I'm guessing they're horse flies.
http://en.wikipedia.org/wiki/Horse-fly
Most short tongued (short proboscid) species of horse flies use their knife-like mandibles to rip and/or slice flesh apart.
@ started
London Fixed Gear and Single-Speed is a community of predominantly fixed gear and single-speed cyclists in and around London, UK.
This site is supported almost exclusively by donations. Please consider donating a small amount regularly.
I'm guessing they're horse flies.
http://en.wikipedia.org/wiki/Horse-fly