An insertion instrument, comprising: a tube having a proximal end and a distal end adapted for insertion; a tip control actuator located at the proximal end; the distal end of the tube including a flexible portion comprising a spine, a plurality of circumferentially extending ribs extending from the spine and being axially spaced apart from a distal tip toward the proximal end, the ribs being spaced apart by wedge shaped partial circumferential openings, the ribs have first and second axial sides (35, 36) 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 including 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 including at least one of a complementary mating recess or a corresponding axial projection; and at least one tension wire extending from the tip control actuator to the distal tip, wherein the axially extending projections and corresponding complementary mating recesses and/or the recesses and the corresponding axial projections in the plurality of ribs are arranged in at least three axially extending rows, with a first one of the rows being located approximately 80 - 100 degrees from the spine, a second one of the rows being located approximately 170 - 190 degrees from the spine, and a third one of the rows being located approximately 260-280 degrees from the spine, the recesses and/or corresponding complementary mating recesses of the first and third rows having a curved path extending from the at least one of the first and second axial sides toward the spine, and the projections and/or corresponding axial projections of the first and third rows having a complementary curved shape to the curved path of the respective recesses or corresponding complementary mating recesses. a tube having a proximal end and a distal end adapted for insertion; a tip control actuator located at the proximal end; the distal end of the tube including a flexible portion comprising a spine, a plurality of circumferentially extending ribs extending from the spine and being axially spaced apart from a distal tip toward the proximal end, the ribs being spaced apart by wedge shaped partial circumferential openings, the ribs have first and second axial sides (35, 36) 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 including 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 including at least one of a complementary mating recess or a corresponding axial projection; and a spine, a plurality of circumferentially extending ribs extending from the spine and being axially spaced apart from a distal tip toward the proximal end, the ribs being spaced apart by wedge shaped partial circumferential openings, the ribs have first and second axial sides (35, 36) 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 including 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 including at least one of a complementary mating recess or a corresponding axial projection; and at least one tension wire extending from the tip control actuator to the distal tip, wherein the axially extending projections and corresponding complementary mating recesses and/or the recesses and the corresponding axial projections in the plurality of ribs are arranged in at least three axially extending rows, with a first one of the rows being located approximately 80 - 100 degrees from the spine, a second one of the rows being located approximately 170 - 190 degrees from the spine, and a third one of the rows being located approximately 260-280 degrees from the spine, the recesses and/or corresponding complementary mating recesses of the first and third rows having a curved path extending from the at least one of the first and second axial sides toward the spine, and the projections and/or corresponding axial projections of the first and third rows having a complementary curved shape to the curved path of the respective recesses or corresponding complementary mating recesses.