Insect Vision and Olfaction: Common Design Principles of Neuronal Organization

Abstract

In insects, the organization of neurons in visual and olfactory neuropils is comparable to arrangements in analogous systems in the brains of higher vertebrates. However, although the two modalities of vision and olfaction are subjectively quite different from each other, in insects they are served by common neuroarchitectures, the glomeruli, which are here suggested to be paramount in the processing of qualitative information. Visual and olfactory systems show other specific similarities with respect to the parallel organization of large- and small-axoned neurons. In the visual system, two parallel channels comprise large color-insensitive and small color-sensitive relay neurons that are linked to two major descending pathways. Color insensitive pathways supply motor circuits mediating visually stabilized flight and optokinetic head movements. This pathway is distinct from the parallel subsystem comprising numerous smaller neurons and many synaptic stations that supply leg and direct flight muscle motor neuropils. These two subsystems provide a simple model of magno- and parvocellular organizations identified in the mammalian visual system. Surprisingly, there exists a similar parallel organization amongst large- and small-axoned neurons in the insect olfactory system. Magnocellular olfactory projection neurons provide a relatively direct route to descending pathways. Smaller parvocellular projection neurons provide the first step in a complex sequence of neurons in which higher brain centers play a cardinal role.