There and back again : The neural basis of migration in the Bogong moth

Abstract: The Bogong moth (Agrotis infusa) is a small, night-active Australian moth that has a remarkable lifestyle. Afterhatching from its pupa in spring, it migrates over 1000 km to the Australian Alps, where it spends the summer incool alpine caves. In the beginning of autumn, the moths emerge from the caves and fly back to their breedinggrounds, where they mate, lay eggs, and die. The following year, a new generation of moths repeats the samejourney to the mountains.Migration is a difficult and dangerous task. If the moths get lost on the way, they will not arrive at the caves in timeand will instead perish in the hot Australian summer. It is therefore crucial that they are efficient and reliablenavigators. However, the brains of these moths are tiny – only 3 mm in diameter. How can such a small braincompute the trajectory of this extraordinary migration?In this thesis, I investigated the neural basis of navigation and migration in the Bogong moth. I began by describingthe Bogong moth brain in detail (Paper I). In insects, neurons in a brain region known as the central complex processspatial information and provide the spatial context for behavioural decisions. The central complex of the Bogongmoth is well developed and can be expected to have the same function as in other insects. From previous studies,we know that brain regions that are of special importance for an animal tend to be bigger. I therefore compared thevolume of several higher processing neuropils, including the central complex, across several moth species (PaperII), including both migrants and non-migrants. I found that that the relative volumes of the central complex acrossspecies were very similar. In fact, the central complex scaled hypo-isometrically, suggesting that the neural networksin this brain region are so fundamentally important that even the smallest moths cannot afford to reduce them further.Therefore, instead of being reflected in the overall volume of the central complex, migratory behaviour may bereflected in the response properties of individual neurons in this brain region. Knowing that the Bogong moth canchose a migratory heading based on the starry sky alone, I recorded from neurons in the central brain whilepresenting the moth with a rotating starry sky (Paper III). I found several neurons that consistently responded to thisstimulus. Some of these neurons had branches in the optic lobes, the central complex or the lateral complex, whichare all associated with visual compass processing. Thus, these neuropils provide a suitable substrate for processingcompass cues during the moths’ nocturnal migration. Finally, I investigated how a compass signal in the centralcomplex is transmitted to downstream motor centres that coordinate wing and leg movement. To this end, I built acomputational model of a proposed steering network (Paper IV). I showed that this network can theoretically steerbased on input from olfaction as well as vision, providing a putative connection between the compass system in thecentral complex and thoracic motor centres. Taken together, these results have not only shed light on the neuralbasis of migration in the Bogong moth, but also on neural processing in the insect central complex and lateralaccessory lobes in general. In the future, combining these results with insights from other insects may lead to acomplete understanding of the neural basis of migration, from the sensory inputs to the behavioural output.