Which life stage of the flea is only found on the host?

The adult flea is the life stage that is exclusively found on the host. This is because adult fleas depend on a host to feed and reproduce. They attach to the host's skin and feed on blood, which is essential for their survival and the development of their eggs.

In contrast, flea larvae and eggs are typically found in the environment, such as in carpets, bedding, or soil, where they can develop away from the host. Larvae feed on organic debris and flea feces, while eggs fall off the host into these surrounding areas. The nymph stage, although it can be somewhat similar in its dependency on a host, still develops in the environment and seeks a host only in the later stages of its life cycle.

Thus, the exclusive association of adult fleas with their hosts underscores the critical role this life stage plays in the flea's lifecycle and the potential for infestation in domestic animals and humans.