They get captured by Jack and his tribe when they go with Piggy and Ralph to receive Piggy's stolen glasses. They are forced to become a part of Jack's tribe and sell out Ralph when they are searching for him across the island. They didn't want to, but they appeared to finally turn on to Jack's side.
Ralph told Samneric that he would hide out in the bushes, so eventually Samneric squeal on Ralph to Jack.
Samneric
Samneric and Piggy.
Samneric tell Ralph that Jack intends to hunt him the following morning and that Jack has "sharpened a stick at both ends."
Jack and his hunters break in and steal piggy's glasses