emerge, the cicadas aren’t yet in their adult form. They’re still nymphs, and they remain so until they molt for the final time. Once their fresh exoskeletons have hardened, they take to the trees with their newfound wings, and the males begin their loud mating calls.</p><span class="MOD6"></span> <p class="topic-paragraph">And now we’re back where we started. Newborn cicada nymphs fall or crawl down from the trees where they hatched and burrow into the ground, not to be seen for another 17 years.</p><span class="MOD7"></span> <span class="AM-END-OF-ARTICLE am-inline"></span></article> </div> </div> </div> <aside class="cm-rightrail d-none d-md-block ad-rail"></aside> </div> </div> </div> </div> </main> <div class="grid gx-0"> <div class="col"> <div class="md-loading hidden display-4 font-weight-normal"></div> <div class="text-center my-20 mb-100"> <button class="md-infinite-load-next btn btn-blue" data-value="next-page">Load More</button> </div> </div> <div class="col-md-da-320 hidden-xs"> </div> </div> </div> </div> </main> <div id="md-footer"></div> <noscript><iframe src="//www.googletagmanager.com/ns.html?id=GTM-5W6NC8" height="0" width="0" style="display:none;visibility:hidden"></iframe></noscript>  <script> window.ap3c = window.ap3c || {}; var ap3c = window.ap3c; ap3c.cmd = ap3c.cmd || []; ap3c.cmd.push(function() { ap3c.init('ZO4siT4cLwnykPnzZWJtd3Byb2Q', 'https://engage.email.britannica.com/'); ap3c.track({v: 0}); }); ap3c.activity = function(act) { ap3c.act = (ap3c.act || []); ap3c.act.push(act); }; var s, t; s = document.createElement('script'); s.type = 'text/javascript'; s.src = "https://engage.email.britannica.com/app.js"; t = document.getElementsByTagName('script')[0]; t.parentNode.insertBefore(s, t); </script> <script class="marketing-page-info" type="application/json"> {"pageType":"Story","templateName":"DEMYSTIFIED","pageNumber":1,"pagesTotal":10,"pageId":9251,"pageLength":null,"initialLoad":true,"lastPageOfScroll":false} </script> <script class="marketing-content-info" type="application/json"> [] </script> <script src="https://cdn.britannica.com/mendel-resources/3-133/js/libs/jquery-3.5.0.min.js?v=3.133.62"></script> <script type="text/javascript" data-type="Init Mendel Code Splitting"> (function() { $.ajax({ dataType: 'script', cache: true, url: 'https://cdn.britannica.com/mendel-resources/3-133/dist/story-page.js?v=3.133.62' }); })(); </script> <script class="analytics-metadata" type="application/json"> {"leg":"B","adLeg":"B","userType":"ANONYMOUS","pageType":"Story","pageSubtype":"DEMYSTIFIED","articleTemplateType":null,"gisted":false,"pageNumber":1,"hasSummarizeButton":false,"hasAskButton":false,"hasAiTopQuestions":false,"hasSimplifyButton":false} </script> <script type="text/javascript"> EBStat={accountId:-1,hostnameOverride:'webstats.eb.com',domain:'www.britannica.com', json:''}; </script> <script type="text/javascript"> ( function() { $.ajax( { dataType: 'script', cache: true, url: '//www.britannica.com/webstats/mendelstats.js?v=1' } ) .done( function() { try {writeStat(null,EBStat);} catch(err){} } ); })(); </script> <div id="bc-fixed-dialogue"></div> </body> </html>

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Why Do Some Cicadas Appear Only Every 17 Years? | Britannica