Flea larvae represent the second development stage of a flea’s life cycle. The first stage is the egg. Once hatched, the flea larvae look like little worms and can turn into an adult flea in about two weeks. After the larval stage, a flea will form a pupa where it can transform into an adult flea. The time it takes for a larva to turn into an adult flea depends on factors including heat, humidity, the availability of a host, and the amount of carbon dioxide present.
When they hatch, flea larvae are usually white and grow to be about ¼ inch (a little over 6 mm) long. Their bodies are separated into 13 segments: three thoracic segments and 10 abdominal segments. They have no eyes but are sensitive to light. If a light is shined on them, they usually crawl away from the light and crawl downward — for example, farther down into a carpet. They also respond to vibration, such as by clinging to their hosts’ fur if they detect the animal is about to scratch.
Even though they don’t have legs, flea larvae have hairs that allow them to move about 20 feet (about 6 m) from where they hatch. Otherwise, the larvae, as well as other stages of the flea, depend on the movement of the host. The food the larvae eat is often the excrement of adult fleas, which consists of undigested dried blood. Once eaten, this food allows the flea to change color from white to yellow or brown. The larvae can also eat other organic material, but adult flea fecal matter is their main food source.
Flea larvae will shed their skin and leave casings behind as they develop. This shedding can happen three times. Since the larvae don’t move very far from where they hatch, the discarded casing can signal where the larvae are located. Once they reach the third shedding, the next stage is the pupae stage. The larvae will gather up debris such as hair, carpet fibers, lint, and other materials to create a camouflaged cocoon in which they can develop into adult fleas.
Both flea eggs and flea larvae are susceptible to drying out or desiccating. Thus, fleas usually live and develop best in higher humidity areas. This phenomenon also makes the larvae and eggs susceptible to desiccants, a substance that can dry them out. Some desiccants can also perform double duty. A manufacturer may coat flea larvae food with a poison which can both dry them out and poison them if ingested.