Newly hatched flea larvae are slender, white, segmented and worm-like. They are sparsely covered with short hairs and are 1 to 2 mm in length. They have an egg-tooth, similar to that of a bird, that they use to cut their way out of the egg. Larvae are free moving, and survive by feeding on organic debris found in their environment and on adult flea feces, which is essential for successful development. Once the larvae have ingested adult flea feces or other material they become darker in color. They have been observed to crawl as far as 6 meter (20 feet) while in this stage of their life cycle.
 

Since larvae are negatively phototactic (avoid light) and positively geotropic (prefer to move downward), they are found deep in carpet fibers, mattress or couch stuffing materials or organic debris (branches, leaves, etc.). They accumulate in areas where the animal spends a great amount of time. These "hotspots" are typically in pet resting areas. They are not usually found in open lawn. Flea larvae are extremely susceptible to heat and desiccation. Moisture in the larval environment is essential for development; relative humidities below 50% cause dessication. Flea larvae undergo two molts before developing into the Pupal stage. While the first larval instar is no more than 2 mm long, the fully developed larvae can be 4 to 5 mm in length. The larval stage usually lasts 5 to 11 days, depending upon the availability of food and the climatic conditions, and will account for about 35% of an infestation in a home.
 

The transition from egg to larva, between molts and from larva to pupa, is controlled by juvenile hormone. When the juvenile hormone level drops, the larva molts to the next stage. The actual molting process is triggered by another hormone, ecdyson.