Fleas, an order of insects once included as a subdivision of the Flies or Diptera, but now separated as the order Aphaniptera. The main characters which distinguish the Fleas from the Flies are that the wings are either absent or are rudimentary, occurring only as scales, and that locomotion is effected by jumping. The typical family is that of the Pulicidce, including the Common Flea (Pulex irritans). The American Sand Flea (Sareopsylla penetrans) bores into the sole of the foot, and there expands considerably by the growth of the eggs. This is the most serious pest belonging to the order.