Siphonaptera are small, wingless insects with the body flattened laterally (from side to side). All the spines on the body point to the rear of the insect which allows them to run through the hair of an animal easily. Mouthparts are formed for piercing and sucking.
The immature or larval stage is elongate and worm-like, quite different from the adults. Larvae are found in the nests of various animals, in carpets in the home or in the soil in areas where animals frequent. They are seldom seen and feed on organic debris.
Fleas are well known as pests of domestic animals and man. One species transmits the bacterium that causes plague. Plague has killed more than 125,000,000 people over the past 3,000 years. These insects are blood-feeders only as adults. They usually feed on animals but will attack humans.
Metamorphosis is complete. Most fleas are under 1/8 inch.