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.

தெள்ளு சைபெனொப்டெரா வகுப்பைச் சேர்ந்த பூச்சி இனமாகும். இறகுகள் அற்றவை. வாயுறுப்பு தோலைக் குற்றி குருதியை அகத்துறுஞ்சுவதற்கு ஏற்றதாக பொருத்தப்பாடடைந்து காணப்படும். பாலூட்டிகள் மற்றும் பறவைகளில் புற ஒட்டுண்ணியாக வாழும்.