During our walk on Aguacatal road in Coatepec, we encountered this beautiful Tailed Sulphur (Phoebis neocypris virgo) at the edge of a puddle on a dirt road perpendicular to Aguacatal road. The "tail" is actually a part of the hindwing that sticks out; see the left bottom of the above photo.
The butterfly was sharing the puddle with another large Sulphur: the White Angled-Sulphur (Anteos clorinde)
This butterfly photo is part of the blog entry Aguacatal road, which has more photos of this specimen.