Olfactory Information Processing in Moths

Insects are the most diverse and abundant animal group, representing more than 70% of all known animal species. They display a range of sophisticated and adaptive behaviors based on the perception of a multitude of stimuli. Within the incoming stream of multimodal sensory information, olfactory signals often serve as key stimuli or releasers for the initiation of behaviors such as orientation toward mating partners, localization of appropriate sites for oviposition, and foraging. The vitally important role of olfaction is a general phenomenon across the animal kingdom. Insects are valuable model systems in neuroscience due to the balance between the moderate complexity of their nervous systems, a rich behavioral repertoire, and the low cost of maintenance as experimental animals. Insect brains contain on the order of 10 to 10 neurons, thus they range slightly above in this measure, but below (>10), which is comparable to small mammals (mouse: ca. 5 × 10). For comparison, the human brain contains on the order of 10 neurons. The concept of individually identifiable neurons and small networks as functional units have been vital for understanding insect brains, whose main properties are processing speed, relative simplicity, and elegant design principles. Moreover, insects are well suited for multidisciplinary studies in brain research involving a combined approach at various levels, from molecules to single neurons to neural networks, behavior, and modeling. These preparations are amenable to a wide variety of methodological approaches, in particular genetic engineering, neuroanatomy, electrophysiology, and functional imaging. The similarity in the construction principles of central olfactory processing areas between insects and vertebrates and the common structural units of olfactory processing, called glomeruli, have made insects valuable model systems for investigating general mechanisms of olfactory information processing (Hildebrand and Shepherd 1997; Rössler et al. 2002). The striking similarity in the design of olfactory systems suggests that there are optimized solutions to deal with this kind of stimulus space, whose relevant metrics are still poorly understood. Odor-induced behaviors and their plasticity in insects have also led to important advances in the understanding of learning and memory (Menzel 2001). Even on shorter timescales of odor-induced orientation, flexibility and reliability are features that characterize insect behavior. In particular, moths have been a model system with a long-standing tradition being able to localize a female or pheromone source over long distances in natural environments despite (or because of?) the intermittent stimulus characteristics caused by turbulent flows. In the present context, we cover, without claiming an exhaustive review of the vast literature, the current state of knowledge concerning moth olfactory behaviors, their plasticity, and the underlying neural mechanisms. These encompass the structure and function of olfactory sensory organs, the molecular mechanisms of olfactory transduction, and the anatomical and physiological properties of olfactory neurons and circuits in the brain, which deliver outputs for the control of behavior. While there is also a large body of work on the development of the olfactory system in moths that is important for our understanding of the generation of the structural characteristics of olfactory systems, we refer the reader to available reviews covering this topic (Keil 1992; Oland and Tolbert 1996; Salecker and Malun 1999; Tolbert et al. 2004).