The first image shows a female Haematopota pluvialis taking a human blood meal. Flight (and hence feeding) activity of Haematopota pluvialis is dependent on a sufficiently high humidity and temperature. Bites from this species can be quite painful and the flies are remarkably resistant to being killed by hitting them. The second image shows a close-up of the eye of a female.
Haematopota pluvialis can be found in a wide range of habitats from May to October. It is distributed from Europe to Russia and China and is the commonest species of tabanid in UK.
Not only are their eyes amazing colours but on the photo above you can see the slicing mouth parts that they use to get to the blood.
See more of these species of fly here