Idiom for mistaking the part for the whole

Question

"Missing the forest for the trees" is close, but moreso refers to failing to consider the whole because you're so focused on the parts. What I'm looking for is an idiom that describes mistaking the whole of something for one of its parts, rather than just failing to consider the whole at all.

A pop-culture example: the meme from this spongebob episode. Sandy is trying to defeat a predatory worm, but mistakes the tongue of the worm for the worm itself. I want a good idiom that expresses this specific failure.

Answer 1

The closest I can think of is the parable of the blind men and an elephant. This refers to a story where a bunch of blind people encounter an elephant. They each feel a different part of its body, and conclude that it's a different object (e.g. the one who feels the trunk thinks it's a big snake).

I'm not sure there's a term for what you describe, since there aren't so many cases where a part of an object is so similar to the whole object that they can be mistaken for each other.

Answer 2

Can't see the wood for the trees (UK) or Can't see the forest for the trees (US) refers to the situation where you are looking at a small component, such as an individual tree, and can't see the forest or wood (meaning a group of trees like a forest, rather than the timber/lumber that the tree is made of) that they are a small part of.

Collins Dictionaries says:

If someone can't see the wood for the trees in British English, or can't see the forest for the trees in American English, they are very involved in the details of something and so they do not notice what is important about the thing as a whole.

can't see the wood for the trees not be able to see the wood for the trees, Collins COBUILD Advanced Learner’s Dictionary. Copyright © HarperCollins Publishers.