A mosquito is a device that emits a high pitched sound that only young people can hear. Its purpose is to prevent young people from congregating in public places. I am really surprised to hear that some people think this is acceptable.
Firstly, young people congregating is not a crime. Young people hang out and talk. That's what they do. They're too young to go to pubs or clubs, and their pocket money doesn't stretch to constant trips to the cinema or restaurants.
Secondly, there are many devices for preventing public congregation if there's a problem with congestion. This is the only one I know of that only prevents one section of society from congregating. It's no different to finding a statistic somewhere that says most muggers are black, and putting up signs saying 'no blacks' on the tube.
If anyone can think of a reason why these evil little things are acceptable, please let me know because so far I'm struggling...