We came across this group of 'puddling' green-veined whites - four of about a dozen - when we were out walking on moorland near Blanchland in Northumberland last week. Each was busy sucking up moisture through its extended proboscis from rapidly drying puddles in the sand soil.
Butterfly 'puddling' is behaviour that's most often seen in wildlife documentaries, with swarms of tropical butterflies congregating around the edge of waterholes, but it's common in our butterflies too. They are sucking up a dilute solution of mineral salts, particularly sodium salts, which is essential for their reproductive success. Males hatch from the chrysalis with a fixed amount of sodium in their bodies and lose some every time they mate, so the longer they live and the more often they mate, the more they need to 'puddle' - otherwise their fertility declines.