Invertebrates
Flea
Ctenocephalides felis · also called flea, cat flea
The flea is a tiny, wingless, blood-drinking insect, and it is built almost entirely around one feat: the jump. A flea can leap fifty times its own body length, straight up, at an acceleration that would black out a fighter pilot — using a spring of elastic protein wound up in advance and released like a catapult. And one flea, the rat flea, carried the bacterium that caused the Black Death.
Overview
There are about 2,500 flea species, feeding on birds and mammals worldwide; the cat flea is the one usually found on pets and people. Adults are 1.5–3 mm long, dark reddish-brown, wingless, and flattened side to side to slip between hairs, with backward bristles that anchor them in fur. The legless larvae live in the host’s bedding and eat debris and dried blood.
The catapult jump
A flea can jump about 50 times its body length, roughly 18 cm, accelerating at around 100 g. The power comes from a pad of the elastic protein resilin that muscle slowly compresses and a catch releases in under a millisecond — the same trick used by trap-jaw ants.
A cocoon that waits
The pupa sits in a sticky, dormant cocoon for months, hatching only when it senses a passing host’s vibration, warmth, and carbon dioxide — which is why an empty house can suddenly seem full of fleas when someone moves in.
The plague
The Oriental rat flea transmits Yersinia pestis, the cause of plague, including the Black Death that killed tens of millions in fourteenth-century Europe. Plague still occurs today. Fleas also spread murine typhus and tapeworms and cause the most common skin disease of dogs and cats.
Status
Fleas are not assessed and of no conservation concern.
Flea: common questions
How can a flea jump so far?
Not by muscle power alone, which could never contract fast enough. Instead the flea stores energy first and releases it all at once. Muscles slowly squeeze a springy pad made of a rubber-like protein called resilin, located in the base of the hind legs, and a catch holds that energy in place. When the flea triggers the catch, the resilin snaps back and drives the leg down against the ground in well under a millisecond, launching the insect at an acceleration of around a hundred times gravity. It is the same catapult principle used by trap-jaw ants and mantis shrimp, and it lets a tiny animal outperform anything relying on muscle directly.
Why do fleas suddenly appear when you move into a house?
Because flea pupae can lie in wait. After the larva spins its cocoon, the developing adult can stay sealed inside, fully formed but dormant, for weeks or months if no host is around. It emerges only when it detects the cues of a warm-blooded animal nearby, chiefly vibrations from footsteps, body heat, and exhaled carbon dioxide. A home left empty by previous occupants and their pets can hold a carpet full of these primed cocoons, and when new people or animals arrive and start walking around, the fleas hatch in a wave.
Did fleas really cause the Black Death?
Fleas were the delivery system. The disease is caused by the bacterium Yersinia pestis, which normally circulates in wild rodents. When an infected rodent dies, its fleas seek a new host, and if they bite a person they can pass on the bacterium, sometimes because the bacteria have multiplied and blocked the flea's gut, making it regurgitate infected material into the wound. In the fourteenth century, this route, aided by rats travelling on ships and grain stores, spread plague across Asia, the Middle East, and Europe, killing an estimated 25 to 50 million people in Europe alone.
What is the difference between a flea, a tick, and a louse?
All three are small parasites that feed on blood or skin, but they belong to different groups. Fleas are insects, wingless and built for jumping, that spend much of their life off the host in the environment. Lice are also insects, but flattened and clawed for clinging, and they live their whole lives on one host, gluing their eggs to hairs or feathers. Ticks are not insects at all but arachnids, relatives of spiders and mites, with eight legs as adults; they attach, feed slowly over days, then drop off. Each transmits its own set of diseases.