#siphonaptera

Genetic evidence places fleas within the Mecoptera (“scorpionflies”), however, confirmation of this is coming out a bit too late for the planning of this tournament. Fossils of large, wingless stem-group fleas with siphonate (sucking) mouthparts from the Middle Jurassic to Early Cretaceous have been found in northeastern China and Russia, showing that they diverged a blood-sucking lifestyle from their predatory/scavenging cousins before this time. 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 birds.

Some species of flea can leap 50 times their body length, a feat second only to jumps made by froghoppers (superfamily Cercopoidea). A flea can jump 60 times its length in height and 110 times its length in distance, equivalent to a 1.8 m (6 ft) adult human jumping 110 m (361 ft) vertically and 200 m (656 ft) horizontally. 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 resilin before releasing it rapidly (like a human using a bow and arrow). 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.

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 armadillos, the Ischnopsyllidae only on bats, and the Chimaeropsyllidae only on elephant shrews. Not all fleas are strictly species-specific, but they may stay within the species’ order (ie. Carnivora, Rodentia, etc.). 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).

The Oriental Rat Flea (Xenopsylla cheopis) is a vector of Yersinia pestis, the bacterium that causes bubonic plague. Major outbreaks included the Plague of Justinian, about 540 AD, and the Black Death, about 1350 AD, each of which killed a sizeable fraction of the world's people. Not the fleas’ faults of course, they didn’t know what they were doing, but crazy that tiny little insects could change so much of world history with tiny little bites.

One theory of human hairlessness is that the loss of hair helped humans to reduce their burden of fleas and other ectoparasites, and especially in old age.

Flea larvae do not always feed on blood like their parents. They emerge from the eggs to feed on any available organic material such as dead insects, the faeces of mature fleas, other flea 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. Another study also showed that 90% of larvae matured into adults when the diet included nonviable (“dud”) eggs.

Given an adequate supply of food, larvae pupate and weave silken cocoons after three larval stages. Fleas will wait in their cocoon for 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. 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. Sometimes a new house may seem clean, quiet, and free of fleas, until a noisy child or dog enters the home and the fleas emerge from their pupae at the presence of the new host.

Generally speaking, an adult flea only lives for 2 or 3 months. Completely developed adult fleas can live for several months without eating, so long as they do not emerge from their puparia. If they emerge too early, without a host to provide a blood meal, a flea's life can be as short as a few days.

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.

Since their larvae must mature in the host’s nest, fleas only infest animals that have a regular nest site. This explains why most rodents (e.g., rats, mice, etc.) and many colonial birds have fleas but most ungulates (e.g., cows, horses, deer, etc.) and solitary birds do not. Humans may not have had issues with fleas before we started settling down in houses!

A flea circus is a circus sideshow attraction in which fleas are attached (or appear to be attached) to miniature carts and other items, and encouraged to perform circus acts within a small housing. The first records of flea performances were from watchmakers who were demonstrating their metalworking skills by making miniature harnesses and chariots. In 1578, the London blacksmith Mark Scaliot produced a lock and chain that were attached to a flea. The first recorded flea circus dates back to the early 1820s, when an Italian impresario named Louis Bertolotto advertised an "extraordinary exhibition of industrious fleas" on Regent Street, London. Bertolotto's flea shows included "four card-playing fleas, a flea orchestra (allegedly playing audibly), and a reenactment of Napoleon's defeat at Waterloo". Flea performances with military themes such as staged "sword fights" among fleas, were popular in the 19th century. Some flea circuses persisted in very small venues in the United States as late as the 1960s. The flea circus at Belle Vue Zoological Gardens, Manchester, England, was still operating in 1970. Real fleas were in fact used in some of these acts, harnessed with a thin gold or copper wire wrapped around their necks, and attached to props. Once in the harness, the fleas usually stayed in it for life. The strong legs of the flea allowed them to pull objects significantly larger than themselves, letting them partake in chariot races and operate carousels. There were (crueler) historical reports of fleas glued to the base of the flea circus enclosure. Miniature musical instruments were then glued to the flea performers and the enclosure was heated. The fleas fought to escape, giving the impression of playing instruments. Mole fleas (family Hystrichopsyllidae) were typically used as performers because they are large (up to 5.5 mm in length) and cannot jump as far as most other fleas. Of course, many other “flea circuses” did not use actual fleas, but were merely electrical, magnetic, and/or mechanical devices made to produce the illusion of a small circus with tiny or invisible performers. These circuses were known as "Humbug" flea circuses. As of 2013, there was still at least one genuine flea circus still performing (at the annual Oktoberfest in Munich, Germany) and Svensons in the UK still occasionally use real fleas.