Fleas: Siphonaptera

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FLEAS: Siphonaptera



Adult fleas are small insects usually measuring 0.04 to 0.3 inches (1.0 to 8 millimeters). The females have shorter antennae (an-TEH-nee) or sense organs and are usually larger than the males. The bodies of fleas are flat from side to side, allowing them to move easily between the fur and feathers of their mammal or bird hosts. Their tubelike mouthparts are used to pierce skin and suck blood. The eyes, if present, are simple and made up of a single lens each. The head and thorax, or midsection, sometimes have special comblike structures, while the legs are spiny. These features help to protect their bodies from damage, allow the fleas to cling to hair and feathers, and prevent them from being removed by the grooming activities of their hosts. The spiny and hairlike structures of bird fleas are longer, more slender, and more numerous than those of mammal fleas. The legs of fleas are used for moving through hair and feathers. Fleas are excellent jumpers and use this method to find new hosts. The abdomen is ten-segmented. In some species the abdomen of the females expands up to one hundred times its original size to accommodate either a blood meal or hundreds of developing eggs. They actually grow new tissue to allow their abdomens to expand without molting, or shedding their exoskeletons or hard outer coverings.

The legless, wormlike larvae (LAR-vee) or young do not resemble the adults at all. They range from 0.02 to 0.4 inches (0.5 to10 millimeters) in length. Their bodies have a distinct head, a three-segmented thorax, and a ten-segmented abdomen.

The legs, wings, antennae, and mouthparts of all flea pupae (PYU-pee) are distinct. These appendages are not attached to the pupae along their entire length. The pupae are surrounded by a silk cocoon and measure 0.008 to 0.4 inches (0.2 to 10 millimeters) long. The pupa is the life stage of the flea between larva and adult.


Fleas are found on all continents, including Antarctica. There are 2,575 species of fleas worldwide, 258 of which occur in North America.


About 5 percent of all flea species occur on birds, while the remaining 95 percent parasitize, or live off of, mammals. They usually do not parasitize amphibians and reptiles. Fleas parasitize hosts in nearly all habitats where their hosts live and are found not only on their bodies but also in their burrows and nests. Bird fleas only parasitize species that reuse their nests year after year, including swallows, seabirds, some ground-dwelling species, and those living in tree holes and cavities. A few flea species that live in coastal, warm and moist, and tropical regions are free-living. Cat, dog, and human fleas all regularly spend time away from their hosts and are commonly found on the floors of homes, foot paths, animal pens, and pet beds. Most larvae are free-living and do not make their home on the body of a bird or mammal. They are usually found in pet beds and nests.


Free-living larvae scavenge scabs, flakes of skin, dried blood produced as waste by adult fleas, and other bits of tissue found in the host's nest. Some larvae live on the body of an animal, where they eat bits of skin and other tissues and fluids. Uropsylla tasmanica is the only larva known to burrow into the skin of its host, the Tasmanian devil. A few species prey on other insects that live in the host nest.

Adult fleas feed on host blood. Some species suck blood directly from a capillary (KAH-peh-LEH-ree), a small blood vessel just under the skin that connects arteries and veins. Others simply cut into the capillary and suck up the blood that pools on the surface of the skin. Males and females require a blood meal to produce sperm and eggs. Females usually drink more blood than males.


The survival of every flea species depends on its ability to find a suitable host. Some species remain in the pupal stage for long periods of time to survive cold weather or to wait until a host comes by. The vibrations of an approaching host often trigger adults to emerge from their pupae (PYU-pee). Although fleas cannot see very well, they do respond to moving shadows by jumping. They are also attracted to the body heat and carbon dioxide, a respiratory gas exhaled by potential hosts. Cave-dwelling species instinctively crawl upward on the walls of their homes to find bats roosting on the ceiling.

Males deposit sperm directly into the reproductive organs of the females. The mating behavior of fleas differs from species to species. In many fleas the male grabs the sides of the female's abdomen with suckerlike structures on the inner surface of his antennae. He may also grasp her hind legs with his. With special claspers at the tip of his abdomen he locks his body to the tip of the female's abdomen.

The reproductive cycles of fleas are usually timed to match the reproductive cycles of their mammal hosts or the nesting and migratory habits of bird hosts. This way, hatching flea larvae will have plenty of young animals available to provide them with food.


A flea can jump one hundred times its body height. That is like a human being jumping 600 feet (182.8 meters) straight up. Instead of muscles, fleas rely mostly on tiny, springlike pads at the bases of their back legs. Made of a rubberlike protein called resilin (REH-zih-lihn), the pads are squeezed as the back legs are pulled up close to the body. When the legs are suddenly extended, energy stored from the squeezing is released, propelling the flea into the air.

The life cycle of fleas includes four very distinct stages: egg, larva, pupa, and adult. Some species lay eggs on the host or in the host's burrow or nest. Others deposit their eggs outdoors in soil rich with decaying plants, in carpets, or in animal beds. The larvae usually molt three times before reaching the pupal stage. Depending on species, temperature, and humidity, flea larvae may take a few weeks to several months to reach adulthood. Adults may live a few weeks to three years.


The bites of the dog, cat, and human fleas are very annoying and may cause itching and allergic reactions in sensitive people. Scratching flea bites often causes further discomfort and leads to infections. Some fleas carry diseases that infect both humans and pets. Plague and other diseases are spread either through the bites of infected fleas or by rubbing the waste of infected fleas into the bite wounds. Eating fleas infected with parasitic worms and blood protozoan parasites can spread these harmful organisms to humans, dogs, rabbits, rats, and other animals.


There are no fleas that are considered endangered or threatened. However, flea species that feed only on one host species are vulnerable to extinction if their hosts were to become threatened by extinction. In these cases the fleas would share the fate of their hosts. No one has yet attempted to identify or list fleas that feed only on species listed by the World Conservation Union (IUCN).


Physical characteristics: The chigoe's body is straw-colored or yellowish. Adults measure up to 0.04 inches (1.0 millimeters) long, but females filled with eggs may reach 0.16 inches (4.0 millimeters). The front of the head is sharply pointed. They lack comblike structures or spines on their legs. The top of each abdominal segment has a single row of hairlike structures. The last four pairs of spiracles, or breathing holes, are large. The base of each back leg has a distinct toothlike projection.

Geographic range: This species lives in the southern United States, Central and South America, the West Indies, and tropical Africa.

Habitat: Chigoes are usually found in places where there is human filth.

Diet: Both males and females bite humans occasionally. They prefer the feet, usually infesting tender areas between toes, under the nails, or along the soles. They will also attack other parts of the body. Only females filled with eggs attach themselves permanently to their hosts.

Behavior and reproduction: Adults will pass through clothing to feed.

After mating the female finds a host and becomes permanently attached. The wound becomes irritated and surrounds the female with swelling tissues. Her body is now filled with eggs. She releases the eggs into the environment where they hatch. The larvae reach the pupal stage in about ten to fourteen days. Under good conditions the adults emerge after about ten to fourteen days.

Chigoes and people: Bites cause extreme irritation. Attached females may form bumps filled with pus on the host's skin. This may lead to infection and the loss of skin and tissue on toes. Extreme cases may result in the removal of toes by a doctor. Embedded females must be removed to help the wound to heal.

Conservation status: This species is not considered endangered or threatened. ∎


Physical characteristics: The body of a sheep and goat flea has light and dark brown markings.

Adult males grow to 0.13 inches (3.3 millimeters) in length, while females are 0.18 inches (4.5 millimeters). Females swollen with eggs may reach 0.6 inches (16 millimeters). The head and thorax lack combs. The mouthparts are very long.

Geographic range: They are found in parts of China, Mongolia, and Russia.

Habitat: They prefer pastures and agricultural fields where domestic sheep, cattle, goats, and similar wild animals live.

Diet: Adults suck blood from their hosts.

Behavior and reproduction: Males attach themselves to skin on the sides of the neck. The legs of females filled with blood are almost useless, so they wriggle through their host's fur like a worm to get around. Females filled with eggs often attach themselves to their hosts just inside the nostrils.

During the winter a single host animal may have as many as one hundred to two hundred fleas, mostly females. The dark oval eggs are dropped into the environment in spring and begin to hatch as temperatures become warmer. The larvae become pupae in late summer, and the adults emerge in early winter. The entire life cycle takes about nine months.

Sheep and goat fleas and people: This species does not bite humans but will attack their animals.

Conservation status: This species is not considered endangered or threatened. ∎



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Tavolacci, J., ed. Insects and Spiders of the World. Volume 4: Endangered Species-Gypsy Moth. New York: Marshall Cavendish, 2003.


Conniff, R. "Body Beasts." National Geographic 194, no. 6 (December 1998): 102–115.

Rothschild, M. "Fleas." Scientific American 213 (1965): 44–53.

Web sites:

Flea News. http://www.ent.iastate.edu/FleaNews/AboutFleaNews.html (accessed on October 22, 2004).

Fleas of the World. http://www.fleasoftheworld.byu.edu (accessed on October 22, 2004).

Fleas (Siphonaptera): Introduction. http://www.zin.ru/Animalia/Siphonaptera/intro.htm (accessed on October 22, 2004).

"Siphonaptera. Fleas." Ecowatch. http://www.ento.csiro.au/Ecowatch/Insects_Invertebrates/siphonaptera.htm (accessed on October 2, 2004).