Pheromones and General Odor Perception in Insects

For insects, finding of a mate or a food source relies often chiefly on olfactory information. The identification of a specific mate or host involves the recognition of a specific odor blend and its discrimination from a complex and changing background. Their highly efficient olfactory systems have evolved to detect behaviorally relevant compounds with a high sensitivity and properly decode the olfactory message to finally lead to adapted behaviors (Martin et al. 2011). Moth sex pheromones, for instance, are precisely defined blends that trigger innate and stable behavioral responses in physiologically competent individuals. We begin this chapter with a description of the general organization of the insect olfactory system from the olfactory organs to the brain (Section 2.2). We will then consider the neuronal bases of the coding of pheromone and general odors (Section 2.3). The chemical specificity of the detection is at first based on the specificity of olfactory receptors (ORs) expressed in precise types of olfactory receptor neurons (ORNs) to detect individual compounds. But to insects as to other organisms, biologically relevant odors are often blends of volatile compounds whose perception involves progressive integration of the input in the central nervous system (CNS). In the wild, different sources release their volatiles simultaneously and their components intermingle to form a complex and fluctuating olfactory environment. Interactions between odor components, or their resulting neural codes, take place at the different levels of the sensory system and are intrinsic to olfaction. Section 2.4 will consider how insects recognize odor multicomponent blends in such chemical complexity. To be pertinent, the response of insects to odors must integrate the physiological and sensory contexts. Even strongly determined odor-guided behaviors are modulated by underlying changes in the internal physiological state of the animal and many behaviors are multimodal (Section 2.5). Very often chemical signals vary in an unpredictable way for the receiver and the chemical environment in which the signal is released is rather complex and changing. Previous experiences also modify the processing of odor signals and the associated behavioral responses. In the last section (Section 2.6), we consider the processing of olfactory signals under the influences of the ecological context and the individual history. Outstanding progress in our understanding of insect olfaction has been accomplished through numerous experimental studies conducted in parallel on several model species, like bees, flies, or moths with their specificities. Although belonging to quite different groups, with diversity in their biology and phylogenetical origin, these model species share many of the general traits of organization of their olfactory system. However, making a synthesis would have been impossible due to the abundance of the literature and the risks of raising specific adaptations to general facts. I arbitrarily chose to focus this chapter on moths. Reproduction in that nocturnal Lepidoptera offers the advantage of providing cases for a highly determined sex pheromone communication system, and a more plastic use of olfaction for feeding and oviposition. Thus, most of the examples herein have been taken from moths and completed by data collected from other classical model insect species when relevant.