Flea eggs are white. Each female can produce hundreds of eggs.
The flea larva (see the drawing above and photograph below) is a 13 segmented eyeless, legless, worm-like grub (similar to some fly larvae) living on bodies, hairs, feathers, nests, bedding, etc. depending on the species. It scavenges on debris, dried blood and adult flea excrement which contains undigested blood.
It pupates in a silk cocoon (see the photograph below) for around 2 weeks before hatching into an adult. The cocoons incorporate debris from their surroundings, this acts as both physical protection and camouflage.
above Pulex irritans, the human flea
All adult fleas (see photographs and drawings above and below) are parasites of warm blooded animals. Their flat shape and backwardly pointing spines (see below) make it easy for them to move through hair and feathers. The largest living flea is 12 mm long, and found on the North American mountain beaver. The largest fossil flea was 20.6 mm long and may have lived on a feathered dinosaur.
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 needs a blood meal before she can produce eggs.
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. If fact we are the only species of great apes to have our own species of flea, he aptly named Pulex irritans.
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 is 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 above).
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 insects 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).