Once a male silkmoth picks up the whiff of a female, very little will deter him from pursuit. Focusing his full attention on the task, the flightless male doggedly walks (surges) along her tantalising pheromone trail, switching to various zigzagging motions when he slips out of the odour plume in a bid to regain entry to track her down. Poonsup Pansopha, Noriyasu Ando and Ryohei Kanzaki from the University of Tokyo, Japan, explain that in addition to using their olfactory senses to track pheromones, insects also use visual cues to help them stay on track, but it wasn't clear how male silkmoths integrate their visual inputs with their olfactory senses to control the surge and zigzag behaviours. According to Kanzaki and his team, insects visually control progress along an odour plume by steering in the direction in which their surroundings appear to move. They do this by moving so that the image of their surroundings – the optic flow – remains static on the eye, and this response is known as the ‘optomotor response’. However, they suspected that the optomotor visual guidance strategy could override the moths' zigzagging behaviour when they slip out of the plume, raising the question of how these males integrate the visual and olfactory senses during a pursuit (p. 1811).
Pulsing puffs of odour to simulate the males' olfactory experience of an odour plume while surging and zigzagging, the team also simulated the optic flow to find out how the insects use this visual information during the different stages of a pursuit. As they suspected, the moths did not use optic flow while zigzagging, but they did use it while surging along the pheromone plume. Kanzaki and colleagues say, ‘Based on these findings, we suggest that the optomoter response is utilised for course control during straight-line walking, whereas the absence of optomotor response during zigzagging is used to effectively perform the programmed walking pattern.’ The team concludes by suggesting that the use of optic flow visual information by the silkmoth is controlled by two different visual pathways as they navigate odour plumes.