Fleas (Order: Siphonaptera)

An illustration of a flea by F. G. A. M. Smit, late custodian of the Rothschild collection of Siphonaptera at Tring.

An illustration of a flea by F. G. A. M. Smit, late custodian of the Rothschild collection of Siphonaptera at Tring.

Fleas are parasitic insects. Parasites live on other creatures, without killing them. The mouthparts of fleas are modified to pierce skin and suck blood. This allows fleas to live on mammals - dogs, cats, hedgehogs, rabbits, pigs, goats, foxes - and even humans, gaining nourishment from their blood. Pulex irritans is the scientific name for the Human Flea - but in fact humans are more likely to be bitten by dog or cat fleas.

The word 'Siphonaptera' comes from the Greek words for 'siphon' (a hollow tube) and 'without wings' - so, fleas are wingless bloodsuckers!

Fleas are easy to recognise. Their bodies are quite hard and are flattened from side to side, which helps them move about in the fur of their host animals, and makes them difficult to catch. They are dark coloured, wingless, 1 - 9mm in length (usually less than 6mm in Europe) and they are great jumpers. For example, The Cat Flea (Ctenocephalides felis) can jump 34cm at up to 130 times the acceleration due to gravity!

Fleas can carry disease. The Rabbit Flea, Spilopsyllus cuniculi, can carry the disease myxomatosis, which is distressing and fatal to rabbits. The Dog Flea, Ctenocephalides canis, can transmit tapeworms to dogs and cats. But the most serious flea borne disease is Yersinia pestis, or plague (known in the Middle Ages as the Black Death). This is a disease of rats and other rodents, and rodent fleas can transmit it to man. Plague carried in this way has probably caused more deaths than all the wars in history. This began when man brought disease carrying rats to Europe in ships from Africa, in the middle ages.

Fleas seem quite different from most other insects, though their larvae are like fly larvae in some ways. There are around 1,800 species of fleas in the world, with around 100 in Europe and 60 or so species in the UK. Fleas are less host specific than lice, for example, because only the adult flea is directly parasitic, while the larva isn't.

Fleas are fast breeders - which is why they need to be controlled on cats and dogs, otherwise one flea will suddenly become hundreds of fleas! The female flea lays several hundred of her white, pearly eggs either on the host (but not stuck to it) or in the host's nest. The eggs laid on the host fall into the nest and hatch in a week or two, to produce white, worm-like larvae, which have no eyes and no legs. They do have biting jaws, for munching away at detritus in the nest, including their parents' droppings. Sometimes the adult passes undigested blood through its body, and the larvae tug at the adult to make it do this.

The larvae moult (i.e. shed their skin so that they can grow) twice over a period of two to three weeks, then they spin a cocoon, inside which they rest (as a 'pre-pupa') for three days before moulting again to form the pupa. Many fleas can pass the winter as pupae. Though many mammal fleas breed all year round, bird fleas must restrict their breeding to the nesting season of their host - which is why birds have fewer fleas than mammals.

The adults emerge from their cocoons when they detect their host moving about. Because the larvae live in the nest or home of their host animals, the human flea only became a parasite of man when he settled into caves. Nomadic people don't retain their fleas, because fleas can only breed when they meet a host with a suitable nesting habit.

Catching fleas

Fleas can be found by examining the host animals, but they can be difficult to catch. Combing will result in many of them jumping off the host. Inspecting old birds' nests and sweepings from animals' nests will produce all life cycle stages. Flea larvae can be bred in captivity given some appropriate detritus, and the right temperatures and humidity.

Siphonaptera Recording Scheme

Not everyone can identify the many flea species, of course. However, if you find a flea you can send it to the Siphonaptera Recording Scheme and they will identify it for you.

Please note, the recording scheme is not part of the Amateur Entomologists' Society and we cannot answer any queries on behalf of the scheme.

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