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I like these guys- they can extend their arms a surprising distance. Personally I've been within a few inches but I can well understand why you respect them.
Why the mantis shrimp is my new favorite animal - The Oatmeal
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Is your statement that you cannot get closer than a few feet a complaint or a warning?
While mantis shrimp may be initially shy, they can also be curious. Just waiting near the shrimp's doorstep may prove helpful. Of course this does take up time and gas.