Flying
about in a crowded airspace is difficult enough, but the biggest problem is
landing. Not just anywhere mark you, but on a precise spot that is coming up
fast and is often moving. Of course, birds do it and bees do it, but so do
fruit flies.
Fruit
flies aren’t very large and have to rely on their in-built guidance systems to
reliably work on the fly. Van Breugel
and Dickinson, in the current edition of the Journal Of Experimental Biology
have been scrutinizing the success and failure of fruit fly landing sequences
with a combination of high-speed photographs and 3-D tracking software (1).
The
touchdown sequence commences with a sharp turn towards the target and a
deceleration that the authors explain is governed by the growth of the image
size on the fly’s retina. The go/no-go decision is made at the point that the
target image takes up an include angle of about 33 degrees of the fly’s vision
field.
If
the decision is go – the deceleration sequence continues until the target image
angle has increased to about 60 degrees and then the landing gear is deployed.
On touch down the feet grasp the target. From gear deployment to touchdown was
about 50 ms – pretty fast reactions are a must.
If
the no-go decision is called for, the fly’s fly-by procedure is a sharp turn
away and off to another target. This occurred in 1065 attempts out of 1242-recorded
measurements.
Flies
flying-by may have been due to too fast an approach or maybe just the
fickleness of the fly. They did record some crash landings when flies came in
too fast and failed to abort, but none included any fatalities.
