Though wingless, fleas evolved from the same winged ancestors as did the scorpionflies (order Mecoptera). Once thought to be close relatives of true flies (order Diptera), fleas are now widely believed to be most closely related to snow scorpionflies (family Boreidae), an unusual group of cold-loving, wingless insects active in the winter. In other words, fleas are thought to have evolved from within the scorpionfly order, although a competing hypothesis holds that the two orders evolved independently from an earlier common ancestor. In either scenario, fleas probably emerged sometime after the origin of the mammals.


Fleas are laterally flattened, meaning they are flat from side to side, like a coin standing on end, and average 1–5 mm in length. Adult fleas are shiny, yellowish-brown to black, and highly specialized for life on furry or feathered animals: their laterally flattened shape and stiff, backward-directed hairs make forward movement through host hair quick and easy, while also making them difficult to dislodge by grooming. Fleas have smaller, simpler eyes than most insects and their antennae are small and short, usually hidden in grooves on the head. Fleas feed by piercing the skin with specialized pointed mouthparts, and their saliva contains an anticoagulant that allows the host’s blood to be drawn up freely, without clotting.

Fleas have characteristically long, muscular legs, each with a pair of claws used to grasp host hairs. Their famous jumping ability results primarily from a highly elastic piece of exoskeleton at the base of their hind legs, made of resilin. Resilin is an elastic protein, responsible for the efficient flight of most other insects. When a flea prepares to jump, this lump of resilin is locked into a compressed state, like a coiled spring. When the spring is released, the resilin expands rapidly, flinging the flea’s hind legs against the ground with incredible speed. Like a pair of long levers, the flea’s legs propel it from the surface, allowing it to jump as far as 32 cm — quite the distance for such a small insect.

Distribution and Habitat

Fleas inhabit all continents, including Antarctica, and are found in all manner of habitats, from rainforest, to desert, to high Arctic tundra, and virtually anywhere else they’re able to thrive in the warm nests of their hosts. While most flea species inhabit Eurasia, close to 300 species are found in North America, with at least 127 species in Canada. Most of these occur in Western Canada, where there is a greater number of rodent and small mammal hosts: British Columbia has around 89 flea species, Alberta around 57, and Saskatchewan about 36, while the remaining provinces and territories have fewer than 30 species apiece. Six of the most common species in Canada are believed to have been introduced from Europe or Asia, and are parasites of humans and domestic animals (such as Ctenocephalides canis on dogs and C. felis on cats).

Reproduction and Development

Fleas undergo complete metamorphosis. Eggs are dropped by female fleas living on the host animal to the ground of the host’s nest or burrow, and after a few days, the egg hatches into a larva. The larva lives in the host animal’s nest and feeds on shed skin and organic debris, especially the blood-rich feces of adult fleas. Eventually, the larva spins a cocoon and transforms into a pupa inside of it. Pupae can remain in their cocoons for anywhere from a week to a year. Adults, once developed, emerge and find their way onto a host animal. Adults of both sexes and of virtually all species depend on blood for nourishment and reproduction. Mating usually takes place on the host animal, and having mated just once, a female flea can continue to fertilize eggs using stored sperm. Most fleas lay eggs in small batches or broods, and the number of broods in a year is highly variable between species. The cat flea, Ctenocephalides felis, can lay close to two dozen eggs per day over the course of a month.

Newly formed adults can lay dormant in a host nest for up to a year or more, until a host animal is detected. In fact, upon entering an empty home where flea-infested pets previously lived, sometimes fleas can be seen suddenly leaping up from the floor. These formerly dormant fleas have detected a potential host (a human) using cues such as low-frequency sound, vibration and increases in temperature and carbon dioxide levels.


Some fleas feed only on specific host animals, but many are less picky. Some flea species, for example, may be found living on as many as 30 different types of hosts. On the other hand, some mammals may be attacked by over 20 different species of fleas. About 94 per cent of all flea species attack mammals and only about 5 per cent attack birds. Common host animals in Canada include cats, dogs, squirrels, racoons, skunks, ground squirrels and other rodents.

Some fleas interact with their hosts in a highly specialized way. For example, a female European rabbit flea (Spilopsyllus cuniculi) will only produce eggs after it has fed on the blood of a pregnant female rabbit, and will only mate and lay eggs after it has fed on the blood of the newborn rabbits. This finely tuned interaction relies on hormones in the pregnant rabbit’s blood which, when consumed, send a signal to the female flea’s ovaries to develop.

Interaction with Humans

Fleas are usually no more than a nuisance to humans. However, the fact they feed on blood make fleas potential transmitters of disease, such as murine typhus, tularemia, cat scratch disease and, most notably, bubonic plague. Today, these diseases are a minor threat to human health, and in general people are unlikely to develop anything more than an itchy welt from a flea bite, usually from dog and cat fleas (Ctenocephalides canis and C. felis). However, during the Middle Ages, the bubonic plague was responsible for the deaths of a large proportion of the world’s population. The bacterium causing plague (Yersinia pestis) was primarily transmitted from rats to humans by the Oriental rat flea (Xenopsylla cheopis). Cunningly, the plague bacterium clogs the digestive tract of an infected flea, so that the more it tries to feed, the more likely it is to regurgitate infected blood and pass the infection onto its host. Nowadays, plague is normally treatable with antibiotics, though outbreaks do still occur in some parts of the world. A few cases are reported each year in the United States, where plague is most likely to be transmitted by fleas that bite rodents. In Canada, the last suspected human case of plague was reported in 1939, and there have only been three confirmed reports of plague in Canadian rodent populations.

How Do You Get Rid of Fleas?

On domestic cats and dogs, adult fleas are effectively removed from fur using a fine-toothed flea comb, and are most easily spotted against a light background, such as a white sheet. Flea infestations are controlled by first eliminating adult fleas, and then by eliminating larvae and pupae. A number of products are available for treating adult fleas both on pets and in the home. With such products, it is important to carefully follow the instructions on the label to ensure that the treatment is both effective and safe. Juvenile stages can be treated by washing pet bedding regularly, by vacuuming and/or steam-cleaning carpets and furniture and by carefully cleaning floors and baseboards, especially around where pets sleep. Products containing insect growth regulators can be used to prevent any remaining larvae from developing into adults, and to avoid recurring infestations. For any treatments on pets (oral or topical), caretakers should always consult with a veterinarian first.