Via Wired:

Harvestmen (also known as daddy long legs) aren’t spiders, and if you could (or wanted to) lean close enough, you’d be able to see one of the few physical features that distinguish them from their arachnid cousins. It’s in the eyes: Spiders usually have 6 or more, but the harvestman has only one set, tightly clustered on the top of its head, making it look like a cyclops. However, scientists report today in the journal Current Biology that the ancestors of modern harvestmen had an extra pair, called median eyes, located on either side of the front of their heads.