Fleas are small, wingless insects with a tough cuticle with many bristles and flattened spines. The adult flea varies from about 0.039 to 0.39 inch in length and feeds only on the blood of mammals that includes humans and birds. With about 2,000 species and subspecies known, this order is still a considered small compared with many other groups of insects. However, it is widely distributed with some specie such as the rat flea and the mouse flea which have been carried all over the world by humans.
Fleas are mostly found in warm and moist climates usually in the south of the United States. Although they inhabit almost every corner of the earth due to human activity(shipping, airplanes, global transportation, etc).
The head and thorax are small compared to the abdomen. Fleas have short but thick antennae, only a pair of simple eyes, and piercing/sucking type of mouth adapted to feeding on the blood of their hosts. The thorax is made of three pairs of long jointed legs. Consisting of the hind pair(especially well adapted for jumping up to 18 cm) or about 200 times their own body length. The abdomen of the flea is made of ten segments. The flea body is hard and has many hairs/short bristles pointing backward helping the flea with easy passage through the hairs of the host. Its tough body is able to hold up against great pressure to survive scratching. Even hard squeezing between the fingers is often not enough to kill a flea.
Fleas attack a wide variety of warm-blooded vertebrates, like dogs, humans, chickens, rabbits, squirrels, rats, and mice.
Fleas are a nuisance to their hosts and causing an itching sensation. This in turn may result in the host attempting to remove the parasite by picking or scratching in the vicinity of the parasite. Besides the problems posed by their sucking of blood, fleas can also act as a transport for many diseases. Oriental rat fleas are the chief transmitter of the Bubonic plague between rodents and humans. Typhus fever and tapeworms can also be transmitted by fleas.
Based on morphological evidences, it is supposed that fleas are related to the scorpion fly family, which have good eyesight and reduced wings. With their simple wings, these flightless snow scorpion flies, do resemble fleas and seem to be close to the common ancestor of the 2000 currently known specie of fleas. It is considered that they split off in evolution about 160 million years ago. Evolution continued to produce adaptations for their parasitic niche like how they have no wings and their eyes are covered over. The large number of flea species can be connected to the wide variety of species they feed on, which provides many specific niches to adapt to.