The itchy welt that forms on the skin when a person has been 'bitten' by a mosquito is, in fact, an allergic reaction to the insects's saliva, which it injects into the skin in order to prevent its victim's blood clotting. A mosquito cannot actually bite because its jaws do not open, but the middle of its proboscis has six needle like parts, called stylets. These are protected by the insect's flexible lower lip, or labium, most of the time. When a mosquito 'bites', the lip slides out of the way like a sleeve as the stylets puncture the skin. The channels made by the stylets allow saliva to flow into the wound, thus allowing the insect to sip blood more easily. Only female mosquitoes bite and only those of a few of the 3,000 or more species attack animals and people. Fact File Some of the worst diseases that people and animals suffer are spread by mosquitoes. Some species are carries of such serious infections as Dengue fever, yellow fever, malaria and filariasis. n/Tell Me Everything