Variations on an ancient theme — the central complex across insects

The central complex (CX) is a highly conserved region of the insect brain, and its ubiquitous occurrence suggests that its neural circuits are of fundamental importance. While its overall layout has not changed since the evolution of insect flight, substantial variations exist in the internal organization of all CX components. By changing the details of a system of repeating columns and layers, these differences affect the almost crystalline internal organization of the CX and thus the characteristic neuroarchitecture that directly links structure with function. While neuropil level changes suggest widespread differences in cellular architecture and circuits, data at these deeper levels are mostly limited to the fruit fly Drosophila. Nevertheless, interspecies neuron-level differences have begun to emerge. Whereas these differences are small compared to the astounding degree of conservation, they reveal highly evolvable aspects of the CX circuitry, providing promising starting points for future research using comparative circuit-level analysis.