Fleas are small (1/16 to 1/8-inch (1.5 to 3.3 mm) long), agile, usually dark colored (for example, the reddish-brown of the cat flea), wingless insects 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). This is around 200 times their own body length, 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 (human anatomical terms), 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 water and may not survive direct contact with anti-flea pesticides.
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 pupal phase, the larva is enclosed in a silken, debris-covered cocoon.
Life cycle and habitat
Fleas are holometabolous insects, going through the four life cycle stages of egg, larva, pupa, and imago (adult). The flea life cycle begins when the female lays after feeding. Adult fleas must feed on blood before they can become capable of reproduction. 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.
Fleas pass through a complete life cycle consisting of egg, larva, pupa and adult. A typical flea population consists of 50% eggs, 35% larvae, 10% pupae and 5% adults. Completion of the life cycle from egg to adult varies from two weeks to eight months depending on the temperature, humidity, food, and species. Normally after a blood meal, the female flea lays about 45 to 50 eggs per day up to 600 in a lifetime usually on the host (dogs, cats, rats, rabbits, mice, squirrels, chipmunks, raccoons, opossums, foxes, chickens, humans, etc.). Eggs loosely laid in the hair coat, drop out most anywhere especially where the host rests, sleeps or nests (rugs, carpets, upholstered furniture, cat or dog boxes, kennels, sand boxes, etc.). Eggs hatch in two days to two weeks into larvae found indoors in floor cracks & crevices, along baseboards, under rug edges and in furniture or beds. Outdoor development occurs in sandy gravel soils (moist sand boxes, dirt crawlspace under the house, under shrubs, etc.) where the pet may rest or sleep. Sand and gravel are very suitable for larval development which is the reason fleas are erroneously called “sand fleas.”
Larvae are blind, avoid light, pass through three larval instars and take a week to several months to develop. Their food consists of digested blood from adult flea feces, dead skin, hair, feathers, and other organic debris. (Larvae do not suck blood.) Pupa mature to adulthood within a silken cocoon woven by the larva to which pet hair, carpet fiber, dust, grass cuttings, and other debris adheres. In about five to fourteen days, adult fleas emerge or may remain resting in the cocoon until the detection of vibration (pet and people movement), pressure (host animal lying down on them), heat, noise, or carbon dioxide (meaning a potential blood source is near). Most fleas overwinter in the larval or pupal stage with survival and growth best during warm, moist winters and spring.
Adult fleas cannot survive or lay eggs without a blood meal, but may live for one year without feeding. There is often a desperate need for flea control after a family has returned from a long vacation. The house has been empty with no cat or dog around for fleas to feed on. When the family and pets are gone, flea eggs hatch and larvae pupate. The adult fleas fully developed inside the pupal cocoon remains in a kind of “limbo” for a long time until a blood source is near. The family returning from vacation is immediately attacked by waiting hungry hordes of fleas. (In just 30 days, 10 female fleas under ideal conditions can multiply to over a quarter million different life stages.)
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 and optimum humidity is 70%. 
Fleas are related to mecoptera, winged insects with good eyesight. The flightless boreid (snow scorpionfly) with its rudimentary wings seems to be close to the common ancestor of the 2000 or so known varieties of flea, which split off in many directions around 160 million years ago. 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 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. 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.
Relationship with host
Fleas attack a wide variety of warm-blooded vertebrates including dogs, cats, humans, chickens, rabbits, squirrels, rats, ferrets, and mice.
Direct effects of bites
Main article: Pulicosis
Fleas (Parasite) are not only a nuisance to humans and their pets, but can cause medical problems including flea allergy dermatitis (FAD), secondary skin irritations and, in extreme cases, anemia, tapeworms, stomach flu. Fleas can transmit murine typhus (endemic typhus) fever among animals and from animal to humans. Also, fleas can transmit bubonic plague and any other disease from human to rodent and from rodent to humans. Tapeworms normally infest in humans in severe cases. Although bites are rarely felt, it is the resulting irritation caused by the flea salivary secretions that varies among individuals. Some may witness a severe reaction (general rash or inflammation) resulting in secondary infections caused by scratching the irritated skin area. Others may show no reaction or irritation acquired after repeated bites over several weeks or months. Most bites usually found on the ankles and legs may cause irritation or pain lasting a few minutes, hours or days depending on one’s sensitivity. The typical reaction to the bite is the formation of a small, hard, red, slightly raised (swollen) itching spot. There is a single puncture point in the center of each spot. (Ants and spiders leave two marks when they bite. Mosquitoes, bees, wasps and bedbugs cause a large swelling or welt.):126
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