Depends what light you look at them in?
Quote from slang dictionary

bluebottle

Definition
noun British
a police officer. A term popular in the 1950s and still heard. It has been used in Britain since the 16th century, well before policemen wore uniforms, and indeed existed in any organised form, which suggests that the original reference was to an annoying pestilential presence.
Citation 'Before you could turn round the place filled up with bluebottles.' (Recorded, pub habitué, London, 1987)