  A method to reverse engineering of a fly brain using the {\it FlyCircuit}
database is presented. This method was designed based on the assumption that
similar neurons could serve identical functions. We thus cluster the neurons
based on the similarity between neurons. The procedures are to partition the
neurons in the database into groups, and then assemble the groups into
potential modules. Some of the modules correspond to known neuropils, including
Medulla were obtained. The same clustering algorithm was applied to analyze
Medulla's structure. Another possible application of the clustering result is
to study the brain-wide neuron connectome by looking at the connectivity
between groups of neurons.
