These are the "fleas". All species are parasitic, most on mammals but a few on birds.
Most fleas live as adults on a host but leave the host to deposit eggs. A few species remain on a host throughout their life cycle. Adults mate and the female produce eggs, usually depositing these in the bedding of its host. A larva hatches from the egg, feeds on detritus, and pupates. The adult that emerges searches for a new host, and feeds on blood.
Fleas are wingless and have laterally-compressed bodies. They are usually 1-4 mm long. The legs are well-developed to allow them to jump onto passing hosts. The body is covered with posteriorly-directed spines and hairs. Mouthparts are adapted for piercing and sucking.