Fleas
are small laterally compressed (flattened from side to side) holometabolous
(having a complete metamorphosis) insects. They are all apterous (flightless
[from a = not, and pteron = wing]). They have no eyes though
2 simple ocelli may be present, their antennae short and stout and their
adult mouthparts are adapted for piercing and sucking. The larvae are eruciform
and apodous meaning they look like a caterpillar with no legs The adults
are all blood sucking ectoparsites ( a parasite which
lives on the outside of its host) of mammals and birds, while in general
the larvae are detritivores feeding on minute particles of discarded organic
matter still adhering to the host, or on the substrate of a commonly used
sleeping place or nest. The bodies of both adults and larvae have many backward
pointing hairs, it is these, and the powerful leg muscles which make it
so hard to hold a flea between you finger tips, they always slip out.
Information provided by: http://www.arthropod.net