How Fleas Work

An adult flea.
Photo courtesy CDC/Vector Ecology & Control Laboratory, Fort Collins, Colo.

Imagine returning to your home after a long vacation. You pick up your pets from the kennel, unload your luggage and head to bed to recover from the long drive. But your sleep is anything but restful. All night, you're plagued by tiny pinpricks and incessant itching. It doesn't take you long to figure out that you're being attacked by a seemingly infinite mob of hungry fleas.

­ What happened? Did your pets pick up an infestation at the kennel? Did the vampire-like insects hitch a ride on your luggage? Or did a swarm of them decide to move in while you were gone?


It's a creepy idea, but the most likely answer is that the fleas were waiting for you. Fleas are parasites -- or life forms that feed on hosts -- often harming the host in some way. Fleas use their hosts' blood as food. They generally prefer the blood of four-legged animals to human blood, so before you went on vacation, the fleas fed on your pets, not on you.

Although newly emerged fleas need to find food within a few days, adults can go for a couple of months without a meal. Flea pupae can also stay in their cocoons for up to a year, waiting to sense the body heat and vibrations that signal the presence of nearby hosts. So when you go on vacation, the fleas don't starve to death -- they simply wait for you and your pets to come back.

When you walk into your home after being away, hungry adult fleas flock to you and to anything else that has a pulse, regardless of how many legs it has. Pupae break out of their cocoons and search for their first blood meal. Your home, which seemed clean and relatively flea-free when you left, is suddenly overrun.

The ability to live without food is just one of a flea's many adaptations. These adaptations make it easier for fleas to move around on their hosts, feed on blood, reproduce and survive when food is scarce. In this article, we'll look at how these adaptations make it harder to kill fleas. We'll also explore how to keep fleas from invading your home and your pets, as well as how to get rid of an infestation.


Flea Anatomy

Photo by Dr. Darlyne A. Murawski/

Fleas are tiny, but anyone who has seen one can usually recognize them with ease. They're tiny, flat, wingless insects that have a knack for jumping away before you can catch them. Their bodies are covered with hard plates called sclerites, so if you do catch one, squashing it can be a challenge. Their hard outer shell protects fleas from everything from an animal's teeth to hitting the floor after a long jump.

To the naked eye, a flea's exoskeleton seems completely smooth, but it's really covered in tiny hairs that point away from the flea's head. Their flattened bodies and these backward-pointing hairs make it easy for fleas to crawl through their hosts' fur. But if something tries to dislodge them, the hairs act like tiny Velcro anchors. This is why a fine-toothed comb removes fleas better than a brush does. The teeth of the comb are too close together for fleas to slip through, so it can pull them from the host's hair, regardless of which way fleas' hairs are pointing.


A flea also has spines around its head and mouth -- the number and shape vary according to the flea's species. The mouth itself is adapted for piercing skin and sucking blood. Several mouthparts come together to form a needlelike drinking tube. Here's a rundown:

  • Two sawlike laciniae cut the skin. They also fit together to form a saliva channel.
  • The epipharynx is like a needle. The laciniae surround the epipharynx, and together they form the stylet, or puncturing organ.
  • The prementum and labial palps form the labium, which supports the stylet.

When a flea bites its host, blood travels from a blood vessel through the epipharynx and into the flea's body. This takes a lot of suction, which comes from pumps in the flea's mouth and gut.

The location of resilin in a flea’s leg (blue).
The location of resilin in a flea’s leg (blue).
Photo courtesy Getty Images

A flea's legs are adapted for jumping. As with all insects, a flea has three pairs of legs that attach to its thorax. The back legs are very long, and the flea can bend them at several joints. The process of jumping is a lot like the action of a crossbow. The flea bends its leg, and a pad of elastic protein called resilin stores energy the way a bowstring does. A tendon holds the bent leg in place. When the flea releases this tendon, the leg straightens almost instantly, and the flea accelerates like a bolt from a crossbow. As it lands, the flea uses tiny claws on the ends of its legs to grasp the surface under it. Aside from these adaptations, fleas look a lot like most other insects, and their reproductive cycles are similar as well. Read on to learn more about the life cycle of fleas and why it makes getting rid of an infestation difficult.

Aside from these adaptations, fleas look a lot like most other insects, and their reproductive cycles are similar as well. Read on to learn more about the life cycle of fleas and why it makes getting rid of an infestation difficult.

The Flea Life Cycle

Fleas reproduce much like butterflies do. Females lay eggs, which hatch into worm-like larvae. The larvae spin cocoons and become pupae. An adult flea emerges from the cocoon. In a population of fleas, about half are eggs, and about five percent are adults.

Female fleas can lay eggs only if they've had a meal. If a female emerges from her cocoon and isn't able to find food, she'll die without reproducing. But once she's eaten, a flea can lay close to 20 eggs at a time, for a total of 500 eggs during her lifetime.


Most of the time, fleas lay their eggs on their host. The eggs are completely smooth, so they slide off of the host and land in its environment. In people's homes, the eggs sink deep into carpet fibers and into floor cracks. Outside, flea eggs settle into the soil. Flea eggs are white -- the black specks you see on flea-infested animals and their bedding are particles of dried blood and flea droppings.

In order to develop, flea eggs need a warm, moist environment: a temperature of about 70 degrees Fahrenheit (21 degrees Celsius) and 70 to 85 percent humidity. In these conditions, the eggs hatch in about 12 days. This 12-day window is one of the reasons why it can be hard to get rid of fleas. Some insecticides kill adult fleas but not eggs, which means a whole new set of fleas can emerge after the adults have died.

Flea larvae are about 1.5 millimeters long and look like white, segmented worms. They avoid light and migrate toward cracks in the floor, where they remain for their development. Unlike their parents, the larvae don't eat blood. Instead, they eat skin cells, flea droppings and other debris. Larvae develop through three stages, or instars, molting after each.

After a week or two, flea larvae spin silk cocoons. They attach pieces of dirt and debris to their cocoons as camouflage. If food is plentiful, the adult flea emerges after about a week. Otherwise, the flea may stay in its cocoon for up to a year.

Although flea larvae and adults have completely different eating patterns, what they eat has a big impact on the diseases fleas can carry. Next, we'll take a look at flea-borne illnesses and complications from flea bites.

Flea Dangers

A tapeworm segment
A tapeworm segment
Photo courtesy CDC

Most people think of fleas as a nuisance -- they bite people and animals, causing itchy, red bumps. This reaction comes from substances in fleas' saliva, and some people react to these substances more strongly than others. Often, fleas draw blood from two or three tightly spaced punctures, causing two or three itchy bumps.

Animals that develop a sensitivity to flea bites can scratch or groom themselves excessively. This can lead to rough skin, baldness and skin infections. Sometimes, these infections can be mistaken for a parasitic infection called mange. Itchy, uncomfortable pets can also be ill-tempered or restless.


In addition to being annoying, fleas can carry diseases. They can even transmit other parasites to people and animals. Flea larvae feed on the eggs of tapeworms. Once swallowed, the tapeworms begin to develop in the gut of the flea. If an animal swallows an infected flea while grooming, the animal becomes infected with tapeworm as well. The tapeworm grows in the intestine of the animal, shedding its egg sacs from the animal's rectum as it sleeps. The sacs land in areas near the animal's bedding -- where flea larvae are likely to live -- and the cycle begins again. People, especially children, can also become infected with tapeworms if they swallow fleas while petting animals or if people come into contact with infected waste and don't wash their hands before eating.

Bubonic plague causes painfully swollen lymph nodes,
Bubonic plague causes painfully swollen lymph nodes,
Photo courtesy CDC

Fleas can also transmit serious diseases. One is bubonic plague. Fleas are vectors -- they transport plague bacteria from rodents, the natural carriers of the disease, to people. A particular species of flea, the Oriental rat flea, is usually the culprit. In addition to preferring to feed on rats, the Oriental rat flea's gut can become blocked by plague bacteria. Then, when it bites its next host, it may regurgitate infected blood into the wound. Symptoms of plague include swollen lymph nodes called buboes, fever, headache and exhaustion. Read How Plague Works to learn more about plague and how fleas spread it.

Fleas can also carry murine typhus, particularly in the southern and southwestern parts of the United States. Murine typhus is caused by the bacteria Rickettsia typhi. The Oriental rat flea and the cat flea can carry these bacteria to people. Usually, the infection comes from the flea's waste rather than the flea's mouth. The fleas defecate while they're eating, and their hosts scratch the infected waste into the bite. People may also break the skin while scratching, and the bacteria can enter the body through the scratch. Murine typhus causes symptoms like fever, headaches and nausea. Both it and bubonic plague are treated with antibiotics.

Since fleas are annoying and can be detrimental for human animal and health, getting rid of them is usually a big priority. Let's take a look at how to keep your home and your pets flea free.

Getting Rid of Fleas

Photo courtesy Consumer Guide Products
Photo courtesy Consumer Guide Products

For many people, seeing fleas prompts an immediate trip to a pet-supply store or to the vet for some type of chemical that will get rid of them. These chemicals can fall into a few basic categories:

  • Insect growth regulators (IGRs) keep flea eggs from hatching by mimicking flea hormones. They may or may not affect pupae, and they have no effect on adult fleas. Some IGRs are topical -- pets absorb them through their skin, and fleas ingest them with the hosts' blood. One IGR, methoprene, is often used as a spray. The World Health Organization has also approved methoprene for use in controlling mosquito larvae in malaria-prone areas.
  • Chitin synthesis inhibitors, also known as insect development inhibitors, keep young fleas from developing properly by disrupting the formation of their exoskeletons. Lufenuron, also known as Program, is a chitin synthesis inhibitor.
  • Pesticides kill adult fleas. Imidacloprid, sold under the brand name Advantage, is an example of a topical pesticide.
  • Repellents like DEET discourage fleas from entering a particular area.

Some chemicals are intended for use in a particular environment, like a room or a yard. Most of the time, you should remove pets and children before applying the chemical, and everyone should stay away from the area until it dries. Other flea treatments are topical or oral medications for pets. Some of these treatments require fleas to bite pets in order to work, while others do not. Typically, you can purchase these flea treatments from a veterinarian. For information on specific chemicals and how to use them, see these overviews from the Texas Agricultural Extension Service and the University of Florida. If you plan to use chemicals to treat your pets or your home, use them sparingly, and follow all accompanying instructions to the letter.


Today, many flea treatments are exceptionally powerful -- they kill fleas and their eggs within a few days. However, sometimes a simple on-the-spot treatment isn't enough to rid your home and your pets of fleas. Here are some simple steps for preventing and dealing with flea infestations:

  • Treat your pets and their environment at the same time.
  • Bathe your pet and use a flea comb to remove fleas. If you plan to use a topical flea treatment on your pet, follow the instructions regarding how long to wait before or after bathing your pet.
  • Vacuum your home thoroughly at least every other day. Immediately empty the canister or replace the bag, and discard the debris in an outdoor garbage can.
  • Launder or replace pets' bedding.
  • Mow your lawn regularly. Rake and discard any leaves or other lawn debris, and keep piles of sand away from your home.
  • Whenever possible, discourage wild animals from entering your yard.

There are a few home remedies for fleas that don't work. While flea collars can help keep fleas off of your pets, they don't generally have much of an effect on established infestations. Feeding brewer's yeast, garlic and vitamin supplements to pets is also unlikely to do much good against fleas. Ultrasonic collars also don't appear to act as a flea deterrent or repellent.

Most of the time, if you treat your pet, lawn and home for fleas at the same time, you can get rid of an infestation on your own. If not, you may need to contact a professional exterminator.

To learn more about fleas, pets and related topics, check out the links on the next page.

Related HowStuffWorks Articles

More Great Links


  • Merchant, Mike. "Controlling Fleas." Texas Agricultural Extension Service. (8/6/2007)
  • Richman, D.L. "Fleas: What They Are, What They Do." University of Florida. (8/6/2007)
  • Fox, Richard. "Invertebrate Anatomy Online: Cat and Dog Fleas." Lander University. 6/30/2006 (8/6/2007).
  • University of Florida IFAS Extension. "Fleas." (8/6/2007)
  • University of Missouri. "Fleas.>" (8/6/2007)
  • Micrographia. "The Life Cycle of the Flea." (8/6/2007)
  • The Tapeworm Page (8/6/2007)
  • Texas Department of State Health Services. "Murine Typhus." 10/1/2005 (8/6/2007)
  • Smith, Keith L. "Fleas." Ohio State University Extension. (8/6/2007)
  • Jacobs, Steve. "Entomological Notes: Fleas." Penn State. (8/6/2007)