From Gifts From Hashem by Rabbi Moshe Goldberger based on the teachings of Rabbi Avigdor Miller
How does a mosquito collect its food from humans and animals (preferably animals)? A mosquito can hover in midair, move vertically, and even fly backward in search of potential prospects. It then utilizes its proboscis (flexible, projecting nose) to drill beneath the surface of the skin in search of a supply of blood. Its proboscis consists of six different shafts: four are cutting and piercing tools; the fifth transports blood from the host to the mosquito; the sixth transports saliva.
The mosquito was perfectly designed by Hashem to drill and pump its food supply in order to survive. Humans, in turn, are served by mosquitos, for they provide a plentiful source of protein (14,000 mosquitos per acre) for birds, fish, and other insects.