The brain of a nocturnal migratory insect, the Australian Bogong moth

Every year, millions of Australian Bogong moths (Agrotis infusa) complete an astonishing journey: in spring, they migrate from their breeding grounds to the alpine regions of the Snowy Mountains, where they endure the hot summer in the cool climate of alpine caves. In autumn, the moths return to their breeding grounds, where they mate, lay eggs and die. Each journey can be over 1000 km long and each moth generation completes the entire roundtrip. Without being able to learn any aspect of this journey from experienced individuals, these moths can use visual cues in combination with the geomagnetic field to guide their flight. How these cues are processed and integrated in the brain to drive migratory behaviour is as yet unknown. Equally unanswered is the question of how these insects identify their targets, i.e. specific alpine caves used as aestivation sites over many generations. To generate an access point for functional studies aiming at understanding the neural basis of these insect’s nocturnal migrations, we provide a detailed description of the Bogong moth’s brain. Based on immunohistochemical stainings against synapsin and serotonin (5HT), we describe the overall layout as well as the fine structure of all major neuropils, including all regions that have previously been implicated in compass-based navigation. The resulting average brain atlas consists of 3D reconstructions of 25 separate neuropils, comprising the most detailed account of a moth brain to date. Our results show that the Bogong moth brain follows the typical lepidopteran ground pattern, with no major specializations that can be attributed to their spectacular migratory lifestyle. Comparison to the brain of the migratory Monarch butterfly revealed nocturnal versus diurnal lifestyle and phylogenetic identity as the dominant parameters determining large-scale neuroarchitecture. These findings suggest that migratory behaviour does not require widespread modifications of brain structure, but might be achievable via small adjustments of neural circuitry in key brain areas. Locating these subtle changes will be a challenging task for the future, for which our study provides an essential anatomical framework.