A surgical instrument for use with a surgical portal apparatus includes an elongate shaft having proximal and distal ends and defining a longitudinal axis and a dilating member disposed at the distal end of the elongate shaft. The dilating member has a first longitudinal arc segment and a second longitudinal arc segment in diametrical opposed relation to the first longitudinal arc segment. The first and second longitudinal arc segments define respective first and second radii of curvature transverse to the longitudinal axis. The first and second radii of curvature decrease from proximal to distal to define a generally tapered configuration of the dilating member. The first radius of curvature is greater than the second radius of curvature at predefined longitudinal positions of the dilating member such that the first longitudinal arc segment extends distally beyond the second longitudinal segment to define a penetrating tip. The penetrating tip has a reduced profile and dimensioned to facilitate initial insertion of the dilating member into tissue.