Landing is tricky at the best of times, but imagine trying to bring down a light aircraft safely without a speedometer or altimeter. That is effectively what a fly does every time it alights. What is more, in order to pull off a successful landing, a fly has to override its instincts. Floris van Breugel from The California Institute of Technology, USA, says, ‘The fly’s natural response is to turn away from looming objects, but for some reason in this case it decides to land and so we can study how the landing process works and learn how the visual system works’. Intrigued by the insect’s graceful manoeuvres, van Breugel and his thesis advisor Michael Dickinson decided to film fruit flies during their final approach and landing to identify the strategy for a successful set down (p. 1783).
Building a flight arena equipped with six 100 frame s–1 cameras and a 1.9 cm wide central landing post, van Breugel filmed 12 hungry flies overnight as they buzzed around and alighted on the post. Adding a high-speed camera with a tracking system and automated focus, activated when a fly neared the post, van Breugel was able to capture the final descent in fine, 5000 frame s–1, detail.
Identifying 177 successful landings and 1065 non-landings when the fly veered off, van Breugel then had to work out how to analyse the data. ‘There wasn’t much to go off in terms of previous papers that looked at things in a similar way’, says van Breugel. ‘At first I was trying to see if I could understand how the image of the post on the fly eye was influencing its flight’, he recalls, but then van Breugel hit upon the idea of analysing the trajectory from the point when the insects began to decelerate.
‘We found that they follow a pattern where both their flight speed and distance from the post were important’, says van Breugel, adding, ‘The flies that were flying really fast slowed down further from the post and the flies that were flying slowly slowed down when they were closer to the post’. But what could this tell van Breugel and Dickinson about how the insects controlled their final approach?
Analysing the fly’s eye view of the post as it closed in, the duo realised that the insects were matching the way that the image expanded across the retina with its visual size, forcing fast flies to decelerate more abruptly than more pedestrian insects. The duo also realised that the flies only extended their legs ready for landing when they were almost upon the landing post. Explaining that the flies extended their legs when the image of the post on the fly’s retina was 60 deg wide, van Breugel adds, ‘The time to touch down after leg extension is on average 50 ms’. However, this is too fast for the final approach to be under visual control, suggesting that the flies switch to autopilot. Describing the final contact, van Breugel says, ‘They extend their legs in a stereotypical way, then the front legs stick out, touch the post and then they grab on and swing towards it and land’.
Having identified the fly’s landing strategy, van Breugel and Dickinson were curious to find out when the insects that pulled out of a landing aborted the manoeuvre and realised that they took evasive action when the pole landing site was 30 deg wide on the insect’s retina. They also investigated the factors that led to less graceful landings and found that the flies that crash landed had not decelerated sufficiently before attempting touch down.