Blood is an excellent source of protein and other nutrients, so it's no surprise that female mosquitoes are adapted to drink it. Females, like males, feed on plant sap, but someone else's blood gives them the necessary elements, which allows them to lay eggs up to 12 times per life.
Only female mosquitoes drink blood, they use it for reproduction. In the body of females, amino acids are synthesized from proteins in large quantities contained in the blood, which serve as building materials for mosquito eggs. If the female mosquitoes did not drink blood, but ate only nectar and pollen, like males, they would not be able to give offspring.
The female mosquito is ready to give birth within 3-4 days after hatching from the egg. Having not yet drunk blood, young females mate with males, and then go in search of prey. Each time a sufficient amount of foreign blood accumulates in the body, the process of synthesizing eggs begins, after the synthesis is completed, the female lays eggs on the surface of the water and again looks for a victim, while mating occurs only once - at the beginning of adulthood, and the sperm obtained are enough for everything remaining breeding cycles.
There are more than 3000 species of mosquitoes in the world, most are adapted to feed on the blood of any warm-blooded animals, including humans, but some specialize only in a particular animal. There are mosquito species that feed exclusively on the blood of frogs or fish. In the tropics, there are mosquitoes that feed not on blood, but on caterpillar lymph.
Although mosquitoes can feed on human blood, they are much better adapted to the blood of animals and birds. Studies have shown that females who drank bird blood lay twice as many eggs as females who use human blood.
In cities, mosquitoes breed in basements, where there is a lot of organic matter, some species of mosquitoes have adapted to feed on it, and not on blood, so they can give birth to offspring without even biting anyone.
Mosquitoes also have their own circulatory system, in which the analogue of mammalian blood, hemolymph, circulates. With its help, useful substances are transferred through the mosquito's body, metabolic products are removed.