In the Amazon rainforest salt is a highly sought after commodity. Tears are full of salt, so these butterflies are literally drinking the turtles tears to get the vital mineral.
This is a fairly common sight in the Amazon, and it’s not restricted to butterflies. Here’s a video of bees drinking turtle tears: http://bit.ly/10q2hG3
More info: http://bit.ly/10u18dk
Thanks to Rainforest Expeditions Lodges for the image.