Adult fleas range in colour from a light brown to a dark mahogany and, when viewed with the naked eye, they are roughly an oval shape. They are flattened laterally (on their side) which allows them to quickly move through their host’s hair. The adults measure around 2 – 8 mm in size and are covered in a series of bristles and combs that enable them to ‘get stuck’ in the hairs of their host so they are not easily removed. They have two short antennae on the head and a very large and well-developed pair of rear legs. These powerful legs enable them to leap over 30 cm to attach themselves to a host, or escape from danger. Both male and female fleas rely on blood for their nutrition, though they can survive for many months in a dormant state if a blood meal is not available. To feed, a flea attaches to its host and crouches down low; it then uses its mouthparts in a sawing motion to cut into the skin. It injects a small amount of an anti-coagulant with its saliva into the wound to keep the blood flowing. Typically fleas are seen biting lower limbs and the bites are usually clustered together.
Females use blood to nourish their eggs and will lay up to 4 eggs after each blood feed. Over their life cycle of several months, a female flea will typically lay around 100 eggs. The eggs are a white to cream colour, oval in shape and only about 0.5mm in length. They can hatch in as little as one week, though they can lie dormant for many months. The larvae that hatch from the eggs are like small white maggots, about 3 mm in length, and sparsely coated with very fine hairs. They have no legs and wriggle around in search of food, usually skin scales or undigested blood excreted by the adults. They will hide away in cracks and crevices for shelter and are rarely seen.
Larvae go through four moults (instars) over a 1 – 3 week period. The life cycle is faster in warmer conditions. The final instar larvae will pupate by spinning a silken cocoon. Inside their cocoon they will transform over a period of 1 – 2 weeks into an adult flea, though they can remain dormant in the cocoon for many months, often emerging only when they feel vibrations from a potential host.