Emergence occurs in response to pressure applied to the cocoon or detection of host warmth, vibrations, or carbon dioxide in the host's breath. Adult fleas are small, brownish insects flattened from side to side, without wings but with powerful jumping legs. Adults can live for several years and go without feeding for months at a time under extreme conditions. Fleas can remain in a structure long after the host mammals have been removed. Depending on the species and environmental conditions, adults can breed from two weeks to two years after emerging.
Adults feed on blood, and females deposit eggs only after a blood meal.
Most species remain on the host only long enough to feed. Nearly all species have host preferences but are not restricted to any one host species. Adults prefer warm humid places and will leave a host if it dies. Outdoors, fleas are most abundant during humid, rainy summers and are more common outside in the southern United States than in the north. Indoors, warmth and high relative humidities are conducive to large populations.