Fleas are insects that form the order Siphonaptera. They are wingless, with mouthparts adapted for piercing skin and sucking blood, and hind legs adapted for jumping. They can jump a distance a few 50 times their body length, a feat second only to the froghoppers. Fleas are external parasites, living by hematophagy off the blood of mammals and birds.

Over 2,500 species have been described worldwide. The Siphonaptera are most closely related to the snow scorpionflies (Boreidae), placing them within the endopterygote insect order Mecoptera.

Fleas arose in the early Cretaceous, most likely as ectoparasites of mammals and marsupials, before moving on to additional groups including birds. Each species of flea is more or less a specialist on its host animal species: a large number of species never breed on any additional host, though a few are less choosy. Some families of fleas are exclusive to a single host group: for example, the Malacopsyllidae are found only on armadillos, the Ischnopsyllidae only on bats, and the Chimaeropsyllidae only on elephant shrews. The cat flea breeds on cats and dogs, and is the vector for Yersinia pestis, the bacterium which causes bubonic plague.

Fleas appear in human culture in such diverse forms as flea circuses, poems like John Donne's erotic The Flea, works of music such as by Modest Mussorgsky, and a film by Charlie Chaplin.

Diversity

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 notwithstanding subsequent descriptions of new species, bringing the total up to around 2500 species, 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 (armadillos), Ischnopsyllidae (bats) and Chimaeropsyllidae (elephant shrews).

Many of the known species are little studied. Some 600 species (a quarter of the total) are known from a single record from a single host. Over 94 percent of species are associated with mammalian hosts, and only about three percent of species can be considered to be specific parasites of birds. The fleas on birds are thought to have originated from mammalian fleas; at least sixteen separate groups of fleas switched to avian hosts throughout the evolutionary history of the Siphonaptera. Occurrences of fleas on reptiles is accidental, and fleas have been known to feed on the hemolymph of ticks.

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 coloured (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. They interestingly don't possess a mandible. Their legs are long, the hind pair well adapted for jumping; a flea can jump vertically up to 7 in (18 cm) and horizontally up to 13 in (33 cm), making the flea one of the best jumpers of all known animals (relative to body size), second only to the froghopper. If humans had the jumping power of a flea, a 1.8-m (6-ft) person could make a jump 90 m (295 ft) long and 49 m (160 ft) high. Flea legs end in strong claws that are designed to grasp a host.

Researchers with the University of Cambridge in England found that fleas take off from their tibiae and tarsi (the insect equivalent of feet) and not their trochantera, or knees. It has been known that fleas don't use direct muscle power, but instead use the muscle to store energy in a protein named resilin before releasing it rapidly (like a human using a bow and arrow), with researchers using high-speed video technology and mathematical models to discover where the spring action actually happens.

Unlike additional insects, fleas don't possess compound eyes but instead only have simple eyespots with a single biconvex lens; a few species lack eyes altogether. 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 covered with hard plates called sclerites. These sclerites are covered with a large number of hairs and short spines directed backward, which additionally 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.

Fleas lay tiny, white, oval-shaped eggs better viewed through a loupe or magnifying glass. 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 various types of organic matter, especially the faeces of mature fleas, which contain dried blood. The adult flea's diet consists solely of fresh blood.

Life cycle and development

Fleas are holometabolous insects, going through the four 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. 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. Some species breed all year round while others synchronise their activities with their hosts' life cycles or with local environmental factors and climatic conditions. Flea populations are distributed with about fifty percent eggs, 35 percent larvae, ten percent pupae, and five percent adults.

Eggs

The number of eggs laid depends on species, with batch sizes ranging from two to several dozen. Fecundity varies from around one hundred to several thousand. In a few species, the flea lives in the nest or burrow and the eggs are deposited on the substrate, 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 habitats of eggs and developing larvae. The eggs take around two days to two weeks to hatch. Experiments have shown that more eggs are laid by fleas with hosts with limited food intakes, and that eggs and larvae show greater survivability under these conditions, perhaps because the host's immune system is compromised.

Larvae

Flea larvae emerge from the eggs to feed on any available organic material such as dead insects, faeces, conspecific eggs, and vegetable matter. In laboratory studies, a few dietary diversity seems necessary for proper larval development. Blood-only diets allow only twelve percent of larvae to mature, whereas blood and yeast or dog chow diets allow almost all larvae to mature. An Additional study additionally showed that ninety percent of larvae matured into adults when the diet included nonviable eggs. They are blind and avoid sunlight, keeping to dark, humid places such as sand or soil, cracks and crevices, under carpets and in bedding.

Pupae

Given an adequate supply of food, larvae pupate and weave silken cocoons after three larval stages. Within the cocoon, the larva moults for a final time and undergoes metamorphosis into the adult form. This can take just four days, but might take much longer under adverse conditions, and there follows a variable-length stage throughout 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 stimuli might indicate the presence of a suitable host. Large numbers of pre-emergent fleas might be present in otherwise flea-free environments, and the introduction of a suitable host might trigger a mass emergence.

Adult

Once the flea reaches adulthood, its primary goal is to find blood and then to reproduce. Female fleas can lay 5000 or more eggs over their life, allowing for phenomenal growth rates. Average 30–90 days. A flea might live a year and a half under ideal conditions. These include the right temperature, food supply, and humidity. Generally speaking, an adult flea only lives for 2 or 3 months. Without a host for food a flea's life might be as short as a few days. With ample food supply, the adult flea will often live for up to 100 days.

Newly emerged adult fleas live for only about one week if a blood meal isn't obtained. Notwithstanding completely developed adult fleas can live for several months without eating, so long as they don't emerge from their puparia. Optimum temperatures for the flea's life cycle are 21 °C to 30 °C (70 °F to 85 °F) and optimum humidity is 70%.

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.

Taxonomy and phylogeny

The snow scorpionflies (Boreidae) are the sister clade to the Siphonaptera.

Fleas are related to the Diptera (true flies) and the Mecoptera (scorpion flies) as shown in the cladogram. The Boreidae (snow scorpionflies) are the sister clade to the Siphonaptera.

part of Endopterygota

Antliophora

Diptera




Mecoptera (scorpionflies, hangingflies)




Boreidae (snow scorpionflies)



Siphonaptera (fleas)







Trichoptera (caddisflies)



Lepidoptera (butterflies and moths)





Hymenoptera (sawflies, wasps, ants, bees)



Cenozoic flea in amber, c. 20 mya, is morphologically modern.

Fossils of wingless "pre-fleas" with siphonate mouthparts from the middle Jurassic to early Cretaceous have been found in northeastern China. These belonged to three proposed extinct families, the Pseudopulicidae, the Saurophthiridae, and the Tarwiniidae. The last common ancestor of modern Siphonaptera separated from the Mecoptera throughout the early Cretaceous. Most flea families formed after the end of the Cretaceous (in the Paleogene and onwards). Fleas probably arose in the southern continental area of Gondwana, and migrated rapidly northwards from there. They most likely evolved with mammal and marsupial hosts, only later moving to birds and monotremes.

Flea phylogeny was long neglected, the discovery of homologies with the parts of additional 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 Tungidae, including the harmful chigoe flea or jigger, is sister to the rest of the Siphonaptera.


Boreidae (snow scorpionflies)


Siphonaptera

Tungidae (inc. chigoe flea or jigger)






Pygiopsyllomorpha




Macropsyllidae, Coptopsyllidae



Neotyphloceratini, Ctenophthalmini, Doratopsyllinae






Stephanocircidae



clade inc. Rhopalopsyllidae, Ctenophthalmidae, Hystrichopsyllidae







Chimaeropsyllidae



Pulicidae (inc. the cat flea, vector of bubonic plague)




Ceratophyllomorpha (inc. the Ceratophyllidae, such as the widespread moorhen flea)





Relationship with host

Flea bites

Fleas feed on a wide variety of warm-blooded vertebrates including humans, dogs, cats, rabbits, squirrels, ferrets, rats, mice and birds. Fleas normally specialise in one host species or group of species, but can often feed but not reproduce on additional species. Ceratophyllus gallinae affects poultry as well as wild birds. The chigoe flea or jigger (Tunga penetrans) causes the disease tungiasis, a major public health problem around the world.

Direct effects of bites

Human foot infested with jigger fleas, Tunga penetrans

In a large number of species, fleas are principally a nuisance to their hosts, causing an itching sensation which in turn causes the host to try to remove the pest by biting, pecking or scratching. Fleas aren't simply a source of annoyance, however. Flea bites cause a slightly raised, swollen itching spot to form; this has a single puncture point at the centre, like a mosquito bite.:126 Besides this, the eczematous itchy skin disease flea allergy dermatitis is common in a large number of host species, including dogs and cats. 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 hair loss as a result of frequent scratching and biting by the animal, and can cause anemia in extreme cases.:126

As a vector

Fleas are vectors for viral, bacterial and rickettsial diseases of humans and additional animals, as well as of protozoan and helminth parasites. Bacterial diseases carried by fleas include murine or endemic typhus.:124 and bubonic plague. Fleas can transmit Rickettsia typhi, Rickettsia felis, and Bartonella henselae, and the myxomatosis virus.:73 They can carry Hymenolepiasis tapeworms and Trypanosome protozoans.:74 Fleas that specialise as parasites on specific mammals might use additional mammals as hosts; thus, humans might be bitten by cat and dog fleas.

Relationship with humans

In literature and art

Robert Hooke's drawing of a flea in Micrographia, 1665

Fleas have appeared in poetry, music and art, as in Robert Hooke's pioneering drawing of a flea in his microscope in 1665, 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.

John Donne's erotic metaphysical 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.

Flea circuses

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

Flea circuses have provided entertainment. These circuses, extremely popular in Europe from 1830 onwards, featured fleas dressed as humans or towing miniature carts, chariots, rollers or cannon. These devices were originally made by watchmakers or jewellers to show off their skill at miniaturization. A ringmaster called a "professor" accompanied their performance with a rapid circus patter.

As a biological weapon

Fleas are seen as a potential biological weapon as they can carry Yersinia pestis. The infected fleas feed on rodents which then infect the human population with the plague. During World War II, the Japanese army dropped fleas infested with Y. pestis in China. The bubonic and septicaemic plague are the most probable form of the plague that would spread as a result of a bioterrorism attack that used fleas as a vector.

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 plague vector flea, Xenopsylla cheopis, additionally known as the oriental rat flea, in 1903. Using what was probably the world's most complete collection of fleas of about 260,000 specimens (representing a few 73 percent 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.

Flea treatments

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.