Three-dimensional atlases of insect brains

The morphological structure of the nervous system is ultimately the basis of its function. Analyses of the anatomical layout of brain areas, single neuron morphologies, and the synaptic connectivity of neurons are therefore essential for a comprehensive understanding of the computational processes implemented in neuronal networks. Insect brains have long served as models to examine neuronal circuits that process sensory information, provide the substrates for learning and memory, or generate motor patterns that drive well-studied behavior. The relatively small number of neurons these brains are composed of (up to one million) and their small overall size make them easily accessible for physiological and anatomical research. To aid the comparison of results within and across species, and thus make it possible to relate function to anatomical structure, printed brain atlases have been used as a common frame of reference for many decades. In recent years, digital, three-dimensional atlases were generated to provide geometrical as well as conceptual reference systems for the brains of several insect species. In this review we compare the different approaches for generating such three-dimensional atlases. We highlight the key problems that must be overcome during this process and the solutions that have been found to achieve this. The advantages and limitations of the different strategies are discussed, and the applications that have so far resulted from the implementation of these atlases are described