Initiation of flight in the unrestrained fly, Drosophila melanogaster

Using high speed cinematography and videography, we determined that unrestrained Drosophila melanogaster can initiate flight in two different ways, which we have designated Type 1 and Type 2 flight initiation. Type 1 behaviour is executed when the fly initiates flight ‘voluntarily’. Type 2 behaviour is executed in response to a visual stimulus and is presumably an escape response. During both types of flight initiation, the fly pushes off from the substratum using only the mesothoracic legs. The mesothoracic legs extend at the coxo‐trochanteric, femoro‐tibial and tibio‐metatarsal joints. Although the movements of the legs are similar during both Type 1 and Type 2 flight initiation, the movements of the wings are different. Prior to the onset of leg extension, during Type 1 flight initiation, the wings are raised and rotated from their resting position to a preflight position dorsal to the thorax. The wings are held in this upright position for several milliseconds and then rapidly depressed during leg extension. Prior to Type 2 flight initiation, by contrast, the wings are not raised and rotated, nor are the wings depressed during leg extension. Thus Type 1 flight initiation involves simultaneous leg extension and wing depression, whereas Type 2 flight initiation involves only leg extension.