Those blending in with surroundings never leave exclusive groups (5,7)

I believe the answer is:

stick insects

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'those blending in with surroundings' is the definition.
(look like sticks)

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for stick insects that I've seen before include "Bugs" , "What flypaper might do" , "Little creatures" , "Kids' pets possibly" .)

I've seen this clue in The Telegraph.

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