Figure 1. (a) Scanning electron micrograph of the head of an adult female Drosophila melanogaster with surface-rendered 3D reconstruction of brain. Major neuropil regions are highlighted in colour or grey. (b) Same brain viewed from a slightly more lateral direction. Whole-mount preparation in which synaptic regions are stained with a fluorescence-tagged antibody. Brain data are obtained by Confocal Laser Scanning Microscopy. Brain regions (for explanations see text) are labelled manually. Courtesy of K. Rein.

Figure 2. Putative motor and premotor organization of the fly brain. Most motor programs are stored in the ventral ganglion. They are represented as options in the brain from where they are activated. Note that in this scheme the valuator does not rigorously determine the choice of the respective option, allowing for a stochastic component in the generation of behavioural modules (see text for further explanation).

Figure 3. Courtship in Drosophila melanogaster. The figure shows five distinct steps in the sequence of interactions. (a) The male (round black abdomen) orients towards the female circling between her abdomen and head. The two flies exchange visual and olfactory cues. (b) Extending and vibrating one wing the male generates a ‘love song’. (c) Tapping the female with his forelegs and licking her abdomen the male samples the female's non-volatile pheromones, the female receives tactile signals. (d) The male tries to mount the female but may still be rejected if the female keeps her vaginal plates closed and kicks him with her hindlegs. (e) Courtship ends in copulation if the male is not ultimately rejected.

Figure 4. (a) Flight simulator. The tethered fly steers with its yaw torque the angular movement of the panorama. (b) In normal operation an intended turn to the right makes the panorama rotate to the left and vice versa (bottom left). If the fly sees a landmark (e.g. a dark vertical stripe) on the side and tries to turn towards it, the stripe moves to a frontal position. The 10 histograms (above) show for successive 4-min intervals how the fly is oriented with respect to the stripe. The peaks in the vicinity of zero indicate that throughout the experiment the flies have the tendency to fly towards the landmark. In the critical part of the experiment the mode of operation of the flight simulator is inverted. Now an intended turn to the right causes the panorama to rotate also to the right (bottom right). As expected, the fly's tendency to fly towards the stripe now leads to frequent flight orientations away from it. However, this applies only initially. In the course of the 40 min the flies learn to invert their motor commands with respect to the motion signals and finally manage to fly in the direction of the stripe (above).