Many aspects of adult flea morphology are closely correlated with their parasitic lifestyle. They are small wingless insects, usually less than five millimetre long, with a heavily sclerotised and distinctive laterally flattened body. Females have a downward sloping abdomen, males have an upward pointed abdomen.
They can be light yellow to almost black in colour and are generally shiny and with varying numbers of bristles. The presence of backward pointing combs in some species helps the flea retain itself in the feathers or fur of its host, and prevents easy removal from the host's body by preening or grooming.
The short antennae have three segments and lie in deep grooves on either side of the head, thus enabling easy movement through the host's feathers or fur.
The mouthparts are of the piercing/sucking type, with two pairs of well-developed palps. The legs are relatively long, the hind ones with especially large coxae to enable jumping, and the five-segmented tarsi have prehensile claws. The adult flea is highly adept at jumping, facilitated by the storage of the elastic protein resilin in the pleural arch, which supplies the necessary energy. Fleas living on larger hosts can attain the greatest heights, some as much as 33cm, or 100 times their own body length, and have been known to jump hundreds of times per hour, for many hours.