An insertion instrument is provided including a tube having a proximal end adapted to be gripped by a user and a distal end adapted for insertion. A tip control actuator is located at the proximal end, and the distal end of the tube includes a flexible portion. This flexible portion includes a spine and a plurality of circumferentially extending ribs extending from the spine. The ribs are axially spaced apart from a distal tip toward the proximal end, with the ribs being spaced apart by wedge shaped partial circumferential openings. The ribs have first and second axial sides defined by the wedge-shaped openings, with the first axial side of one of the ribs facing the second axial side of an adjacent one of the ribs. For each of the ribs, at least one of the first or second axial sides includes at least one of an axially extending projection or a recess and a facing one of the at least one of the first or second axial sides of an adjacent one of the ribs includes at least one of a complementary mating recess or a corresponding axial projection. At least one tension wire extends from the tip control actuator to the distal tip
