Glossosoma intermedium are benthic grazers (Cavanaugh, Haro and Jones 2004).  Since the larvae lives in the bottom of rocky streams they consume the periphyton that grows down there on the rocks.  They graze the periphyton with a hard scraping and digging mandible (Cavanaugh, Haro and Jones 2004).  As they feed they are protected by their case that they carry along with them.  Since Glossosoma intermdium are effective grazers they can sometime deplete their food sources.  Due to this they would sometime consume the algae that grows on another larvae's case.  Some larvae will even leave the protection of their case and drift to a new location downstream where food sources is more abundant.

                                  Green algae growin at the bottom of a stream by Justin Murdok
                                                      Image of algae on rocks courtesy of Justin Murdok