1 Fleas are wingless insects with a laterally compressed body of about 1.5-4 mm length. Like all insects they possess six legs and three body segments. Taxonomically they belong to the order Siphonaptera (Eckert et al. 2000) (Table 1). This family contains several species and subspecies. Fleas represent one of the most important ectoparasites (Mehl- horn 2000; Mehlhorn et al. 2001b). At the moment there are more than 2000 described species and subspecies throughout the world (Borror et al. 1981). These species belong to the families Pulicidae, including Pulex spp., Ctenocephalides spp., Spilopsyllus spp. and Archaeopsyllus spp., or the familia Ceratophyllidae with the genuses Ceratophyllus or Nosopsyllus to mention only some of the most important veterinary and human representatives. Fleas have a history of about 60 million years and were already found on prehistoric mammals. While becoming parasitic the original exterior of the two-wing insects, also designated as the order Diptera, has changed by losing the wings in the adults, whereas the larval form still has similarity with the larva of the order Diptera (Strenger 1973). About 95% of the -2000 different flea species parasitize on mammals, 5% live on birds. Table 1. Taxonomy of fleas Systematic Taxonomy Phylum Arthropoda Tracheata (=Antennata) Subphylum Classis Insecta (Hexapoda) Ordo Siphonapterida Familia Pulicidae Familia CeratophyUidae Genus Ctenocephalides. Genus Ceratophyllus. Nosopsyllus Pulex.

The book describes in great detail the complex life cycle of fleas based on the example of the cat flea ("Ctenocephalides" "felis"), the most important ectoparasite worldwide. Besides being the cause of painfully itching bites and allergic skin diseases, it is also a vector for viruses, bacteria, nematodes and cestodes. Over the years the market for insecticide use in small animals has become a major segment of the chemical-pharmaceutical industry.
Insecticides of the new generation should not only fight the existing infestation (therapy) but should also effectively prevent new infestations for weeks (prophylaxis). The latest class of chemicals developed to this effect are chloronicotinyls (syn. neonicotinoides). Imidacloprid is the first member of this class of insecticides with a high selectivity towards the site of action within an insect. Applied to the skin it combats flea infestation persistently. The book describes the development of the compound from its discovery to its use in veterinary medicine.