It is believed that when man was nomadic and naked he had no fleas. It was only when he settled in caves and started to dress that fleas could breed and use him as a host.
The adult is Brown/black, shiny and wingless. The hind pair of legs store energy in special pads of resilin protein. The thoracic muscles compress the resilin pad, and when the muscles relax the resilin springs back to its original size releasing energy which is transmitted to the rear legs and the flea jumps. The mouthparts are adapted for blood sucking. Adult fleas can detect smells and carbon dioxide given off by animals. The female need a blood meal before she can produce eggs.

Family Pulicidae (common fleas), there are about 200 species worldwide and all feed on mammalian blood. Some are unspecialised and can live on as many as 30 different mammal species. The adults range in size from 1 - 8 mm long.
The eggs. Females lay their eggs as they feed. and the eggs drop into the host's nest, burrow or bedding.
The larvae are pale and worm-like grubs with biting jaws which usually live for 2 -3 weeks.
Adult fleas can remain inside their cocoon for week or even months until they sense the presence of a suitable host (usually by vibrations). Then they emerge and jump aboard the host. Even if the host is not moving, so that there are no vibrations, the flea can sense it by its body heat. However, if they do not find a host they can lie and wait for one for months.
Flea bites can cause allergic reactions and discomfort, and the flea can be host to parasites that it passes on to its host. For example bubonic plague (Black Death) was passed to humans in bites from the rat flea. The plague bacterium, Pasturella pestis is transmitted by rodent flea bites. The infected fleas often develop a blockage in their digestive tracts caused by the bacterium multiplying. This blockage makes the flea constantly hungry, but when it takes a blood meal the blood flows up through the flea's mouthparts, hits the blockage, then flows back to the host carrying some plague bacteria with it. Such fleas bite more often, and the bacteria spreads even more rapidly. Murine typhus is also caused by rodent fleas. Myxomatosis is passed to rabbits from Spilopsyllus cuniculi ( the rabbit flea). The largest of British fleas is the mole flea, Hystricopsylla talpae.

Pulex irritans, right, drinks human blood and is commonly known as the human flea. It has the typical flea body being compressed sideways. The maxillary palps are often mistaken for antenna but the true antennae are minute and rest in grooves. Note the rear pair of legs which are modified for jumping. P. irritans can jump more than 200 times its own height. Like all fleas the body has numerous backward-pointing spines and combs. These are used in identifying the different species, and they also make the animal difficult to remove from hair. Even after you have caught you flea killing it is not so easy as the exoskeleton is hard.

Pulex irritans was once the most common flea found on humans, and it also lives on pigs, goats, foxes and badgers. It can jump 30 cm.

Humans can be parasitized by the dog, cat and rat fleas.

Today cat (Ctenocephalides felis, see adult, larva and pupa below left) and dog (C. canis) fleas are the most common fleas to bite humans. The dog flea adult is just 2 - 3 mm long. Dog fleas can carry a tapeworm (Dipylidium caninum) that can infect dogs, cats and humans. The fleas' larva (see right) are infected with tapeworm eggs, the dog swallows the adult flea while grooming and the tapeworm now infects the dog.

Does your dog/cat have fleas?

Usually the first sign that your dog or cat has fleas - apart from scratching - is finding little dark specks of flea faeces in their hair. Around the base of the tail and the neck are places fleas like to live. The dark specks are easily seen in light hair. In dark-haired dogs you can clean the brush or comb over a piece of white paper. If you find dark specks, put a drop of water on them. If they dissolve into a reddish brown colour they are flea faeces, as the faeces are largely composed of blood.

The cat flea is one of the most powerful jumpers and can reach a height of 34 cm. This is the equivalent of a human jumping 600 feet! Though, of course it is the cross-sectional area and small size of the flea as well as the resilin pad that makes this possible.

Below is a drawing of a dog flea larva, Ctenocephalides canis.