motion and vision

[Nicholas Roy Longrich <longrich@phoenix.Princeton.EDU>]

        I had the good fortune to run into a praying mantis this fall, 
and kept it as a pet for a few days, feeding it moths, crickets and 
grasshoppers. When I threw a grasshopper into its cage, the mantis took 
note, and began to move towards it. When the grasshopper stopped, the 
mantis stood still and stopped moving towards it. When the 
grasshopper resumed motion, the mantis struck and began to eat. I 
wouldn't discount shape as having some role here, but I'd guess that 
motion was the primary means by which my pet tracked its prey. Now, the 
interesting part is that the mantis, instead of moving in a direct, fluid 
manner towards its prey, would move in fits and starts, jerking towards 
and away while slowly creeping up on the grasshoper. As I understand it, 
this motion somehow avoids setting off the motion detectors that other 
insects use to see predators, by resembling motion patterns commonly 
encountered in nature, such as vegetation, which would move back and forth 
with the wind. This has nothing to do with dinosaurs, actually, but 
mantids are still pretty neat.