Neurogenetic studies can be used to identify genes required for specific behaviors and can lead to the identification of neural circuits and molecular/physiological mechanisms underlying the behavior. The fruit fly, Drosophila melanogaster, is an ideal model organism for investigating the molecular mechanisms underlying male and female sexual behaviors owing to the availability of vast genetic information and clearly defined behaviors. Neurogenetic studies on D. melanogaster have resulted in the identification of genes regulating male courtship behaviors, and neural circuits in the brain relevant to male courtship have been identified. On the other hand, brain mechanisms that control sexual behavior in virgin females remain largely unknown. In this review, I introduce the analysis method of sexual behaviors in Drosophila virgin females and the genes regulating the behaviors. In addition, I discuss the molecular and physiological mechanisms underlying sexual behaviors in virgin females.

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  The nervous system of the nematode Caenorhabditis elegans is comprised of 302 neurons. Although the structure of the nervous system is quite different from that of vertebrates or insects, many evolutionally conserved molecules are used in the neural circuits. C. elegans use chemosensation to survive in a continuously changing environment. They migrate to comfortable places by smelling for bacterial food and approaching the source of the smell. They also sense noxious stimuli, such as toxic chemicals and mechanical stimuli, and avoid them. Some of the chemotaxis responses are altered by prior chemical exposure in a given context (i. e., high population density, bacterial infection, and food status). Sugar derivatives termed ascarosides are continuously secreted from C. elegans and used as mediators of population density information in the plasticity of chemotaxis response. Evolutionally conserved molecules, such as components of insulin and monoamine signaling pathways, function in neural circuits to regulate chemotaxis plasticities.
  C. elegans has been used as a useful model animal not only in molecular genetics studies but also in physiological studies using live imaging of neurons. In this review, I introduce the overall structure of the nervous system and regulatory mechanisms of neural circuits and molecular pathways in chemical sensation and information processing to generate an appropriate sensory response.

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