Why do insects bite? A review of blood sucking behaviour.

Introduction There is considerable variation in the extent of the association between a blood sucking insect and its host. Blood sucking Diptera such as mosquitoes are temporary ectoparasites, only visiting the host for long enough to obtain a blood meal. Temporary ectoparasites such as tsetse flies, tabanids and mosquitoes tend to take relatively large blood meals, thus limiting the danger from the host by minimising the number of visits paid to it, and acting as an insurance policy in case hosts are difficult to locate in the future. Permanent ectoparasites such as lice are able to take small and frequent blood meals because they spend their whole life on the host, and are therefore not subject to the same feeding pressures or dangers as temporary ectoparasites. Natural selection has ensured that these temporarily ectoparasitic insects have optimised their locomotory systems to maximise the size of meal they can take, while at the same time minimising the risks attendant in carrying it off. This concept is demonstrated in tetse flies (1), and mosquitoes (2), which frequently take a blood meal two to three times their unfed body weight. Such a meal seriously impairs the insect's manoeuvrability and reduces flight speed, which is also affected by the cool ambient temperatures occurring at dawn and dusk when many species of both mosquitoes and tsetse flies normally feed. To compensate, the fed tsetse fly generates heat endogenously by 'buzzing' with its wings, raising the thoracic temperature and enabling it to maximise its lift and flight speed (3). In tsetse flies for mostly encounter host-derived stimuli in a particular sequence. The various behaviour patterns involved in host location can be divided into three stages (5).