On May 14, 2012, Daniel Gerstgrasser photographed beautiful interference colours on the wings of a fly near Zürich. (Camera Canon , 100-mm-macro-lens, f11/125sec, ISO 100). The colours are caused by interferences. The structure of the wings which consist of thin, stacked layers with different refraction indices reflect the light from each boundary where the refraction index changes. So the incoming light is split up into a wave reflected from the front side of the layer and another wave reflected from its rear side. Both waves meet in front of the layer causing the interference.
This is because one wave has to pass through the layer while the other one is already reflected from the front side. So both reflected waves are shifted against each other due to the difference in time they need for these different ways. As the different colours are caused by a difference in light wavelength, it depends from the colour of the light if this shifting causes an amplification or an extinction of the respective colour. If, for example, red is extinguished, the complementary colour, which in this case is green, remains. So the wing shimmer greenish.
Light crossing the layer in a flat angle has to cover a longer distance inside the layer. This causes a different shifting against the wave reflected from the front side of the layer than it is compared to light leaving the layer in a more pointed angle. This is why there appear different colours when looking at the wing from different directions.