To take the example of the cat flea, there are four main stages in the life cycle.
Life Cycle of FLea
Once hatched from the egg, the flea larva feeds on the faecal material of the adult flea. In this way, the adult population on the host nutritionally supports the growing juvenile larvae. They actively move away from light sources and hide against the skin in the darker areas of the body of the animal. Equally, in a domestic environment many eggs will fall off the host and land in the carpet and then work their way deep inside. The larvae metamorphose four times before finally creating a cocoon and entering the pupal stage.
The length of time the flea remains as a pupa varies depending on how whether it detects the presence of a host or not. This might be through vibration or heat. Once emerged the young adult will instinctively jump on the nearest host within seconds, unless of course if finds itself already there.
The female reaches full reproductive age very quickly, mates with the male and lays its eggs on the host. It may lay somewhere in the region of 200 eggs in a week.
Once the eggs have dried out, they tend to work their way out of the animals hairs and down to the skin surface where they are more protected. They will hatch after approximately one to two weeks depending on environmental conditions.
The nature of the life cycle, in terms of its short duration and the number of offspring involved, mean that it is extremely unlikely that fleas once in your home in any number will disappear of their own accord. In the past numbers did drop substantially in winter months but with the advent of central heating, the impact is much less. One of a raft of flea treatments is generally recommended, including the use of flea bombs otherwise known as flea foggers.