Re: motion and vision

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

On Tue, 28 Feb 1995, Mickey Rowe wrote:
> Nicholas Roy Longrich <longrich@phoenix.Princeton.EDU> wrote that a
> praying mantis he kept relied on motion to find a grasshopper he fed
> it.  I have two responses, a) I too have temporarily housed praying
> mantises.  The last one that I had was perfectly willing to eat pieces
> of beef heart that I pulled from my freezer (I originally bought the
> beef heart for my turtles--honest, you can get the stuff from fish
> stores...)  I didn't have to wiggle the beef heart to help him find
> it. b) When swatting flies, I use a technique rather similar to that
> Nick described for his mantis.  Move towards them while they're
> moving, remain motionless as soon as they freeze.  Move closer again
> when they start to walk or clean themselves. Stop if they stop.  You
> can almost always catch flies this way.  I suspect mantises also have
> learned (via more hard-wired pathways, than what I use :-) that this
> is an effective strategy to pursue while stalking other insects.

        I haven't ruled this possibility out but my general impression 
was that the mantid was no longer aware of the grasshopper when it ceased 
motion. It could well be that motion in prey is a sign that the prey is 
alerted and thus the predator stops. But motionless prey seemed to be 
pretty unappetizing to the insect. 
 
> I don't know where you heard the story about that behaviour being an
> attempt at deception -- it's well documented that mantises perform
> these motions in order to gain distance cues.  They rely on motion
> parallax to tell them whether an object is near or far.  Usually the
> motions are combined with side to side movements, though.  The mantis
> "knows" that when it wobbles, objects which appear to move quickly are
> closer than those that appear to move slowly.
        I've heard about this "motion camouflage", if I recall, from an 
article or film talking about walking-sticks, which use it to avoid 
rather un-sticklike motions that might cue in a bird or other predator. I 
assume it would also protect the mantid from detection (by its prey) as 
well. The grasshopper didn't seem too alarmed until the mantis struck. It 
sems only natural that there must be certain forms of motion that eyes 
are cued to react to and certain forms they tend to ignore. Scallops, for 
example, are ringed with motion-detecting eyes. Fast motions of fish tend 
to set them off, while very slow motions (the speeds at which starfish 
move) trigger their swimming response as well. They more or less ignore 
everything else. Point being, an insect or a scallop or a vertebrate 
smply can't respond to or be aware of every single motion going on, or else 
they would constantly be panicking or on alert for prey. Brains probably 
evolve to recognize certain speeds, patterns and kinds of motion. 
Predators and prey can benefit by moving in such a way so as not to set 
off the motion detection systems of their prey and predators.
         As for 
dinosaurs, I once crept up, inch by inch, on a crowd of feeding birds, 
over several minutes, and watched as they ate sunflower seeds and hopped 
on my feet. Even for animals with the visual acuity of birds, motion is 
pretty important.