During the growth of InAs nanowires from Pd catalyst particles on InAs(111)A substrates, two distinct classes of nanowires are observed with smooth or zigzagged sidewalls. It is shown that this is related to a bimodal distribution of the wire-tip diameter: above a critical diameter wires grow with smooth sidewalls, and below with zigzagged morphology. Transmission electron microscopy analysis shows that the catalyst particles at the tip of zigzagged wires are smooth and have a higher aspect ratio than those at the tip of smooth wires. Zigzagged wires grow from liquid particles in the vapor- liquid-solid (VLS) mode whereas the smooth ones grow from solid particles in the vapor-solid-solid (VSS) mode.