Flea

Fleas are a specialized group of small, wingless insects known as the order Siphonaptera. There are approximately 2,500 known species within this order. Most fleas live as external parasites on mammals and birds. They survive by ingesting the blood of their hosts.

Because they rely on hosts for food, many species are highly specialized. Some flea families only live on specific animals. For example, Malacopsyllidae live only on armadillos. Ischnopsyllidae are found exclusively on bats. Chimaeropsyllidae live only on elephant shrews. This specialization ensures that each species has a consistent food source.

A flea's body is expertly designed for a parasitic life. Their bodies are laterally compressed, meaning they are flattened sideways. This shape allows them to move easily through thick fur or feathers.

Their bodies are covered in hard plates called sclerites. These plates protect the insect from being crushed when a host scratches. Sclerites are also covered in hairs and short spines that point backward. These spines help the flea move forward through hair. Instead of compound eyes, fleas have simple eyespots or may lack eyes entirely. They use a proboscis, or stylet, to pierce skin and suck blood through an epipharynx.

One of the most incredible features of a flea is its ability to jump. Fleas are among the best jumpers in the animal kingdom relative to their size. A flea can jump 60 times its body length vertically. It can also jump 110 times its length horizontally. This feat is second only to the froghoppers.

To achieve this, fleas use an elastic protein called resilin. They do not rely solely on muscle power for the jump itself. Instead, muscles contract to deform the resilin pad, storing energy like a drawn bow and arrow. A catch mechanism holds the joint closed until the energy is released. This rapid release powers the extension of the legs for propulsion.

The life cycle of a flea involves four distinct stages of complete metamorphosis. It begins when a female lays tiny, white, oval eggs. These eggs may be laid on the host or in the host's nest. The eggs hatch into larvae, which are small, pale, and worm-like.

Larvae lack limbs and eyes and prefer dark, humid places like soil or bedding. They feed on organic debris, such as feces from adult fleas containing dried blood. After three larval stages, the insect enters the pupa stage. The larva weaves a silken cocoon to undergo metamorphosis. Inside, the adult form develops. Emergence from the pupa can be triggered by heat, vibrations, or carbon dioxide.

Adult fleas have the primary goals of feeding and reproduction. Most adult fleas live for only two or three months. However, under ideal conditions of temperature and humidity, they can live up to 1.5 years.

A single female can lay 5,000 or more eggs in her lifetime. Many species require a blood meal to become sexually mature. In female rabbit fleas, specifically Spilopsyllus cuniculi, the fleas can even detect hormones in the rabbit. When they sense the rabbit is about to give birth, the fleas prepare to feed on the newborns. This shows how closely flea life cycles are tied to their hosts.

Fleas have a long and complex evolutionary history. Genetic evidence suggests they are a specialized lineage of parasitic scorpionflies, known as Mecoptera. They are most closely related to the family Nannochoristidae.

The earliest known fleas lived during the Middle Jurassic period. Modern-looking fleas appeared later during the Cenozoic era. Fossils like Pseudopulex wangi from the Early Cretaceous show their ancient forms. Scientists believe fleas likely originated on mammals before expanding to birds. They probably migrated from the southern landmass of Gondwana toward the north.

Fleas have also had a massive impact on human history and culture. The oriental rat flea, Xenopsylla cheopis, is a vector for Yersinia pestis. This is the bacterium that causes the bubonic plague.

When infected fleas bite rodents like the black rat, the disease can spread to humans. This led to massive historical outbreaks, such as the Plague of Justinian around 540 and the Black Death around 1350. These events killed a large fraction of the global population. Beyond history, fleas appear in art, from the poems of John Donne to the films of Charlie Chaplin.