The control of the Ga droplet during the MBE growth and its impact on the crystal structure of self-catalyzed GaAs nanowires (NWs) were investigated. The consumption of the droplet proceeds in two steps. First, the contact angle decreases to 90 degrees keeping the NW diameter constant. The crystal structure changes from zinc blende (ZB) to wurtzite (WZ). Then, the contact angle keeps constant while the top radius of the NW decreases and the NW grows again in ZB configuration. During the last step, {110}, {211}B and {100} facets develop at the top. Calculations show that the Ga desorption from the droplet has to be taken into account during its consumption. With this information, several WZ segments of different lengths were placed into ZB GaAs NWs via partial droplet consumption. For this purpose, we supplied As and Ga separately, in order to partially consume and refill the Ga droplet. The same mechanism was applied to self-catalyzed InAs NWs resulting in short WZ segments inserted in a ZB twinning superlattice. (c) 2012 Elsevier B.V. All rights reserved.