Fleas don't have a very good image to the general public. They are associated with dogs, rats, unclean conditions and disease. But one has to admit that they are amazing little survivors. They have adapted themselves to an ectoparasitic lifestyle that has been highly successful. Worldwide there are about 2000 described species of fleas that inhabit a wide variety of mammalian and avian hosts.
Fleas are laterally flattened and steamlined for moving through fur or feathers. Even their heads are keel shaped to help them with this task. They have piercing mouthparts adapted for sucking the blood of their hosts. They have combs used to attach themselves to fur or feathers. Their eyes are small and antennae are short as they are not needed in this habitat. The back legs are adapted for jumping and are quite strong. They are so good at jumping that they have been described as insects that fly with their legs. Fleas have secondarily lost their wings. There is a special structure, called the pleural arch, that is a modification of the wing hinge that was present in the fleas' winged ancestors. The pleural arch compresses and the flea uses this compressed energy for its jumps. Jumping is stimulated by the carbon dioxide breathed out by potential hosts. Hungry, homeless fleas will jump for hours looking for a host. Fleas that are only 9 mm long have been measured to have jumped over 300 mm or over thirty times their own body length.
Fleas probably evolved from scorpion flies (Mecoptera) some 150 million years ago. There are similarities in muscle arrangements, skeletal structures and chromosomes to indicate a relationship. Mecopteran ancestors may also have given rise to butterflies and flies as well at around the same time. In the case of fleas, their ancestors turned from a free-living lifestyle to parasitising mammals and this has lead to the changes and adaptations for this speciality. Only about ten percent of fleas live on birds. All the rest live on mammals. Most of their hosts build nests or live in burrows. Aquatic mammals like seals are not troubled by fleas.
Fleas go through a complete life cycle. The adults live on a host but the larvae live in the nest. The females leave the host to lay sticky eggs which hatch into small larvae. The larvae are found in the nests or burrows of the host and many eat the feces of the adult flee. Rat flea larvae will even 'beg' for food from adults by grasping the adult by posterior bristles and stimulating it to release blood from its anus. Flea larvae are much more vulnerable to climatic changes than the adults. They can drown in a drop of water or become desiccated and die. If they survive, they pupate and become adult fleas which return to the host to feed and mate.
Fleas are less host specific than lice and can live on an alternate host if the regular host is not available. Cat and dog fleas don't prefer humans but will bite them if they are hungry. The plague bacteria, Yersinia, spread from rats to humans via the rat flea. Rats still infest cities and plague outbreaks still occur. Insecticides have been used to reduce rat fleas but outbreaks are still possible and the World Health Organisation takes plague very seriously. Fleas are not just annoying nuisances. They can also be dangerous to our health.
For pictures and more information: http://anic.ento.csiro.au/insectfamilies/order_overview.aspx?OrderID=42515&PageID=overview