It is used in Academy City as a way to draw blood out of someone, most often a student who is temporarily leaving Academy City, to be checked for authenticity of identity using a micro machine. It is also used to inject a nanomachine that acts as a transmitter into the student. This is a countermeasure against the abduction of students by a possible agent or spy leaving the premises of Academy City.
Baba Yoshio uses mosquito robots to insert nanomachines into a body to take it over, weakening them, giving them a fever, and even making them fall unconsious. It has been shown that it cannot be felt when the mosquito has bitten the target, so it can be presumed that it too uses the Mosquito Needle.