Abstract
Animals must quickly recognize objects in their environment
and act accordingly. Previous studies indicate that looming
visual objects trigger avoidance reflexes in many species 1,
2, 3, 4 and 5; however, such reflexes operate over a
close range and might not detect a threatening stimulus at a
safe distance. We analyzed how fruit flies (Drosophila
melanogaster) respond to simple visual stimuli both in free
flight and in a tethered-flight simulator. Whereas Drosophila,
like many other insects, are attracted toward long vertical
objects 6, 7, 8, 9 and 10, we found that smaller
visual stimuli elicit not weak attraction but rather strong
repulsion. Because aversion to small spots depends on the
vertical size of a moving object, and not on looming, it can
function at a much greater distance than expansion-dependent
reflexes. The opposing responses to long stripes and small
spots reflect a simple but effective object classification
system. Attraction toward long stripes would lead flies toward
vegetative perches or feeding sites, whereas repulsion from
small spots would help them avoid aerial predators or
collisions with other insects. The motion of flying Drosophila
depends on a balance of these two systems, providing a
foundation for studying the neural basis of behavioral choice
in a genetic model organism.
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