Whatever our reservations about them, it cannot be denied that fleas are superbly designed. Their tough bodies have flattened sides that are well suited to tunnelling through a host’s hairs to reach the skin and blood that lies beneath. Comb-like bristles and claws give them a good grip, and a network of fine sensory organs at the back of the abdomen enables them to detect the slightest change in body heat or smell. Fleas are powered by extra-large coxae (upper segments of the leg) and leap with astounding acceleration from rest to the body of a passing host, or from host to host. The human flea can clear 120 times its own height, spinning through the air to land, hooked claws at the ready, on flesh.
It is their method of feeding that makes fleas a serious hazard for, like the mosquito, they pierce the skin and inject their anticoagulant saliva into the blood to keep it flowing. In this way they pass on microorganisms from one creature to another and so spread disease. When human beings are forced to lie in cramped and unhygienic conditions the effects may be devastating. By transmitting bubonic plague from rats to humans, the oriental rat flea was responsible for many epidemics, of which the most devastating was the Black Death during the 14th century that wiped out more than a quarter of the population of Europe.
The human flea, which lays its eggs in carpets and furnishings, can, in larval form, survive for years without food. It simply waits until a new host arrives, then rapidly completes the final stage of its development into adulthood and emerges to feast upon the blood of the newcomer.