to flay.-an insect belonging to the genus pulex, of the order aphaniptera. fleas are destitute of wings, but have the power of leaping energetically. the bite is poisonous to most persons. the human flea (pulex irritans), abundant in europe, is rare in america, where the dog flea (p. canis) takes its place. see aphaniptera, and dog flea. see illustration in appendix.