If we are talking about a mosquito's own blood (hemolymph), then it performs the same functions as human blood - it transfers nutrients, harmful metabolic products, hormones, and provides protection against infections. Only oxygen and carbon dioxide it does not tolerate - the mosquito has a tracheal respiratory system, and oxygen is delivered directly to the cells through thin tubes-trachea. The larvae of bell mosquitoes ("bloodworms") are a rare case among insects when the hemolymph is colored red due to hemoglobin. These aquatic larvae breathe through the integument, the trachea are poorly developed and do not open outward with holes. They live in silt at the bottom of water bodies, where there is often very little oxygen, and hemoglobin allows them to bind and store additional amounts of oxygen. (See also the answer to the question "Do insects have blood?")
If we are talking about the blood that female mosquitoes drink when sucking blood, then they need it first of all for reproduction. Females, like males, can drink water and nectar and live without blood feeding. But most species and populations of kulicid mosquitoes (Culicidae; blood-sucking mosquitoes belong to this family) are not capable of reproduction without blood feeding. Blood, unlike nectar, is a protein-rich food. The proteins contained in the plasma (liquid part of the blood) and erythrocytes are digested in the intestines of the "mosquito", and the resulting amino acids are used to synthesize the proteins of its eggs.
3–4 days after hatching from pupae, female mosquitoes mate with males. Fertilized females look for their victims. After drinking blood, females digest it for 2-3 days. During this time, eggs ripen in their ovaries, and then the female finds a suitable reservoir and lays eggs on the surface of the water. A certain percentage of females then die, and the survivors can drink blood again and only after that lay a new portion of eggs. (They do not need to mate again, since they retain sperm reserves in the seminal receptacles - a special part of the reproductive system.) The cycle "nutrition - digestion of food - laying eggs" is scientifically called "gonotrophic harmony".
But some mosquitoes don't need blood. For example, female large mosquitoes of the genus Toxorhynchites feed only on nectar. This is due to the nutrition of their larvae. The larvae of most mosquitoes eat bacteria and small particles of dead organic matter - detritus. And the larvae of Toxorhynchites eat the larvae of other mosquitoes - protein animal food. Therefore, at the larval stage, they store enough proteins for the female to lay eggs, and she does not have to risk her life by extracting blood.
The ability to autogeny (oviposition without bloodsucking) also appeared in the so-called "city mosquitoes" - Culex pipiens pipiens forma molestus. Populations of these mosquitoes have adapted to life in the half-flooded basements of city houses. The water in the basements often contains enough organic substances, they are stored when the larva feeds, and the females of these mosquitoes can lay the first clutch of eggs without sucking blood. For the next clutches, blood supply is needed, but the population can exist and increase its number indefinitely without it. Unfortunately, the “blood-sucking instinct” in the females of this mosquito has not disappeared, and they pester the inhabitants of some cities even in winter …
Sergey Glagolev