Behavioral Analysis of Navigation Behaviors in the Drosophila Larva

Functional and anatomical dissection of neural circuits is often hindered by the complexity of such systems. With only 10,000 neurons, the central nervous system of the Drosophila larva is at least one order of magnitude simpler than its adult counterpart. Despite this numerical simplicity, the behavioral repertoire of the larva contains a surprisingly diverse array of sophisticated behaviors. Larvae demonstrate robust orientation behavior toward light and odors (phototaxis and chemotaxis). The sensory organs and circuits underlying these behaviors are greatly reduced in comparison with the adult: the larval eye is composed of just 12 photoreceptor neurons, the nose of just 21 olfactory sensory neurons. While the larval olfactory pathway displays remarkable structural similarities with the adult system its numerical simplicity facilitates the analysis of individual, genetically identifiable neurons at anatomical and functional levels. The use of information arising from different modalities allows for investigation of the principles controlling multisensory integration. In this chapter, we review a series of assays to study light and odor-driven behaviors. The advent of high-resolution machine-vision algorithms to analyze behavior in real time is likely to revolutionize our knowledge of how organization of the larval brain mediates distinct behaviors. The simplicity of the larval sensory systems allows us to aim for a comprehensive and systems-level understanding of the relationships between circuit anatomy and function, from afferent sensory neurons through to higher brain centers where orientation decisions are made and communicated to efferent motor neurons.