Fleas are small, laterally flattened, wingless, and highly specialized holometabolous insects. The are of great importance as vectors of pathogens in many parts of the world. Both adult males and females are obligate hematophagous ectoparasites of mammals and birds. Adult fleas are strongly sclerotized, and have backward-directing spines on their legs and bodies that facilitate forward movement through fur, hair or feathers. Those spines also prevent them from being easily dislodged by grooming and preening efforts of the host. Compound eyes are absent. Fleas (like all insects), have three thoracic segments sustaining each a pair of legs. They have strongly developed hind legs, that permit them to jump

up to 100 times their own body length.  This feature is related to the presence of resilin (biomaterial), that is compressed during the flexion of the coxa in the metathorax, permitting a rapid relax. Larval morphology is inconspicuous, and newly hatched larvae are slender, white, segmented and worm-like. Flea eggs are pearly white, oval, with rounded ends, and approximately 0.5mm long.

As holometabolous insects, fleas complete a cycle from egg, through 3 larval stages, a pupal stage, to the adult.

The majority of characters used for morphological identification of flea species are based on the shape and structure of their extraordinarily complex genitalia, and the presence and distribution of setae, spines and ctenidia (combs) on the body. Identifications require an extensive knowledge of flea morphology. Whiting (2002) showed that the order Siphonaptera is most closely related to Boreidae (snow fleas), a family within the order Mecoptera. Therefore, one can assume that Siphonaptera are just derived Mecoptera, and thus don’t necessarily deserve their own order. However, Siphonaptera is monophyletic, and recent studies show that fleas likely evolved at the base of mammalia, radiating with rodents (Whiting et al., 2008).