Realistic learning situations always comprise interactions of skill- and fact-learning
components (composite learning). So far, these interactions have escaped thorough scrutiny.
Fixed flying Drosophila melanogaster at the torque meter provide one of the very few
systems where the relationship of operant and classical predictors in composite learning can
be studied with sufficient rigor. Experiments with wildtype, mutant and transgenic flies show
that there is an interaction between predictive stimuli (classical component) and goal-directed
actions (operant component) which makes composite conditioning more effective than the
operant and classical components alone. Rutabaga (rut) mutants are impaired in learning
about the (classical) stimuli, but show improved (operant) behavior learning. This is the first
evidence that operant and classical conditioning differ not only at the circuit, but also at the
molecular level. The interaction between operant and classical components is reciprocal and
hierarchical, such that an impaired classical component (in rut flies) suppresses retrieval and
an intact classical component suppresses acquisition of the operant component. Experiments
with transgenic flies demonstrate that this suppression of operant acquisition is mediated by
the mushroom-bodies and serves to ensure that the classical memories can be generalized for
access by other behaviors. Extended training can overcome this suppression and transforms
goal-directed actions into habitual responses. In conclusion, composite conditioning consists
of two components with reciprocal, hierarchical interactions. Acquisition of the rut-dependent
classical component suppresses acquisition of the rut-independent operant component via the
mushroom-bodies. The operant component facilitates acquisition of the classical component
via unknown, non-mushroom-body pathways. This interaction leads to efficient learning,
enables generalization and prevents premature habit-formation. Habit formation after
extended training reveals the gate-keeping role of the mushroom-bodies, allowing only well-
rehearsed behaviors to consolidate into habits.