Flea is the common name for any of the small wingless insects of the order Siphonaptera (some authorities use the name Aphaniptera because it is older, but names above family rank do not follow the rules of priority, so most taxonomists use the more familiar name). Fleas are external parasites, living by hematophagy off the blood of mammals and birds, and 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 the past, however, it was most commonly supposed that fleas had evolved from the flies (Diptera), based on similarities of the larvae.
Note: There is also a genus of foraminiferan Protozoa named Siphonaptera
Some well known flea species include:
- Cat flea (Ctenocephalides felis)
- Dog flea (Ctenocephalides canis)
- Human flea (Pulex irritans)
- Northern rat flea (Nosopsyllus fasciatus)
- Oriental rat flea (Xenopsylla cheopis)
In most cases, fleas are just a nuisance to their hosts, but some people and some animals suffer allergic reactions to flea saliva resulting in rashes. Flea bites generally result in the formation of a slightly-raised swollen itching spot with a single puncture point at the center. The bites often appear in clusters or lines, and can remain itchy and inflamed for up to several weeks afterwards. Fleas can also lead to hair loss as a result of frequent scratching and biting by the animal, and can cause anemia in extreme cases.
However, fleas can also act as a vector for disease. One possible example of this was the bubonic plague, which may have been transmitted between rodents and humans. Murine typhus (endemic typhus) fever, and in some cases tapeworms, Hymenolepis, can also be transmitted by fleas.
Morphology and behavior
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 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 clothes). Their legs are long, the hind pair well adapted for jumping (vertically up to seven inches (18 cm); horizontally thirteen inches (33 cm)) - around 200 times their own body length, making the flea one of the best jumpers of all known animals (in comparison to body size), second only to the froghopper. The flea body is hard, polished, and covered with many hairs and short spines directed backward, which also assists its movements on the host. Its tough body is able to withstand great pressure, likely an adaptation to survive attempts to eliminate them such as scratching. Even hard squeezing between the fingers is normally insufficient to kill the flea; it may be necessary to capture them with adhesive tape, crush them between the fingernails, roll them between the fingers, or put them in a fire-safe area and burn them with match or lighter. They can also be drowned.
Fleas lay tiny white oval shaped eggs. Their larvae are small and pale with bristles covering their worm-like body. They lack eyes, and have mouthparts adapted to chewing. While the adult flea's diet consists solely of blood, the larvae feed on various organic matter, including the feces of mature fleas. In the pupal phase the larvae are enclosed in a silken, debris-covered cocoon.
Life cycle and habitat
Fleas are holometabolous insects, going through the three life cycle stages of 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 easily roll onto the ground. As such, 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.
Micrograph of a flea larva.
Flea larvae emerge from the eggs to feed on any available organic material such as dead insects, feces, and vegetable matter. They are blind and avoid sunlight, keeping to dark places like sand, cracks and crevices, and bedding. Given an adequate supply of food, larvae should pupate and weave a silken cocoon within 1-2 weeks after 3 larval stages. After another week or two, the adult flea is fully developed and ready to emerge from the cocoon. They may however remain resting during this period until they receive a signal that a host is near - vibrations (including sound), heat, and carbon dioxide are all stimuli indicating the probable presence of a host. Fleas are known to overwinter in the larval or pupal stages.
Once the flea reaches adulthood its primary goal is to find blood - adult fleas must feed on blood in order to reproduce. Adult fleas only have around a week to find food once they emerge, though they can survive two months to a year between meals. A flea population is unevenly distributed, with 50 percent eggs, 35 percent larvae, 10 percent pupae, and 5 percent adults. Their total life cycle can take as little as two weeks, but may be lengthened to many months if conditions are favorable. Female fleas can lay 500 or more eggs over their life, allowing for phenomenal growth rates.
Adult female rabbit fleas, Spilopsyllus cuniculi, can detect the changing levels of cortisol and corticosterone, hormones in the rabbit's blood that indicate she 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.
Fleas and classification
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 currently 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.
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.