Serial tunneling behavior of Coptotermes formosanus Shiraki (Isoptera: Rhinotermitidae), the Formosan subterranean termite, includes excavation, loading, transportation, and deposition of sand particles. Functions of mouthparts, including mandibles, maxillae, labrum, and labium, for the tunneling behaviors were described. The four mouthparts form a buccal cavity for loading three to four sand particles (ranging 0.300–0.355 mm in diameter) during sand displacement. The maxillae, not the mandibles, are the major appendages for sand excavation and deposition. Previous studies speculated a “soil-compaction hypothesis” that subterranean termites might press the sand particles with their head to build the tunnel. Examination of video recordings of tunnel excavation through a microscope indicated sand was not compacted. When two photographs of tunnel tips taken before and after excavation were superimposed, sand particles surrounding tunnel tips remained in place, which demonstrated that termites did not press sand to either side. Observations and experiments indicate that particle displacement is the major mechanism for subterranean termites to build tunnels rather than particle compaction.