Fleas are not, or only rarely, found in nomadic animals such as the gorilla as the larva need a safe place to live and grow, and this is usually found in the place where the host sleeps. It is believed that when man was nomadic and naked he had no fleas. It was only when he settled in caves and started to dress that fleas could breed and use him as a host.

The adult is Brown/black, shiny and wingless. The hind pair of legs store energy in special pads of resilin protein. The thoracic muscles compress the resilin pad, and when the muscles relax the resilin springs back to its original size releasing energy which is transmitted to the rear legs and the flea jumps. The mouthparts are adapted for blood sucking. Adult fleas can detect smells and carbon dioxide given off by animals. The female need a blood meal before she can produce eggs.

Family Pulicidae (common fleas), there are about 200 species worldwide and all feed on mammalian blood. Some are unspecialised and can live on as many as 30 different mammal species. The adults range in size from 1 - 8 mm long. Both males and females suck blood. The eggs. Females lay their eggs as they feed. and the eggs drop into the host's nest, burrow or bedding. The eggs are pearly white, ovel and about 0.5 mm long. 4 - 8 eggs are laid after each blood meal. A female can lay several hundred in her lifetime. The larvae are pale, legless, and worm-like grubs with biting jaws which usually live for 2 -3 weeks. They are about 1.5 mm long when they first hatch; growing to around 5 mm before they pupate. See the human flea, Pulex irritans larva on the left, and the dog and cat flea larvae below. They don't have eyes, but they do have jaws and backwardly-facing bristles. The larva eats partly

digests blood in the faeces of the adult fleas, particles in the host's faeces, and any other food particles.

The pupa can lie dormant until they are disturbed. So it is possible to move into a house that has been empty for months and suddenly find adult fleas everywhere. The behaviour is of great advantage to the fleas, as it would die of starvation if it had hatched out in an empty house. See the cat flea larva, Ctenocephalides felis below. Particles of debris stick to the silken cocoon of the pupa and hide it. The pupa are triggered to hatch into adults by vibration and body heat. This usually signals the return of the host to its sleeping area or den.

Adult fFlea bites can cause allergic reactions and discomfort, and the flea can be host to parasites that it passes on to its host. For example bubonic plague (Black Death) was passed to humans in bites from the rat flea, see below for more disease transmitted by fleas.

Pulex irritans, right, drinks human blood and is commonly known as the human flea. It has the

typical flea body being compressed sideways. The maxillary palps are often mistaken for antenna but the true antennae are minute and rest in grooves. Note the rear pair of legs which are modified for jumping. P. irritans can jump more than 200 times its own height. Like all fleas the body has numerous backward-pointing spines and combs. These are used in identifying the different species, and they also make the animal difficult to remove from hair. Even after you have caught you flea killing it is not so easy as the exoskeleton is hard.

Pulex irritans was once the most common flea found on humans, and it also lives on pigs, goats, foxes and badgers. It can jump 30 cm.

Humans can be parasitized by the dog, cat and rat fleas.

Today cat (Ctenocephalides felis, see adult, left larva and pupa below) and dog (C. canis) fleas are the most common fleas to bite humans. The dog flea adult is just 2 - 3 mm long. Dog fleas can carry a tapeworm (Dipylidium caninum) that can infect dogs, cats and humans. The fleas' larva (see below) are infected with tapeworm eggs, the dog swallows the adult flea while grooming and the tapeworm now infects the dog.

Does your dog/cat have fleas?

Usually the first sign that your dog or cat has fleas - apart from scratching - is finding little dark specks of flea faeces in their hair. Around the base of the tail and the neck are places fleas like to live. The dark specks are easily seen in light hair. In dark-haired dogs you can clean the brush or comb over a piece of white paper. If you find dark specks, put a drop of water on them. If they dissolve into a reddish brown colour they are flea faeces, as the faeces are largely composed of blood.

The cat flea is one of the most powerful jumpers and can reach a height of 34 cm. This is the equivalent of a human jumping

600 feet! Though, of course it is the cross-sectional area and small size of the flea as well as the resilin pad that makes this possible.

Left is a drawing of a dog flea larva, Ctenocephalides canis.

The plague bacterium, Pasturella pestis is transmitted by rodent flea bites. The infected fleas often develop a blockage in their digestive tracts caused by the bacterium multiplying. This blockage makes the flea constantly hungry, but when it takes a blood meal the blood flows up through the flea's mouthparts, hits the blockage, then flows back to the host carrying some plague bacteria with it. Such fleas bite more often, and the bacteria spreads even more rapidly. Murine typhus is also caused by rodent fleas. Myxomatosis is passed to rabbits from Spilopsyllus cuniculi ( the rabbit flea). The largest of British fleas is the mole flea, Hystricopsylla talpae. The tropical rat flea, Xenopsylla cheopis still transmits plague and murine typhus today, also it will bite man if it has the chance.

How insects breathe

Unlike our blood insect blood rarely has any special substances, such as haemoglobin, for absorbing oxygen, and the blood plays only a minor part in the breathing process. Oxygen in carried directly to the tissues through branching tubes called trachea, which are found in all but the most primitive of insects, and some very specialised internal parasites. A trachea is a flexible tube which branches into smaller tubes called tracheoles (see the drawing of a flea showing the trachea and spiracles on the right). The tracheal opening to the exterior is called a spiracle. These usually run down the sides of the insect body, and are most easily seen in a large caterpillar.In small insects diffusion through the tracheoles is sufficient to supply their needs. In larger insects abdominal pumping is necessary. This can be seen by watching a stationary insect, especially a bumblebee, its abdomen will pulsate. The spiracles act as valves to the outside, and most can be partly or completely closed. This also helps to reduce water loss from the body. Carbon dioxide escapes through the exoskeleton. Some insect living in water have a siphon, or breathing tube, (Water stick insect, Water scorpion), others have gills, (damsel fly nymphs, may fly nymphs), and others hold a bubble of water, (water beetles).