A mosquito was a small insect native to the planet Earth. They carried the disease malaria.
The Klingon glob fly was half the size of a mosquito. (TNG: "The Outrageous Okona")
Mosquitoes were common in the Everglades. When Charles Tucker III and Kaitaama crash landed on a jungle planet in 2152, Tucker remarked that at least there were no mosquitoes. (ENT: "Precious Cargo")
In 2366 Doctor Paul Stubbs compared sentient nanites that were created accidentally by Wesley Crusher to mosquitoes and said, when Commander William T. Riker asked why the nanites would attack the USS Enterprise-D: "Why does a mosquito bite your ear and who cares? The answer is simple. Call an exterminator!" (TNG: "Evolution")
Miles O'Brien used to worry about having to swat a mosquito when he was a child. He had never killed anything before shooting a Cardassian on Setlik III. (TNG: "The Wounded")
Stinger was a giant mosquito in The Adventures of Flotter children's holonovel series. (VOY: "Once Upon a Time")
Dr. Beverly Crusher used the metaphor "What if we look at it from a mosquito's point of view... if we sting them in a tender spot, they might stop for a minute to scratch," in reference to forcing the Borg cube that kidnapped Captain Jean-Luc Picard out of warp. (TNG: "The Best of Both Worlds")