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{{Short description|Insects of the order Siphonaptera}}
{{Other uses}}
{{Redirect|Siphonaptera|the poem|Siphonaptera (poem)}}
{{Good article}}
{{Use dmy dates|date=January 2017}}
{{Automatic taxobox
| name = Fleas
| fossil_range = {{Fossil range|Middle Jurassic | Recent}}
| image = Flea Scanning Electron Micrograph False Color.jpg
| image_caption = [[Scanning electron microscope#Color in SEM|Scanning electron micrograph]]
| display_parents = 3
| taxon = Siphonaptera
| authority = [[Latreille]], 1825
| subdivision_ranks = Suborders
| subdivision = * {{extinct}}[[Pseudopulicidae]]
* {{extinct}}[[Saurophthiridae]]
* {{extinct}}[[Tarwiniidae]]
{{Plain list|
* Ceratophyllomorpha
* Hystrichopsyllomorpha
* Pulicomorpha
* Pygiopsyllomorpha
}}
| synonyms = Aphaniptera
}}

'''Flea''', the common name for the [[order (biology)|order]] '''Siphonaptera''', includes 2,500 species of small flightless [[insect]]s that live as external [[parasites]] of [[mammal]]s and [[bird]]s. Fleas live by [[hematophagy|ingesting the blood]] of their hosts. Adult fleas grow to about {{convert|3|mm|in|frac=32|abbr=off}} long, are usually dark in color, and have bodies that are "flattened" sideways or narrow, enabling them to move through their hosts' fur or feathers. They lack wings; their hind legs are extremely well adapted for jumping. Their claws keep them from being dislodged, and their mouthparts are adapted for piercing skin and sucking [[blood]]. Some species can leap 50 times their body length, a feat second only to jumps made by another group of insects, the [[superfamily (taxonomy)|superfamily]] of [[froghopper]]s. Flea larvae are worm-like, with no limbs; they have chewing mouthparts and feed on organic debris left on their hosts' skin.

Genetic evidence indicates that fleas are a specialised lineage of parasitic [[scorpionflies]] (Mecoptera) ''sensu lato'', most closely related to the family [[Nannochoristidae]]. The earliest known fleas lived in the Middle [[Jurassic]]; modern-looking forms appeared in the [[Cenozoic]]. Fleas probably originated on mammals first and expanded their reach to birds. Each species of flea specializes, more or less, on one species of host: many species of flea never breed on any other host; some are less selective. Some families of fleas are exclusive to a single host group; for example, the Malacopsyllidae are found only on [[armadillo]]s, the [[Ischnopsyllidae]] only on [[bat]]s, and the Chimaeropsyllidae only on [[elephant shrew]]s.

The oriental rat flea, ''[[Xenopsylla cheopis]]'', is a [[Vector (epidemiology)|vector]] of ''[[Yersinia pestis]]'', the [[bacterium]] that causes [[bubonic plague]]. The disease was spread to humans by rodents, such as the [[black rat]], which were bitten by infected fleas. Major outbreaks included the [[Plague of Justinian]], about 540, and the [[Black Death]], about 1350, each of which killed a sizeable fraction of the world's people.

Fleas appear in human culture in such diverse forms as [[flea circus]]es; poems, such as [[John Donne]]'s erotic "[[The Flea (poem)|The Flea]]"; works of music, such as those by [[Modest Mussorgsky]]; and a film by [[Charlie Chaplin]].

== Morphology and behavior ==
Fleas are wingless insects, {{convert|1.5|to|3.3|mm|in|frac=32|abbr=off}} long, that are agile, usually dark colored (for example, the reddish-brown of the [[cat flea]]), with a [[proboscis]], or stylet, adapted to feeding by piercing the skin and sucking their host's blood through their epipharynx. Flea legs end in strong claws that are adapted to grasp a host.<ref name=":1" />

Unlike other insects, fleas do not possess [[compound eye]]s but instead only have simple eyespots with a single biconvex lens; some species lack eyes altogether.<ref>{{Cite journal |last1=Taylor |first1=Sean D. |last2=Cruz |first2=Katharina Dittmar de la |last3=Porter |first3=Megan L. |last4=Whiting |first4=Michael F. |date=May 2005 |title=Characterization of the Long-Wavelength Opsin from Mecoptera and Siphonaptera: Does a Flea See? |journal=Molecular Biology and Evolution |volume=22 |issue=5 |pages=1165–1174 |doi=10.1093/molbev/msi110 |issn=0737-4038 |pmid=15703237|doi-access=free }}</ref> Their bodies are laterally compressed, permitting easy movement through the hairs or feathers on the host's body. The flea body is covered with hard plates called sclerites.<ref name=":1">{{Cite web |title = Wiley: The Insects: An Outline of Entomology, 5th Edition – Gullan, P.J.; Cranston, P.S. |url = http://www.wiley.com/WileyCDA/WileyTitle/productCd-111884615X.html |website = wiley.com |access-date = 11 November 2016 |archive-date = 5 December 2017 |archive-url = https://web.archive.org/web/20171205054521/http://www.wiley.com/WileyCDA/WileyTitle/productCd-111884615X.html |url-status = dead }}</ref> These sclerites are covered with many hairs and short spines directed backward, which also assist its movements on the host. The tough body is able to withstand great pressure, likely an [[adaptation]] to survive attempts to eliminate them by scratching.<ref name="Florida">[http://edis.ifas.ufl.edu/IG087 Fleas]. Koehler, P.G.; Oi, F.M. Printed July 1993, revised February 2003. Provided by the [[University of Florida]]</ref>

Fleas lay tiny, white, oval eggs. The larvae are small and pale, have bristles covering their worm-like bodies, lack eyes, and have mouth parts adapted to chewing. The larvae feed on organic matter, especially the feces of mature fleas, which contain dried blood. Adults feed only on fresh blood.<ref name=BugGuide>{{cite web |url=http://bugguide.net/node/view/7040 |title=Order Siphonaptera – Fleas |publisher=BugGuide.Net |access-date=11 November 2016}}</ref>

===Jumping===
Their legs are long, the hind pair well adapted for jumping; a flea can jump vertically up to {{convert|18|cm|in|frac=2|abbr=off}} and horizontally up to {{convert|33|cm|in|frac=2|abbr=on}},<ref name=Crosby>{{cite web |url=http://vetmedicine.about.com/od/parasites/f/FAQ_fleacycle.htm |title=What is the Life Cycle of the Flea? |author=Crosby, J.T. |work=Veterinary Parasites |publisher=About Home |access-date=4 November 2016 |archive-date=19 September 2005 |archive-url=https://web.archive.org/web/20050919195615/http://vetmedicine.about.com/od/parasites/f/FAQ_fleacycle.htm |url-status=dead }}</ref> making the flea one of the best jumpers of all known animals (relative to body size), second only to the [[froghopper]]. A flea can jump 60 times its length in height and 110 times its length in distance, equivalent to a {{convert|1.8|m|ft|frac=2|abbr=on}} adult human jumping {{convert|110|m|ft|frac=2|abbr=on}} vertically and {{convert|200|m|ft|frac=2|abbr=on}} horizontally. Rarely do fleas jump from dog to dog. Most flea infestations come from newly developed fleas from the pet's environment.<ref>{{cite web |title=Fleas and Ticks: Facts about Fleas |url=https://www.mypet.com/fleas-and-ticks/facts-about-fleas.aspx#:~:text=A%20flea%20can%20jump%20more,fleas%20from%20the%20pet's%20environment. |website=mypet |publisher=Merck Animal Health |access-date=21 March 2022}}</ref> The flea jump is so rapid and forceful that it exceeds the capabilities of muscle, and instead of relying on direct muscle power, fleas store muscle energy in a pad of the elastic protein named [[resilin]] before releasing it rapidly (like a human using a bow and arrow).<ref name="sciencenews.org"/>  Immediately before the jump, muscles contract and deform the resilin pad, slowly storing energy which can then be released extremely rapidly to power leg extension for propulsion.<ref name=JEB>{{cite journal|last1= Burrows|first1=M.|title =How Fleas Jump|journal =Journal of Experimental Biology|volume =212|issue =18|year= 2009|pages= 2881–2883|doi= 10.1242/jeb.022855|pmid=19717668|doi-access=free}}</ref>  To prevent premature release of energy or motions of the leg, the flea employs a "catch mechanism".<ref name=JEB />  Early in the jump, the tendon of the primary jumping muscle passes slightly behind the coxa-trochanter joint, generating a [[torque]] which holds the joint closed with the leg close to the body.<ref name=JEB />  To trigger jumping, another muscle pulls the tendon forward until it passes the joint axis, generating the opposite torque to extend the leg and power the jump by release of stored energy.<ref name=JEB /> The actual take off has been shown by high-speed video to be from the tibiae and tarsi rather than from the [[Arthropod leg#Trochanter|trochantera]] (knees).<ref name="sciencenews.org">{{cite magazine |url=http://www.sciencenews.org/view/generic/id/69664/title/Fleas_leap_from_feet%2C_not_knees |title=Fleas leap from feet, not knees |magazine=Science News |date=2 October 2011 |access-date=11 November 2016 |archive-date=8 August 2011 |archive-url=https://web.archive.org/web/20110808003410/http://www.sciencenews.org/view/generic/id/69664/title/Fleas_leap_from_feet,_not_knees |url-status=dead }}</ref>

== Life cycle and development ==
[[File:A dog flea (Ctenocephalides canis); adult, pupa, egg and lar Wellcome V0022501EL.jpg|thumb|upright|[[Ctenocephalides canis|Dog flea]] (from top) larva, egg, pupa and adult]]

Fleas are [[holometabolous]] insects, going through the four [[Biological life cycle|lifecycle]] stages of [[egg]], [[larva]], [[pupa]], and [[imago]] (adult). In most species, neither female nor male fleas are fully mature when they first emerge but must feed on blood before they become capable of reproduction.<ref name="Florida"/> The first blood meal triggers the maturation of the ovaries in females and the dissolution of the testicular plug in males, and copulation soon follows.<ref name=Krasnov44>{{cite book|author=Krasnov, Boris R. |title=Functional and Evolutionary Ecology of Fleas: A Model for Ecological Parasitology |url=https://books.google.com/books?id=tkvr8h8zxFwC&pg=PA44 |year=2008 |publisher=Cambridge University Press |isbn=978-1-139-47266-1 |pages=44–54}}</ref> Some species breed all year round while others synchronise their activities with their hosts' life cycles or with local environmental factors and climatic conditions.<ref name=Krasnov64>{{cite book|author=Krasnov, Boris R. |title=Functional and Evolutionary Ecology of Fleas: A Model for Ecological Parasitology |url=https://books.google.com/books?id=tkvr8h8zxFwC&pg=PA44 |year=2008 |publisher=Cambridge University Press |isbn=978-1-139-47266-1 |pages=64–67}}</ref> Flea populations consist of roughly 50% eggs, 35% larvae, 10% pupae, and 5% adults.<ref name="Crosby" />

=== Egg ===
The number of eggs laid depends on species, with batch sizes ranging from two to several dozen. The [[fecundity|total number of eggs produced in a female's lifetime]] (fecundity) varies from around one hundred to several thousand. In some species, the flea lives in the host's nest or burrow and the eggs are deposited on the substrate,<ref name=Krasnov44/> but in others, the eggs are laid on the host itself and can easily fall off onto the ground. Because of this, areas where the host rests and sleeps become one of the primary [[habitat]]s of eggs and developing larvae. The eggs take around two days to two weeks to hatch.<ref name="Crosby"/>

=== Larva ===
[[File:Flea Larva.jpg|thumb|upright|Flea larva]]

Flea larvae emerge from the eggs to feed on any available organic material such as dead insects, faeces, [[Biological specificity|conspecific]] eggs, and vegetable matter, some species are subdermal endoparasites on their parent’s hosts. In laboratory studies, some dietary diversity seems necessary for proper larval development. Blood-only diets allow only 12% of larvae to mature, whereas blood and yeast or dog chow diets allow almost all larvae to mature.<ref>{{cite journal |last1=Silverman |first1=Jules |last2=Appel |first2=Arthur |date=March 1994 |title=Adult Cat Flea (Siphonaptera: Pulicidae) Excretion of Host Blood Proteins in Relation to Larval Nutrition |url=http://www4.ncsu.edu/~jsilver/Silverman%20and%20Appel%201994.pdf |journal=Journal of Medical Entomology |issue=2 |pages=265–271 |doi=10.1093/jmedent/31.2.265 |access-date=18 July 2014 |volume=31 |pmid=7910638 |archive-date=25 July 2014 |archive-url=https://web.archive.org/web/20140725150050/http://www4.ncsu.edu/~jsilver/Silverman%20and%20Appel%201994.pdf |url-status=dead }}</ref> Another study also showed that 90% of larvae matured into adults when the diet included nonviable eggs.<ref>{{Cite journal |last=Shryock |first=J. |date=2006 |title=Time Spent by Ctenocephalides felis (Siphonaptera: Pulicidae) Larvae in Food Patches of Varying Quality |journal=Environmental Entomology |doi= 10.1603/0046-225x-35.2.401|volume=35 |issue=2 |pages=401–404|doi-access=free }}</ref> They are blind and avoid sunlight, keeping to dark, humid places such as sand or soil, cracks and crevices, under carpets and in bedding.<ref name=Hinkle/> The entire larval stage lasts between four and 18 days.<ref>{{Cite web|url=https://www.orkin.com/other/fleas/fleas-life-cycle|title=Flea Life Cycle: Eggs, Larvae, etc.|website=Orkin.com|date=11 April 2018|language=en|access-date=2019-09-13}}</ref>

=== Pupa ===
Given an adequate supply of food, larvae [[pupate]] and weave [[silk]]en cocoons after three larval stages. Within the cocoon, the larva molts for a final time and undergoes metamorphosis into the adult form. This can take just four days, but may take much longer under adverse conditions, and there follows a variable-length stage during which the pre-emergent adult awaits a suitable opportunity to emerge. Trigger factors for emergence include vibrations (including sound), heat (in warm-blooded hosts), and increased levels of [[carbon dioxide]], all of which may indicate the presence of a suitable host.<ref name="Crosby" /> Large numbers of pre-emergent fleas may be present in otherwise flea-free environments, and the introduction of a suitable host may trigger a mass emergence.<ref name=Hinkle/>

=== Adult ===
Once the flea reaches adulthood, its primary goal is to find blood and then to reproduce.<ref>{{cite web|url=http://edis.ifas.ufl.edu/IG087 |title=Fleas |author1=Koehler, P.G. |author2=Pereira, R.M. |author3=Diclaro, J.W. |publisher=Edis.ifas.ufl.edu |access-date=11 November 2016}}</ref> Female fleas can lay 5000 or more eggs over their life, permitting rapid increase in numbers.<ref name="loc.gov">{{cite web|url=https://www.loc.gov/rr/scitech/mysteries/flea.html |title=How long is the life span of a flea? |work=Everyday Mysteries: Fun Science Facts from the Library of Congress |publisher=Loc.gov |date=2 July 2013 |access-date=11 November 2016}}</ref> Generally speaking, an adult flea only lives for 2 or 3 months. Without a host to provide a blood meal, a flea's life can be as short as a few days. Under ideal conditions of temperature, food supply, and humidity, adult fleas can live for up to a year and a half.<ref name="loc.gov"/> Completely developed adult fleas can live for several months without eating, so long as they do not emerge from their [[puparium|puparia]]. Optimum temperatures for the flea's life cycle are {{convert|21|to|30|C|F}} and optimum humidity is 70%.<ref name="Piper, Ross 2007"/>

Adult female rabbit fleas, ''[[Spilopsyllus cuniculi]]'', can detect the changing levels of [[cortisol]] and [[corticosterone]] hormones in the rabbit's blood that indicate it is getting close to giving birth. This triggers sexual maturity in the fleas and they start producing eggs. As soon as the baby rabbits are born, the fleas make their way down to them and once on board they start feeding, mating, and laying eggs. After 12 days, the adult fleas make their way back to the mother. They complete this mini-migration every time she gives birth.<ref name="Piper, Ross 2007">[[Ross Piper|Piper, Ross]] (2007), ''Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals'', [[Greenwood Press]].</ref>

== Taxonomy and phylogeny ==
=== History ===
Between 1735 and 1758, the Swedish naturalist [[Carl Linnaeus]] first classified insects, doing so on the basis of their wing structure. One of the seven orders into which he divided them was "Aptera", meaning wingless, a group in which as well as fleas, he included [[spider]]s, [[woodlice]] and [[myriapod]]s. It wasn't until 1810 that the French zoologist [[Pierre André Latreille]] reclassified the insects on the basis of their mouthparts as well as their wings, splitting Aptera into [[Thysanura]] (silverfish), [[Anoplura]] (sucking lice) and Siphonaptera (fleas), at the same time separating off the arachnids and [[crustacean]]s into their own subphyla.<ref name=Gillott>{{cite book|author=Gillott, Cedric|title=Entomology |url=https://books.google.com/books?id=latCAAAAQBAJ&pg=PA97 |year=2005|publisher=Springer Science & Business Media|isbn=978-1-4020-3183-0|page=97}}</ref> The group's name, Siphonaptera, is zoological Latin from the Greek ''siphon'' (a tube) and ''aptera'' (wingless).<ref>{{cite web|last1=Meyer|first1=John R.|title=Siphonaptera|url=https://projects.ncsu.edu/cals/course/ent425/library/compendium/siphonaptera.html|publisher=North Carolina State University|access-date=3 December 2016|date=28 March 2016|archive-date=25 September 2022|archive-url=https://web.archive.org/web/20220925114314/https://projects.ncsu.edu/cals/course/ent425/library/compendium/siphonaptera.html|url-status=dead}}</ref>

=== External phylogeny ===
It was historically unclear whether the Siphonaptera are sister to the [[Mecoptera]] (scorpionflies and allies), or are inside that clade, making "Mecoptera" paraphyletic. The earlier suggestion that the Siphonaptera are sister to the [[Boreidae]] (snow scorpionflies)<ref name=Whiting2008>{{cite journal |last1=Whiting |first1=Michael F. |author1-link=Michael F. Whiting|last2=Whiting |first2=Alison S. |last3=Hastriter |first3=Michael W. |last4=Dittmar |first4=Katharina |title=A molecular phylogeny of fleas (Insecta: Siphonaptera): origins and host associations |journal=Cladistics |date=2008 |volume=24 |issue=5 |pages=677–707 |doi=10.1111/j.1096-0031.2008.00211.x |citeseerx=10.1.1.731.5211  |s2cid=33808144 }}</ref><ref name=Whiting2002>{{cite journal |last1=Whiting |first1=Michael F. |author1-link=Michael F. Whiting|year=2002 |title=Mecoptera is paraphyletic: multiple genes and phylogeny of Mecoptera and Siphonaptera |journal=Zoologica Scripta |volume=31 |issue=1 |pages=93–104 |url=http://www3.interscience.wiley.com/journal/118916123/abstract |archive-url=https://archive.today/20130105095659/http://www3.interscience.wiley.com/journal/118916123/abstract |url-status=dead |archive-date=2013-01-05 |doi=10.1046/j.0300-3256.2001.00095.x |s2cid=56100681 |url-access=subscription }}</ref><ref name=WiegmannYates2012>{{cite book |last1=Wiegmann |first1=Brian |last2=Yeates |first2=David K. |title=The Evolutionary Biology of Flies |url=https://books.google.com/books?id=rElP5sNn6IYC&pg=PA5 |year=2012 |publisher=Columbia University Press |isbn=978-0-231-50170-5 |page=5 |quote=Recently, a close affinity between Siphonaptera and Mecoptera has been convincingly demonstrated via morphology (Bilinski et al. 1998) and molecular data (Whiting 2002), rendering Mecoptera paraphyletic, but making the clade including Mecoptera and Siphonaptera monophyletic}}</ref> is not supported. A 2020 genetic study recovered Siphonaptera within Mecoptera, with strong support, as the sister group to [[Nannochoristidae]], a small, [[Relict (biology)|relictual]] group of mecopterans native to the Southern Hemisphere. Fleas and nannochoristids share several similarities with each other that are not shared with other mecopterans, including similar mouthparts as well as a similar sperm pump organisation.<ref name=":0">{{Cite journal|last1=Tihelka|first1=Erik|last2=Giacomelli|first2=Mattia|last3=Huang|first3=Di-Ying|last4=Pisani|first4=Davide|last5=Donoghue|first5=Philip C. J.|last6=Cai|first6=Chen-Yang|date=2020-12-21|title=Fleas are parasitic scorpionflies|url=https://www.mapress.com/pe/article/view/palaeoentomology.3.6.16|journal=Palaeoentomology|language=en|volume=3|issue=6|pages=641–653–641–653|doi=10.11646/palaeoentomology.3.6.16|issn=2624-2834|hdl=1983/8d3c12c6-529c-4754-b59d-3abf88a32fc9|s2cid=234423213|hdl-access=free}}</ref>

Relationships of Siphonaptera per Tihelka et al. 2020.<ref name=":0" />

{{clade|{{clade 
   |1=[[Diptera]] (true flies) [[File:Common house fly, Musca domestica.jpg|70px]] 
   |2={{clade
      |1=[[Boreidae]] (snow scorpionflies, 30 spp.) [[File:Boreus hiemalis2 detail.jpg|85px]]
      |2={{clade
         |1={{clade
            |1=[[Nannochoristidae]] (southern scorpionflies, 8 spp.) <!--NC img http://v3.boldsystems.org/index.php/Taxbrowser_Taxonpage?taxid=892130-->
            |2='''Siphonaptera''' (fleas, 2500 spp.) [[File:British Entomologycutted Plate114.png|50px]]
            }}
         |2=[[Pistillifera]] (scorpionflies, hangingflies, 400 [[Species|spp]].) [[File:Gunzesrieder Tal Insekt 3.jpg|90px]] 
         }}
      }}
   }}|label1=[[Antliophora]]}}

=== Fossil history ===
[[File:Flea in amber.jpg|thumb|upright|[[Cenozoic]] flea in [[amber]], c. 20 [[mya (unit)|mya]], is morphologically modern.]]
[[File:P.wangi fossil image.png|thumb|''[[Pseudopulex|Pseudopulex wangi]]'', a primitive flea from the Early Cretaceous of China]]

Fleas likely descended from [[scorpionflies]], insects that are predators or scavengers.<ref name=":0" /> Fossils of large, wingless [[stem-group]] fleas with siphonate (sucking) mouthparts from the Middle [[Jurassic]]<ref>{{cite journal |author=Huang, D. |author2=Engel, M.S. |author3=Cai, C. |author4=Wu, H. |author5=Nel, A. |date=8 March 2012 |title=Diverse transitional giant fleas from the Mesozoic era of China |journal=Nature |volume=483 |issue=7388 |pages=201–204 |doi=10.1038/nature10839 |pmid=22388812|bibcode=2012Natur.483..201H |s2cid=4415855 }}</ref> to Early [[Cretaceous]] have been found in northeastern China and Russia, belonging to the families [[Saurophthiridae]] and [[Pseudopulicidae]], as well as ''[[Tarwinia]]'' from the Early Cretaceous of Australia. Most flea families formed after the end of the Cretaceous (in the [[Paleogene]] and onwards). Modern fleas probably arose in the southern continental area of [[Gondwana]], and migrated rapidly northwards from there. They most likely evolved with [[mammal]] hosts, only later moving to [[bird]]s.<ref>{{cite journal |last1=Zhu |first1=Qiyun |last2=Hastriter |first2=Michael |last3=Whiting |first3=Michael |last4=Dittmar |first4=Katherina |title=Fleas (Siphonaptera) are Cretaceous, and Evolved with Theria |journal=Molecular Phylogenetics and Evolution |date=September 2015 |volume=90 |pages=129–139 |biorxiv=10.1101/014308 |doi=10.1016/j.ympev.2015.04.027 |pmid=25987528|bibcode=2015MolPE..90..129Z |s2cid=13433327 }}</ref>

Siphonaptera is a relatively small order of insects: members of the order undergo complete metamorphosis and are secondarily wingless (their ancestors had wings which modern forms have lost). In 2005, Medvedev listed 2005 species in 242 genera, and despite subsequent descriptions of new species, bringing the total up to around 2500 species,<ref name=Whiting2008/> this is the most complete database available. The order is divided into four infraorders and eighteen families. Some families are exclusive to a single host group; these include the Malacopsyllidae ([[armadillo]]s), Ischnopsyllidae ([[bat]]s) and Chimaeropsyllidae ([[elephant shrew]]s).<ref name=Krasnov3>{{cite book|author=Krasnov, Boris R. |title=Functional and Evolutionary Ecology of Fleas: A Model for Ecological Parasitology |url=https://books.google.com/books?id=tkvr8h8zxFwC&pg=PA3 |year=2008 |publisher=Cambridge University Press |isbn=978-1-139-47266-1 |pages=3–9}}</ref>

Many of the known species are little studied. Some 600 species (a quarter of the total) are known from single records. Over 94% of species are associated with [[mammal]]ian hosts, and only about 3% of species can be considered to be specific parasites of [[bird]]s. The fleas on birds are thought to have originated from mammalian fleas; at least sixteen separate groups of fleas switched to avian hosts during the evolutionary history of the Siphonaptera. Occurrences of fleas on reptiles is accidental, and fleas have been known to feed on the [[hemolymph]] (bloodlike body fluid) of [[tick]]s.<ref name=Krasnov3/>

=== Internal phylogeny ===
Flea phylogeny was long neglected, the discovery of [[Homology (biology)|homologies]] with the parts of other insects being made difficult by their extreme specialization. Whiting and colleagues prepared a detailed molecular phylogeny in 2008, with the basic structure shown in the cladogram. The [[Hectopsyllidae]], including the harmful [[Tunga penetrans|chigoe flea or jigger]], is sister to the rest of the Siphonaptera.<ref name=Whiting2008/>

{{clade
|label1='''Siphonaptera'''
|1={{clade
   |1=[[Hectopsyllidae]] (inc. [[Tunga penetrans|jigger]]) [[File:ChiggerBMNH (cropped).jpg|75px]]
   |2=<!--9-->{{clade
      |1=<!--10-->{{clade
         |1=<!--11-->{{clade
            |1=[[Pygiopsyllomorpha]]
            |2={{clade
               |1=[[Macropsyllidae]], [[Coptopsyllidae]]
               |2=[[Neotyphloceratini]], [[Ctenophthalmini]], [[Doratopsyllinae]]
               }}
            }}
         |2=<!--26-->{{clade
            |1=[[Stephanocircidae]] [[File:Craneopsylla minerva.jpg|75px]]
            |2=clade inc. [[Rhopalopsyllidae]], [[Ctenophthalmidae]], [[Hystrichopsyllidae]] [[File:British Entomologycutted Plate114.png|75px]]
            }}
         }}
      |2={{clade
         |1={{clade
            |1=[[Chimaeropsyllidae]]
            |2=[[Pulicidae]] (inc. the [[rat flea]], vector of [[bubonic plague]]) [[File:NHMUK010177265 The plague flea - Xenopsylla cheopis cheopis (Rothschild, 1903).jpg|75px]]
            }}
         |2=[[Ceratophyllomorpha]] (inc. the [[Ceratophyllidae]], such as the widespread [[moorhen flea]]) [[File:NHMUK010177289 The moorhen flea - Dasypsyllus Dasypsyllus gallinulae gallinulae (Dale, 1878).jpg|75px]]
         }}
      }}
   }}
}}

==Taxonomy==
{{As of|2023}}, there are 21 recognized families within the order Siphonaptera, 3 of which are extinct.<ref>{{cite web |title=Siphonaptera |url=https://www.catalogueoflife.org/data/taxon/43J |website=Catalogue of Life |access-date=18 July 2023}}</ref> In addition, some researchers have suggested that the subfamily [[Stenoponiinae]] should be elevated to its own family ([[Stenoponiidae]]).<ref name="Zurita2015">{{cite journal |last1=Zurita |first1=A. |last2=Callejón |first2=R. |last3=De Rojas |first3=M. |last4=Gómez López |first4=M.S. |last5=Cutillas |first5=C. |title=Molecular study of Stenoponia tripectinata tripectinata (Siphonaptera: Ctenophthalmidae: Stenoponiinae) from the Canary Islands: taxonomy and phylogeny |journal=Bulletin of Entomological Research |date=December 2015 |volume=105 |issue=6 |pages=704–711 |doi=10.1017/s0007485315000656|pmid=26282009 |s2cid=35756267 }}</ref>
{{Div col|colwidth=24em}}
*[[Ancistropsyllidae]] <small>Toumanoff & Fuller, 1947</small>
*[[Ceratophyllidae]] <small>Dampf, 1908</small>
*[[Chimaeropsyllidae]] <small>Ewing & I. Fox, 1943</small>
*[[Coptopsyllidae]] <small>Wagner, 1928</small>
*[[Ctenophthalmidae]] <small>Rothschild, 1915</small>
*[[Hystrichopsyllidae]] <small>Tiraboschi, 1904</small>
*[[Ischnopsyllidae]] <small>Wahlgren, 1907</small>
*[[Leptopsyllidae]] <small>Rothschild & Jordan, 1915</small>
*[[Lycopsyllidae]] <small>Baker, 1905</small>
*[[Malacopsyllidae]] <small>Baker, 1905</small>
*[[Pseudopulicidae]]† <small>Gao, Shih & Ren, 2012</small>
*[[Pulicidae]] <small>Billberg, 1820</small>
*[[Pygiopsyllidae]] <small>Wagner, 1939</small>
*[[Rhopalopsyllidae]] <small>Oudemans, 1909</small>
*[[Saurophthiridae]]† <small>Ponomarenko, 1986</small>
*[[Stephanocircidae]] <small>Wagner, 1928</small>
*[[Stivaliidae]] <small>Mardon, 1978</small>
*[[Tarwiniidae]]† <small>Huang, Engel, Cai & Nel, 2013</small>
*[[Tungidae]] <small>Fox, 1925</small>
*[[Vermipsyllidae]] <small>Wagner, 1889</small>
*[[Xiphiopsyllidae]] <small>Wagner, 1939</small>
{{Div col end}}

== Relationship with host ==
[[File:Fleabite.JPG|thumb|Flea bites in humans.]]

Fleas feed on a wide variety of [[warm-blooded]] [[vertebrate]]s including dogs, cats, rabbits, squirrels, ferrets, rats, mice, birds, and humans. Fleas normally specialise in one host species or group of species, but can often feed but not reproduce on other species. ''[[Ceratophyllus gallinae]]'' affects poultry as well as wild birds.<ref name=Krasnov72/> As well as the degree of relatedness of a potential host to the flea's original host, it has been shown that avian fleas that exploit a range of hosts, only parasitise species with low immune responses. In general, host specificity decreases as the size of the host species decreases. Another factor is the opportunities available to the flea to change host species; this is smaller in colonially nesting birds, where the flea may never encounter another species, than it is in solitary nesting birds. A large, long-lived host provides a stable environment that favours host-specific parasites.<ref name=Poulin>{{cite book |last=Poulin |first=Robert |author-link=Robert Poulin (zoologist) |title=Evolutionary Ecology of Parasites |edition=Second |url=https://books.google.com/books?id=eegdgaR81k0C&pg=PA68 |year=2011 |publisher=Princeton University Press |isbn=978-1-4008-4080-9 |page=68}}</ref>

Although there are species named dog fleas ([[Dog flea|''Ctenocephalides canis'']] Curtis, 1826) and cat fleas ([[Cat flea|''Ctenocephalides felis'']]), fleas are not always strictly species-specific. A study in [[Virginia]] examined 244 fleas from 29 dogs: all were cat fleas. Dog fleas had not been found in Virginia in more than 70 years, and may not even occur in the US, so a flea found on a dog in the United States is likely a cat flea (''Ctenocephalides felis'').<ref>{{Cite journal|last=Eckerlin|first=Ralph P.|date=2011|title=WHAT KIND OF FLEAS DOES YOUR DOG HAVE?|url=http://virginianaturalhistorysociety.com/banisteria/pdf-files/ban37/Banisteria37_Fleas.pdf|journal=Banisteria|volume=37|pages=42–43|access-date=7 September 2020|archive-date=5 July 2021|archive-url=https://web.archive.org/web/20210705093120/http://virginianaturalhistorysociety.com/banisteria/pdf-files/ban37/Banisteria37_Fleas.pdf|url-status=dead}}</ref><ref>{{Cite journal |last1=Marchiondo|first1=A. A. |last2=Holdsworth |first2=P. A. |last3=Green |first3=P. |last4=Blagburn |first4=B. L. |last5=Jacobs |first5=D. E. |date=2007-04-30 |title=World Association for the Advancement of Veterinary Parasitology guidelines for evaluating the efficacy of parasiticides for the treatment, prevention and control of flea and tick infestation on dogs and cats |url=http://www.sciencedirect.com/science/article/pii/S0304401706006236 |journal=Veterinary Parasitology |volume=145 |issue=3 |pages=332–344 |doi=10.1016/j.vetpar.2006.10.028 |pmid=17140735 |issn=0304-4017|url-access=subscription }}</ref>

One theory of human hairlessness is that the loss of hair helped humans to reduce their burden of fleas and other ectoparasites.<ref>{{cite journal |author=Rantala, M.J. |year=2006 |title=Evolution of nakedness in ''Homo sapiens'' |journal=Journal of Zoology |volume=273 |pages=1–7 |doi=10.1111/j.1469-7998.2007.00295.x |issn=0952-8369 |url=https://wiki.bio.purdue.edu/MultidiscPhysioLab/images/b/b3/Evolution_of_nakedness.pdf |archive-date=30 November 2016 |access-date=30 November 2016 |archive-url=https://web.archive.org/web/20161130191205/https://wiki.bio.purdue.edu/MultidiscPhysioLab/images/b/b3/Evolution_of_nakedness.pdf |url-status=dead }}</ref>

=== Direct effects of bites ===
{{Main|Pulicosis}}
[[File:Jigger infested foot (2).jpg|thumb|Human foot infested with jigger fleas, ''[[Tunga penetrans]]'']]

In many species, fleas are principally a nuisance to their hosts, causing an [[itch]]ing sensation which in turn causes the host to try to remove the pest by biting, pecking or scratching. Fleas are not simply a source of annoyance, however. Flea bites cause a slightly raised, swollen, irritating nodule to form on the epidermis at the site of each bite, with a single puncture point at the centre, like a [[mosquito]] bite.<ref name="Mullen">{{cite book |title=Medical and Veterinary Entomology |last=Mullen |first=Gary R. |author2=Mullen, Gary|author3=Durden, Lance |year=2009 |publisher=Academic Press |isbn=978-0-12-372500-4 |page=637 |url=https://books.google.com/books?id=6R1v9o-uaI4C }}</ref>{{rp|126}} This can lead to an [[eczema]]tous itchy skin disease called [[flea allergy dermatitis]], which is common in many host species, including dogs and cats.<ref name=Krasnov72>{{cite book|author=Krasnov, Boris R. |title=Functional and Evolutionary Ecology of Fleas: A Model for Ecological Parasitology |url=https://books.google.com/books?id=tkvr8h8zxFwC&pg=PA72 |year=2008 |publisher=Cambridge University Press |isbn=978-1-139-47266-1 |pages=72–74}}</ref> The bites often appear in clusters or lines of two bites, and can remain itchy and inflamed for up to several weeks afterwards. Fleas can lead to secondary hair loss as a result of frequent scratching and biting by the animal. They can also cause [[anemia]] in extreme cases.<ref name="Mullen"/>{{rp|126}}

=== As a vector ===
Fleas are [[Vector (epidemiology)|vectors]] for [[virus|viral]], [[bacteria]]l and [[rickettsia]]l [[disease]]s of humans and other animals, as well as of [[protozoa]]n and [[helminth]] parasites.<ref name="Krasnov">{{cite book |title=Functional and evolutionary ecology of fleas: a model for ecological parasitology |last=Krasnov |first=Boris R.|year=2008 |publisher=Cambridge University Press |isbn=978-0-521-88277-4 |page=593 |url=https://books.google.com/books?id=tkvr8h8zxFwC}}</ref> Bacterial diseases carried by fleas include [[Murine typhus|murine or endemic typhus]]<ref name="Mullen" />{{rp|124}} and [[bubonic plague]].<ref>{{cite book |title=Tending animals in the global village: a guide to international veterinary medicine |last=Sherman |first=David M. |year=2002 |publisher=Wiley-Blackwell |isbn=978-0-683-18051-0 |page=209 |url=https://books.google.com/books?id=ax_lLPG634AC}}</ref> Fleas can transmit ''[[Rickettsia typhi]]'', ''[[Rickettsia felis]]'', ''[[Bartonella henselae]]'', and the [[myxomatosis]] virus.<ref name="Krasnov"/>{{rp|73}} They can carry ''[[Hymenolepiasis]]'' [[tapeworms]]<ref>{{cite book |title=Anorectal and colon diseases: textbook and color atlas of proctology |last=Stein |first=Ernst |year=2003 |publisher=Springer |isbn=978-3-540-43039-1 |page=478 |url=https://books.google.com/books?id=yrlD7i4A68IC}}</ref> and [[Trypanosoma|Trypanosome]] protozoans.<ref name="Krasnov"/>{{rp|74}} The chigoe flea or jigger (''[[Tunga penetrans]]'') causes the disease [[tungiasis]], a major public health problem around the world.<ref name="Gibbs">{{cite web |url=http://emedicine.medscape.com/article/231037 |title=Tungiasis |author=Smith, Darvin Scott |publisher=Medscape |access-date=11 November 2016}}</ref> Fleas that specialize as parasites on specific mammals may use other mammals as hosts; thus, humans may be bitten by cat and dog fleas.<ref>{{cite book |title=Diseases and Human Evolution |last=Barnes |first=Ethne |year=2007 |publisher=UNM Press |isbn=978-0-8263-3066-6 |page=253 |url=https://books.google.com/books?id=wnlB7EMtqFYC}}</ref>

== Relationship with humans ==
===In literature and art===
Fleas have appeared in poetry, literature, music and art; these include [[Robert Hooke]]'s drawing of a flea under the [[microscope]] in his pioneering book ''[[Micrographia]]'' published in 1665,<ref>{{cite book |last=Neri |first=Janice |editor1-last=O'Malley |editor1-first=Therese |editor2-last=Meyers |editor2-first=Amy R. W. |title=The Art of Natural History |publisher=National Gallery of Art |date=2008 |pages=83–107 |chapter=Between Observation and Image: Representations of Insects in Robert Hooke's Micrographia |isbn=978-0-300-16024-6}}</ref> poems by Donne and [[Jonathan Swift]], works of music by [[Giorgio Federico Ghedini]] and [[Modest Mussorgsky]], a play by [[Georges Feydeau]], a film by [[Charlie Chaplin]], and paintings by artists such as [[Giuseppe Crespi]], [[Giovanni Battista Piazzetta]], and [[Georges de La Tour]].<ref>{{cite journal |last1=Roncalli |first1=Amici R.|title=La storia della pulce nell'arte e nella letteratura |journal=Parasitologia |date=June 2004 |volume=46 |issue=1 |pages=15–18 |pmid=15305680 |trans-title=The history of the flea in art and literature|language=it}} See also the [https://web.archive.org/web/20161105032455/http://www.soipa.it/index.php/it/informazioni-joomla/index.php?view=article&catid=47%3Apubblicazioni&id=57%3Ala-storia-della-pulce-nellarte-e-nella-letteratura&format=pdf&option=com_content&Itemid=65&lang=it 2009 version].</ref>

John Donne's erotic metaphysical poem "[[The Flea (poem)|The Flea]]", published in 1633 after his death, uses the [[conceit]] of a flea, which has sucked blood from the male speaker and his female lover, as an extended [[metaphor]] for their sexual relationship. The speaker tries to convince a lady to sleep with him, arguing that if the mingling of their blood in the flea is innocent, then sex would be also.<ref>{{cite book |editor=Black, Joseph |title=The Broadview Anthology of British Literature, Volume 2 |date=2010 |edition=2nd |publisher=Broadview Press |isbn=978-1-55481-290-5}}</ref>

The comic poem [[Siphonaptera (poem)|''Siphonaptera,'']] written in 1915 by the mathematician [[Augustus De Morgan]], describes an infinite chain of parasitism made of ever larger and ever smaller fleas.<ref>{{Cite web|url=https://www.gutenberg.org/files/26408/26408-h/26408-h.htm|title=The Project Gutenberg eBook of A Budget of Paradoxes, Volume II (of II), by Augustus de Morgan|website=gutenberg.org|access-date=2019-10-30}}</ref>

<gallery widths="200px" heights="155px">
File:HookeFlea01.jpg|[[Robert Hooke]]'s drawing of a flea in ''[[Micrographia]]'', 1665
File:The development of the flea from egg to adult Wellcome M0016633.jpg|Development of the flea from egg to adult. [[Antonie van Leeuwenhoek]], c. 1680
</gallery>

===Flea circuses===
{{Main|Flea circus}}

[[Flea circus]]es provided entertainment to nineteenth century audiences. These circuses, extremely popular in Europe from 1830 onwards, featured fleas dressed as humans or towing miniature carts, [[chariot]]s, [[Roller (agricultural tool)|rollers]] or [[cannon]]. These devices were originally made by [[watchmaker]]s or [[jeweller]]s to show off their skill at miniaturization. A ringmaster called a "professor" accompanied their performance with a rapid circus patter.<ref>{{Cite journal |title=The flea in cultural history and first effects of its control |journal=Berliner und Münchener Tierärztliche Wochenschrift |date=1 April 2000 |issn=0005-9366 |pmid=10816916 |pages=152–160 |volume=113 |issue=4 |first=R. |last=Schmäschke}}</ref><ref name="BBC">{{cite web |title=The rise and demise of the flea circus |url=https://www.bbc.co.uk/programmes/articles/2LF04Y9n5hJTHK1l6ffLhPc/the-rise-and-demise-of-the-flea-circus |publisher=BBC Radio 4 |work=Natural Histories |access-date=2 November 2016}}</ref>

[[File:The Go-As-You-Please Race, as seen through a Magnifying Glass.jpg|thumb|center|upright=1.5<!--width for very detailed image-->|A [[flea circus]]: "The Go-As-You-Please Race, as seen through a Magnifying Glass", engraved by J. G. Francis, from an article by [[C. F. Holder]] in ''[[St. Nicholas Magazine]]'', 1886]]

===Carriers of plague===
[[File:Great plague of london-1665.jpg|thumb|upright|[[The Great Plague of London]], in 1665, killed up to 100,000 people.]]

[[Oriental rat flea]]s, ''Xenopsylla cheopis'', can carry the [[coccobacillus]] ''[[Yersinia pestis]].'' The infected fleas feed on rodent vectors of this bacterium, such as the [[black rat]], ''Rattus rattus'', and then infect human populations with the [[Plague (disease)|plague]], as has happened repeatedly from ancient times, as in the [[Plague of Justinian]] in 541–542.<ref>{{cite book |author=Rosen, William |date=2007 |url=https://archive.org/details/justiniansfleapl00rose/page/3 |title=Justinian's Flea: Plague, Empire, and the Birth of Europe |publisher=Viking Adult |page=[https://archive.org/details/justiniansfleapl00rose/page/3 3] |isbn=978-0-670-03855-8 |url-access=registration }}</ref>  Outbreaks killed up to 200 million people across Europe between 1346 and 1671.<ref>{{cite book |last=Hays |first=J. N. |title=The Burdens of Disease: Epidemics and Human Response in Western History |url=https://archive.org/details/burdensofdisease0000hays |url-access=registration |year=1998 |publisher=Rutgers University Press |isbn=978-0-8135-2528-0 |pages=[https://archive.org/details/burdensofdisease0000hays/page/58 58] and following}}</ref> The [[Black Death]] pandemic between 1346 and 1353 likely killed over a third of the population of Europe.<ref>{{cite book | last=Austin Alchon | first=Suzanne | title=A pest in the land: new world epidemics in a global perspective | url=https://books.google.com/books?id=YiHHnV08ebkC&pg=PA21 | publisher=University of New Mexico Press | year=2003 | page=21 | isbn=978-0-8263-2871-7 }}</ref>

Because fleas carry plague, they have seen service as a [[biological weapon]]. During [[World War II]], the [[Kaimingjie germ weapon attack|Japanese army dropped fleas]] infested with ''Y. pestis'' in China. The [[Bubonic plague|bubonic]] and [[Septicemic plague|septicaemic plagues]] are the most probable form of the plague that would spread as a result of a [[bioterrorism]] attack that used fleas as a vector.<ref>{{Cite journal |last=Bossi |first=P. |display-authors=etal |date=2004 |title=Bichat guidelines for the clinical management of plague and bioterrorism-related plague |journal=Eurosurveillance |volume=9 |issue=12 |page=Article 12 |url=http://www.eurosurveillance.org/ViewArticle.aspx?ArticleId=501}}</ref>

===The Rothschild Collection===
The banker [[Charles Rothschild]] devoted much of his time to [[entomology]], creating a large collection of fleas now in the Rothschild Collection at the [[Natural History Museum, London]]. He discovered and named the [[pandemic|plague]] [[Vector (epidemiology)|vector]] flea, ''[[Xenopsylla cheopis]]'', also known as the oriental rat flea, in 1903.<ref name="TWT">{{cite web |title=Charles Rothschild |url=http://www.wildlifetrusts.org/who-we-are/history/centenary/charles-rothschild |publisher=[[The Wildlife Trusts]] |access-date=1 November 2016}}</ref> Using what was probably the world's most complete collection of fleas of about 260,000 specimens (representing some 73% of the 2,587 species and subspecies so far described), he described around 500 species and subspecies of Siphonaptera. He was followed in this interest by his daughter [[Miriam Rothschild]], who helped to catalogue his enormous collection of the insects in seven volumes.<ref name="NYT">{{cite news |last1=Sullivan |first1=Walter |title=Miriam Rothschild Talks of Fleas |url=https://www.nytimes.com/1984/04/10/science/miriam-rothschild-talks-of-fleas.html |newspaper=[[The New York Times]] |access-date=1 November 2016 |date=10 April 1984}}</ref><ref name="NHM">{{cite web |title=Siphonaptera collections |url=http://www.nhm.ac.uk/our-science/collections/entomology-collections/siphonaptera-collections.html |publisher=[[Natural History Museum, London]] |access-date=1 November 2016}}</ref>

=== Flea treatments ===
{{Main|Flea treatments}}

Fleas have a significant economic impact. In America alone, approximately $2.8 billion is spent annually on flea-related veterinary bills and another $1.6 billion annually for flea treatment with pet groomers. Four billion dollars is spent annually for prescription flea treatment and $348 million for flea pest control.<ref name=Hinkle>{{Cite book |publisher=Springer Netherlands |year=2008 |isbn=978-1-4020-6242-1 |pages=797–801 |first1=Nancy C. |last1=Hinkle |first2=Philip G. |last2=Koehler |title=Encyclopedia of Entomology |chapter=Cat Flea, Ctenocephalides felis felis Bouché (Siphonaptera: Pulicidae) |editor=Capinera, John L.|doi=10.1007/978-1-4020-6359-6_536 }}</ref>

== See also ==
{{Portal|Arthropods}}
* [[Trombiculidae|Chigger]]
* [[Louse]]

==References==
{{Reflist}}

==External links==
{{Commons category|Siphonaptera}}
{{Wikispecies|Siphonaptera}}
* [https://en.wikibooks.org/wiki/Parasitic%20Insects,%20Mites%20and%20Ticks:%20Genera%20of%20Medical%20and%20Veterinary%20Importance/Fleas Parasitic Insects, Mites and Ticks: Genera of Medical and Veterinary Importance]

{{Flea-borne diseases}}
{{Orders of Insects}}
{{Mecoptera & Siphonaptera}}
{{Taxonbar|from=Q388162}}
{{Authority control}}

[[Category:Fleas| ]]
[[Category:Extant Middle Jurassic first appearances]]
[[Category:Hematophages]]
[[Category:Insect vectors of human pathogens]]
[[Category:Insects in culture]]
[[Category:Parasites of cats]]
[[Category:Parasites of dogs]]
[[Category:Veterinary entomology]]