Small white butterflies, Pieris rapae (Lepidoptera: Pieridae), feeding on minerals in mud. Animals (such as butterflies) that feed mainly on plants, which are generally poor sources of minerals, need to supplement their diet in other ways. If you look closely (click through the image and then select the magnifying glass for a larger view), you will see that each of the butterflies has its proboscis inserted into the mud. This way of taking in minerals is known as mud-puddling.
Photo taken at Highcliff Gate, near Guisborough, North Yorkshire, UK, on 2009-08-06.