Blood is not a primary component of a mosquito’s diet. In fact, almost the entire diet of male and female mosquitoes alike consists of nectar, a fact which many plants take advantage of in order to cross-pollinate. The male mosquito doesn’t even have a proboscis, that part of the mosquito which sticks into your skin to extract the blood. It is even possible for both genders of mosquito to go through their entire life cycle and even reproduce without once feeding on anything other than nectar.
However, a female mosquito will lay many more eggs and the resulting young will be much stronger if she has fed on blood during the week of their early development, right after mating with the male. Blood is richer by far than nectar in the protein and nutrients the eggs need, to the point where a single bite can balance out more than a week of feeding on nectar alone. Most mosquitoes can digest the blood of any mammalian species, or even bird or reptile or amphibian. A few species are far more selective.
Thus the female mosquito has evolved a hunting strategy for use during her reproductive period only, thereby optimizing the risk of haemophagic parasitism against the reproductive benefit. Initially she tracks her prey by scent, following the trail of exhaled carbon dioxide and octenol. For unknown reasons, some people produce more octenol than others, and are thus more attractive to mosquitoes. Some perfumes are also made using octenol. (It is the olefactory detection of octenol which is blocked by the insect repellent DEET.) Once close enough to living prey, the female mosquito can directly detect the infrared radiation of the body’s heat.
When biting, the female first injects some of her saliva, which contains a complex chemical mixture including an anti-clotting agent, an anti-platelet agent, and a vasodilatory agent. This is necessary because the puncture wound is so small, it would otherwise immediately be blocked off by the body’s own defence mechanisms. The familiar itching of a mosquito bite is due to the reaction of the body’s immune system against one or more parts of this compound.
The females of most species of mosquito only need to bite twice in their lifetime in order to lay over three hundred strong eggs. Unfortunately, this is also the minimum number of bites needed for female mosquitoes to transmit disease from one person to another. Common mosquito-transmitted diseases are several different forms of encephalitis, yellow fever, and especially malaria, which is endemic in most tropical and subtropical parts of the world. Most such diseases are specific to a single mosquito species and host species, although West Nile virus can infect most warm-blooded creatures. The mosquito bite itself may also become infected, especially after having been scratched.
Fortunately the vast majority of blood-borne diseases, such as AIDS, cannot be transmitted from one person to another via mosquito bite, because the pathogens which carry them are destroyed in the mosquito’s digestive system.