Flea, the common name for the order Siphonaptera, includes 2,500 species of small flightless insects that live as external parasites of mammals and birds. Fleas live by ingesting the blood of their hosts. Adult fleas grow to about 3 millimetres long, are usually brown, and have bodies that are "flattened" sideways or narrow, enabling them to move through their hosts' fur or feathers. They lack wings; their hind legs are extremely well adapted for jumping. Their claws keep them from being dislodged, and their mouthparts are adapted for piercing skin and sucking blood. They can leap 50 times their body length, a feat second only to jumps made by another group of insects, the superfamily of froghoppers. Flea larvae are worm-like, with no limbs; they have chewing mouthparts and feed on organic debris left on their hosts' skin.

पिस्सू (Flea) साइफोनाप्टेरा (Siphonaptera) गण के कीट हैं। इनके पंख नहीं होते तथा मुख-अंग त्वचा को छेदने एवं रक्त चूसने के लिये विशेष रूप से बने होते हैं। है पिस्सू वाह्य परजीवी है और स्तनधारियों एवं पक्षियों का खून चूसकर अपना पोषण करते हैं। इसके मुख्य चुभने एवं चूसने वाले होते हैं