Fleas are wingless insects, 1/16 to 1/8-inch (1.5 to 3.3 mm) long, that are agile, usually dark colored (for example, the reddish-brown of the cat flea), with tube-like mouth parts adapted to feeding on the blood of their hosts. Their legs are long, the hind pair well adapted for jumping; a flea can jump vertically up to 7 in (18 cm) and horizontally up to 13 in (33 cm), making the flea one of the best jumpers of all known animals (relative to body size), second only to the froghopper. If humans had the jumping power of a flea, a 1.8-m (6-ft) person could make a jump 90 m (295 ft) long and 49 m (160 ft) high.
Researchers with the University of Cambridge in England found that fleas take off from their tibiae and tarsi (the insect equivalent of feet) and not their trochantera, or knees.It has been known that fleas do not use direct muscle power, but instead use the muscle to store energy in a protein named resilin before releasing it rapidly (like a human using a bow and arrow), with researchers using high-speed video technology and mathematical models to discover where the spring action actually happens.
Their bodies are laterally compressed, permitting easy movement through the hairs or feathers on the host’s body (or in the case of humans, under clothing). The flea body is hard, polished, and covered with many hairs and short spines directed backward, which also assist its movements on the host.
The tough body is able to withstand great pressure, likely an adaptation to survive attempts to eliminate them by mashing or scratching. Even hard squeezing between the fingers is normally insufficient to kill a flea. However, rolling them back and forth a dozen times disables their legs, resulting in death.
Fleas lay tiny, white, oval-shaped eggs better viewed through a loupe or magnifying glass. The larva is small and pale, has bristles covering its worm-like body, lacks eyes, and has mouth parts adapted to chewing. The larvae feed on various organic matter, especially the feces of mature fleas. The adult flea’s diet consists solely of fresh blood. In the pupal phase, the insect is enclosed in a silken, debris-covered cocoon.