Temporal range: Middle Jurassic–Recent 
|Scanning electron microscope (SEM) depiction of a flea|
Fleas are the insects forming the order Siphonaptera. They are wingless, with mouthparts adapted for piercing skin and sucking blood. Fleas are external parasites, living by hematophagy off the blood of mammals and birds.
Some flea species include:
- Cat flea (Ctenocephalides felis)
- Dog flea (Ctenocephalides canis)
- Human flea (Pulex irritans)
- Moorhen flea (Dasypsyllus gallinulae)
- Northern rat flea (Nosopsyllus fasciatus)
- Oriental rat flea (Xenopsylla cheopis)
Over 2,000 species have been described worldwide.
Morphology and behavior 
Fleas are wingless insects (1/16 to 1/8-inch (1.5 to 3.3 mm) long) that are agile, usually dark colored (for example, the reddish-brown of the cat flea), with tube-like mouth-parts adapted to feeding on the blood of their hosts. Their legs are long, the hind pair well adapted for jumping: a flea can jump vertically up to 7 inches (18 cm) and horizontally up to 13 inches (33 cm), making the flea one of the best jumpers of all known animals (relative to body size), second only to the froghopper. According to an article in Science News, "researchers with the University of Cambridge in England have shown that fleas take off from their tibiae and tarsi—the insect equivalent of feet—and not their trochantera, or knees. The researchers report their conclusion in the March 1 Journal of Experimental Biology." It has been known that fleas do not use muscle power but energy stored in a protein named resilin but the researchers used high-speed video technology and mathematical models to discover where the spring action actually happens. Their bodies are laterally compressed, permitting easy movement through the hairs or feathers on the host's body (or in the case of humans, under clothing). The flea body is hard, polished, and 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 mashing or scratching. Even hard squeezing between the fingers is normally insufficient to kill a flea. It is possible to eliminate them by pressing individual fleas with adhesive tape or softened beeswax (or "cheese" wax) or by rolling a flea briskly between the fingers to disable it then crushing it between the fingernails. Fleas also can be drowned in soapy water.
Fleas lay tiny white oval-shaped eggs better viewed through a loupe. The larva is small, pale, has bristles covering its worm-like body, lacks eyes, and has mouthparts adapted to chewing. The larvae feed on various organic matter, especially the feces of mature fleas. The adult flea's diet consists solely of fresh blood. In the pupa phase, the larva is enclosed in a silken, debris-covered cocoon.
Life cycle and habitat 
The flea life cycle begins when the female lays after feeding. Eggs are laid in batches of up to 20 or so, usually on the host itself, which means that the eggs can easily roll onto the ground. Because of this, areas where the host rests and sleeps become one of the primary habitats of eggs and developing fleas. The eggs take around two days to two weeks to hatch.
Flea larvae emerge from the eggs to feed on any available organic material such as dead insects, feces, and vegetable matter. They are blind and avoid sunlight, keeping to dark places like sand, cracks and crevices, and bedding. Given an adequate supply of food, larvae should pupate and weave a silken cocoon within 1–2 weeks after 3 larval stages. After another week or two, the adult flea is fully developed and ready to emerge from the cocoon. They may however remain resting during this period until they receive a signal that a host is near - vibrations (including sound), heat, and carbon dioxide are all stimuli indicating the probable presence of a host. Fleas are known to overwinter in the larval or pupal stages.
Once the flea reaches adulthood, its primary goal is to find blood and then to reproduce. Adult fleas only have about a week to find food once they emerge, but after that they can survive two years to three years between meals. Flea populations are evenly distributed, with about 50% eggs, 35% larvae, 10% pupae, and 5% adults. Their total life span can be as short as one year, but may be several years in ideal conditions. Female fleas can lay 5000 or more eggs over their life, allowing for phenomenal growth rates.
Adult female rabbit fleas, Spilopsyllus cuniculi, can detect the changing levels of cortisol and corticosterone, hormones in the rabbit's blood that indicate she 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.
Newly emerged adult fleas live only about one week if a blood meal is not obtained. However, completely developed adult fleas can live for several months without eating, so long as they do not emerge from their puparia. Optimum temperatures for the flea's life cycle are 70°F to 85°F (21°C to 30°C) and optimum humidity is 70%.
In the past, it was most commonly supposed that fleas had evolved from the flies (Diptera), based on similarities of the larvae. (Some authorities use the name Aphaniptera because it is older, but names above family rank need not follow the ICZN rules of priority, so most taxonomists use the more familiar name). Genetic and morphological evidence indicates that they are descendants of the Scorpionfly family Boreidae, which are also flightless; accordingly it is possible that they will eventually be reclassified as a suborder within the Mecoptera. Their evolution continued to produce adaptations for their specialized parasitic niche, such that they now have no wings and their eyes are covered over. The large number of flea species may be attributed to the wide variety of host species they feed on, which provides so many specific ecological niches to adapt to. In any case, all these groups seem to represent a clade of closely related insect lineages, for which the names Mecopteroidea and Antliophora have been proposed.
Flea systematics are not entirely fixed. While, compared to many other insect groups, fleas have been studied and classified fairly thoroughly, details still remain to be learned about the evolutionary relationships among the different flea lineages.
- Suborder Pulicomorpha
- Superfamily Pulicoidea
- Superfamily Malacopsylloidea
- Family Malacopsyllidae
- Family Rhopalopsyllidae—hosts
- Family Vermipsyllidae—hosts: carnivores
- Superfamily Coptopsylloidea
- Family Coptopsyllidae
- Superfamily Ancistropsylloidea
- Family Ancistropsyllidae
- Suborder Pygiopsyllomorpha
- Superfamily Pygiopsylloidea
- Family Lycopsyllidae
- Family Pygiopsyllidae
- Family Stivaliidae
- Superfamily Pygiopsylloidea
- Suborder Hystrichopsyllomorpha
- Superfamily Hystrichopsylloidea
- Family Hystrichopsyllidae—hosts: rats and mice. Includes Ctenopsyllidae, Amphipsyllidae
- Family Chimaeropsyllidae
- Superfamily Macropsylloidea
- Family Macropsyllidae
- Superfamily Stephanocircidoidea
- Family Stephanocircidae
- Superfamily Hystrichopsylloidea
- Suborder Ceratophyllomorpha
- Superfamily Ceratophylloidea
- Family Ceratophyllidae
- Superfamily Ceratophylloidea
Relationship with host 
Direct effects of bites 
Fleas are a nuisance to their hosts, causing an itching sensation which in turn may result in the host attempting to remove the pest by biting, pecking, scratching, etc. in the vicinity of the parasite. Fleas are not simply a source of annoyance, however. Some people and animals suffer allergic reactions to flea saliva resulting in rashes and or impotence. Flea bites generally result in the formation of a slightly raised, swollen itching spot with a single puncture point at the center (similar to a mosquito bite). 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 also lead to hair loss as a result of frequent scratching and biting by the animal, and can cause anemia in extreme cases.:126
As a vector 
Besides the problems posed by the creature itself, fleas can also act as a vector for disease. Fleas transmit not only a variety of viral, bacterial and rickettsial diseases to humans and other animals, but also protozoans and helminths.:72–73
- bacteria: Murine or endemic typhus.:124 Fleas have helped cause epidemics by transmitting diseases such as the bubonic plague between rodents and humans by carrying Yersinia pestis bacteria. Fleas can transmit Yersinia pestis, Rickettsia typhi, Rickettsia felis, and Bartonella henselae.
- virus: myxomatosis.:73
- helminth: infestation of Hymenolepiasis tapeworm.
- protozoa: Trypanosome protozoans such as those of the subgenus Herpetosoma, use a variety of flea species opportunistically as vectors.:74
Fleas that specialize as parasites on specific mammals may use other mammals as hosts; therefore humans are susceptible to the predation of more than one species of flea.
A misconception concerning the carrying/transmission of the HIV/AIDS by fleas has been debunked by the Centers for Disease Control and Prevention (CDC 2003), which stated that fleas cannot carry the virus and spread it to other humans.
Flea treatments 
For humans 
Fleas can settle in a person's hair in less than ten minutes, causing soreness and itching. The itching associated with flea bites can be treated with anti-itch creams, usually antihistamines or hydrocortisone. Calamine lotion has been shown to be effective for itching.
For pets 
Modern flea control is approached using Integrated Pest Management (IPM) protocols at the host (pet) level. IPM is achieved by targeting fleas during at least two separate life stages, with at least two separate molecules. This is typically achieved using an adulticide to kill adult fleas and an insect development inhibitor (IDI), like lufenuron, or insect growth regulator (IGR), like methoprene, to prevent development of immature stages. Flea adults, larvae, or eggs can be controlled with insecticides. Lufenuron is a veterinary preparation (known as Program) that attacks the larval flea's ability to produce chitin, necessary for the adult's hard exoskeleton, but does not kill fleas. Flea medicines need to be used with care because many of them also affect mammals.
Flea and tick ointment is hazardous to humans; the label of a commercial preparation warns: “First aid: If on skin or clothing, take off contaminated clothing, rinse skin immediately with plenty of water for 15 – 20 minutes; call a poison control center or doctor for treatment advice. . . Although (the product is) applied only between the shoulder blades and at the base of the tail, the dog’s skin and hair oils carry the product over the entire body . . . Wash thoroughly with soap and water after handling . . .”
Since more than three-quarters of a flea's life is spent somewhere other than on the host animal, it is not adequate to treat only the host; it is important also to treat the host's environment. Thorough vacuuming, washing linens in hot water, and treating all hosts in the immediate environment (the entire household, for example) are essential and if possible on a regular basis.
Contemporary commercial products for the topical treatment of flea infestations on pets contain pesticides such as imidacloprid, permethrin, and (S)-methoprene. All flea control products are recommended to be used at least half-yearly because the lifecycle of flea and tick can last to up to 6 months, and by using one of the flea and tick control products for so long, the infestation is highly prevented and, in the end, stopped. Although all these products are effective in fighting against flea and tick infestations, they have different active ingredients and, because cats cannot metabolize some of the compounds of the product, care must be taken in their use.
For the home 
Combatting a flea infestation in the home takes patience because for every flea found on an animal, there could be many more developing in the home. A spot-on insecticide will kill the fleas on the pet and in turn the pet itself will be a roving flea trap and mop up newly hatched fleas. The environment should be treated with a fogger or spray insecticide containing an insect growth regulator, such as pyriproxyfen or methoprene to kill eggs and pupae, which are quite resistant against insecticides. Frequent vacuuming is also helpful, but the vacuum bag must be disposed of immediately afterwards.
Diatomaceous earth can also be used as a home flea treatment in lieu of acetylcholinesterase inhibitory treatments or insecticides which carry with them a risk of poisoning for both humans and other animals. However, Diatomaceous earth is at least potentially dangerous to pets and people when inhaled, so care in use is recommended. Application is effective on both the interior and exterior of one's property, but the efficacy of Diatomaceous earth is diminished when introduced to water. Because Diatomaceous earth is commonly available in food grade quality, it is also possible to leave it exposed in areas typically vulnerable to fleas and other insects.
Baking Soda is also used to kill via dehydration, it's available in large amounts as a food grade material and safe for family and pets when used inside the home (carpets and floors) a layer can be sprinkled onto a carpet and worked into the fibers down to where the larvae and eggs are to dehydrate and kill them. It can be easily vacuumed up afterwards with safe disposal... often multiple weekly treatments will be required to remove an infestation completely.
Borax is sold as a "Natural Laundry Booster" and can also be used as another home treatment for flea infestations. Borax contains sodium borate which kills fleas by dehydrating them, but its safety for pets is untested.
Using dehumidifiers with air conditioning and vacuuming all may interrupt the flea life cycle. Vacuuming on a frequent basis, not only the places where the pet lies, but extensively, is particularly effective.
Humidity is critical to flea survival. Eggs need relative humidity of at least 70–75% to hatch, and larvae need at least 50% humidity to survive. In humid areas, about 20% of the eggs survive to adulthood; in arid areas, less than 5% complete the cycle. Fleas thrive at higher temperatures, but need 70° to 90°F (21° to 32°C) to survive. Lower temperatures slow down or completely interrupt the flea life-cycle. A laboratory study done at the University of California showed that vacuuming catches about 96% of adult fleas. A combination of controlled humidity, temperature, and vacuuming should eliminate fleas from an environment. Altering even one of these environmental factors may be enough to drastically lower and eliminate an infestation.
See also 
- Huang, D., Engel, M.S., Cai, C., Wu, H., Nel, A. (2012). "Diverse transitional giant fleas from the Mesozoic era of China". Nature, in press. doi:10.1038/nature10839.
- Fleas: What They Are, What To Do D. L. Richman and P. G. Koehler, University of Florida IFAS Extension. Accessed 10 December 2010
-  Crosby, J.T. What is the Life Cycle of the Flea. Accessed 6 August 2012
- Fleas. P.G. Koehler and F. M. Oi. Printed July 1993, revised February 2003. Provided by the University of Florida
- Order Siphonaptera – Fleas – BugGuide.Net Accessed 28 December 2006
- Piper, Ross (2007), Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals, Greenwood Press.
- Grimaldi, D. and Engel, M.S. (2005). Evolution of the Insects. Cambridge University Press. ISBN 0-521-82149-5.
- Mullen, Gary R.; Mullen, Gary; Durden, Lance (2009). Medical and Veterinary Entomology. Academic Press. p. 637. ISBN 978-0-12-372500-4. Retrieved 2010-04-09.
- Krasnov, Boris R. (2008). Functional and evolutionary ecology of fleas: a model for ecological parasitology. Cambridge University Press. p. 593. ISBN 978-0-521-88277-4. Retrieved 2010-04-09.
- [David M.] (2002). Tending animals in the global village: a guide to international veterinary medicine. Wiley-Blackwell. p. 209. ISBN 978-0-683-18051-0. Retrieved 2010-04-09.
- Stein, Ernst (2003). Anorectal and colon diseases: textbook and color atlas of proctology. Springer. p. 478. ISBN 978-3-540-43039-1. Retrieved 2010-04-09.
- Barnes, Ethne (2007). Diseases and Human Evolution. UNM Press. p. 253. ISBN 978-0-8263-3066-6. Retrieved 2010-04-09.
- Scott, Susan; Thomas, Craig (2000). Pests of Paradise: First Aid and Medical Treatment of Injuries from Hawaii's Animals. University of Hawaii Press. p. 58. ISBN 978-0-8248-2252-1. Retrieved 2010-04-08.
- Jacoby, David B.; Youngson, R.M. (2004). Encyclopedia of Family Health. Marshall Cavendish. p. 647. ISBN 978-0-7614-7486-9. Retrieved 2010-04-08.
- Label of Bio Spot spot on flea and tick control for dogs. 2007, 2010. Phoenix, AZ: Farnam Companies, Inc.
- "Lights Out For Bed Bugs as "BEST YET" Cedar Oil Solution Solves International Bed Bug Dilemma". http://www.prweb.com/. 2008-11-25. Retrieved 2010-04-08.
- Das, S. S. (2000). "Efficacy of pestoban aerosol spray against Ctenocephalides canis on dogs". Indian Veterinary Journal 77 (4): 290–292.
- Natural Flea Control, 1987, Mother Earth News, Retrieved 2007-12-18
- Warnings About Essential Oils
- Fleas Management Guidelines. Univ. of California Division of Agriculture and Natural Resources, Nov. 2000.
|Wikimedia Commons has media related to: Siphonaptera|
|Wikispecies has information related to: Siphonaptera|
- Flea at the Open Directory Project
- Flea Fact Sheet highlighting habits, habitat, threats and prevention tips
- Integrated Flea Control from University of Nebraska-Lincoln Extension in Lancaster County
- How to Get Rid of Fleas
- National Pesticide Information Center Flea Control
- Advice about fleas from the Berkeley Parents Network