The Mysterious Fence-Building Insect of Peru

Troy Alexander, a graduate student at Georgia Tech, spent his summer in the Peruvian rain forest assisting with the Tambopata Macaw Project. While he was there he kept noticing these fascinating structures attached to tree trunks and even a tarp. The look like egg sacs left by a spider or insect, but Alexander never saw the organism that made them. He took these pictures and brought them home, hoping somebody would be able to tell him what they are. So far, nobody has.

When they find it, they can name it the White Picket Fence Spider/Moth/Whatever.

UPDATE: it seems that the builders are spiders. Species not yet established.