Fleas: Siphonaptera

The survival of every flea species depends on its ability to find a suitable host. Some species remain in the pupal stage for long periods of time to survive cold weather or to wait until a host comes by. The vibrations of an approaching host often trigger adults to emerge from their pupae (PYU-pee). Although fleas cannot see very well, they do respond to moving shadows by jumping. They are also attracted to the body heat and carbon dioxide, a respiratory gas exhaled by potential hosts. Cave-dwelling species instinctively crawl upward on the walls of their homes to find bats roosting on the ceiling.

Males deposit sperm directly into the reproductive organs of the females. The mating behavior of fleas differs from species to species. In many fleas the male grabs the sides of the female's abdomen with suckerlike structures on the inner surface of his antennae. He may also grasp her hind legs with his. With special claspers at the tip of his abdomen he locks his body to the tip of the female's abdomen.

The reproductive cycles of fleas are usually timed to match the reproductive cycles of their mammal hosts or the nesting and migratory habits of bird hosts. This way, hatching flea larvae will have plenty of young animals available to provide them with food.

The life cycle of fleas includes four very distinct stages: egg, larva, pupa, and adult. Some species lay eggs on the host or in the host's burrow or nest. Others deposit their eggs outdoors in soil rich with decaying plants, in carpets, or in animal beds. The larvae usually molt three times before reaching the pupal stage. Depending on species, temperature, and humidity, flea larvae may take a few weeks to several months to reach adulthood. Adults may live a few weeks to three years.