Adult: body dark, laterally flattened, wingless; hind legs adapted for jumping; mouthparts adapted for piercing skin and sucking blood; row of large bristles often present on head and/or thorax (called genal and pronotal combs)
Larva: pale legless worm-like body covered with sparse bristles; head reduced, eyeless, with chewing mouthparts.
Eggs are laid in the host nest, bedding, carpet, upholstery, or cracks in the floor, usually hatching in 7 to 14 days. After a larval period that includes two molts, fleas pupate within a thin silken cocoon. Under favorable conditions, the life cycle can be completed in less than a month. Most fleas spend a considerable amount of time away from their host. Adults may live for two years or more and can survive for weeks or months without a blood meal.
The laterally flattened body allows easy movement among the host’s fur or feathers, and backward-pointing bristles of the hard cuticle prevent fleas from falling off or being easily captured by the host. Fleas may be extremely irritating to the host, causing skin inflammation and itching.
Fleas transmit pathogens that cause disease in humans and other animals. The Cat Flea and Dog Flea are intermediate hosts for a tapeworm (Dipylidium caninum) that infects dogs, cats, and humans. The Rabbit Flea spreads a myxomatosis virus within rabbit populations, and the Oriental Rat Flea is the primary vector of Yersinia (=Pasturella) pestis, the bacterial pathogen for bubonic plague.
The Cat Flea commonly infests dogs, and the Dog Flea may infest cats; both species may bite humans.
Fleas only infest animals that have a regular nest site, which is why most rodents (rats, mice, etc.) have fleas but most ungulates (cows, horses, deer, etc.) do not.