Regeneration. The mosquito regains any hit points it's attack has drained at the start of its turn. If the mosquito takes fire, necrotic, or radiant damage, this trait doesn’t function at the start of its next turn. The mosquito dies only if it starts its turn with 0 hit points and doesn’t regenerate.

Blood drain. Melee Weapon Attack: +5 to hit, reach 5 ft., 1 target. Hit: 5 (1d2 + 3) [piercing] damage. If the prey fails a DC 14 strength save the mosquito attaches to the target. While attached, the mosquito doesn't attack. Instead, at the start of each of the stirge's turns, the target loses 5 (1d2 + 3) hit points due to blood loss.

The mosquito can detach itself by spending 5 feet of its movement. It does so after it drains 10 hit points of blood from the target or the target dies. A creature, including the target, can use its action to detach the mosquito with a DC 14 strength check.