Like all fleas, human fleas go through a four stage life cycle of egg, larva, pupa, and adult. The flea life cycle helps explain how prodigiously quickly an infestation can spread and how difficult it is to stop. The human flea was once commonly carried by just about every human in Europe. In fact, women wore furs around their necks not just for fashion, but for capturing human fleas as well. Today, thanks largely to changes in hygiene, human fleas do not often infest humans and are more often found on pigs.
The eggs of fleas are, not surprisingly, small, about .5 millimetres. They are often laid on the animals but can either remain there or, because they are not sticky, fall from the animal into the environment. Along with each egg, the female also leaves excrement composed mostly of dried blood from the host animal, on which the newborn larvae will feed. A large number of eggs, accompanied by the excrement, may look like a powdery salt and pepper substance, sometimes called "flea dirt." Flea eggs are particularly sensitive to climate and very few will hatch at temperatures below fifty degrees Fahrenheit.
In two days to two weeks, depending on temperature and humidity, a flea larva will use a little horn on the end of its head to force its way out of the flea egg. Just 6mm in length, the larva will immediately set about eating, consuming, along with the excrement, other organic matter including dried bits of skin and dried blood. After eating for about a week, or longer if food is scarce, and after moulting several times, the larva will begin to form a pupa around itself. To form the pupa, the larva will use organic materials from its environment as well as its own silk. As in its egg form, the larva is somewhat sensitive to environmental conditions and will not survive temperatures near freezing.
After forming a casing around itself, the flea larva will begin to metamorphose into a fully adult flea. The transformation takes about ten days. However, the adult flea will not emerge from the pupa until it senses the warmth, vibration, or the increased carbon dioxide and humidity that accompany a mammalian host animal. When the flea, from inside its pupa, senses these changes in the environment, it can be assured of an immediate meal and will emerge and attack immediately. Flea pupae are very robust and can survive a range of environmental conditions. Fleas can remain dormant within pupae for several months, waiting for hosts to happen by.
The newly hatched adult flea is very hungry and will move faster and jump higher in order to get food than it will later in life. The human flea adult is 1- 10 millimetres in length and brown in colour. Like other fleas its most pronounced features are its blood sucking mouthparts and its powerful hind legs. The adult flea must have blood from its host to survive. With a blood source the adult flea can live up to several months. Without blood it will die in a matter of days. A flea population, at any given time will only have about five per cent of its members in the adult stage. All others will be eggs, larva or pupa.
Female fleas will greatly outnumber male fleas. After mating and feasting a female flea will begin laying eggs at a rate of around twenty per day, totally about 600 in a lifetime. It is this prodigious reproduction that makes flea infestations spread so quickly. Twenty-five female fleas can multiply in just thirty days to 250,000 fleas.