the spots on its tail

[diamondcutter]

This is the Banggai Cardinalfish which lives around the islands of Indonesia. It’s easy to see because of the spots on its tail and its striped body.
Source: Kid’s Box 5, CUP

The preposition “on” means “on the surface of” but the spots are not removable. Why is “on” used here? Can we use any other preposition?

 

[5jj]

'On' is the correct and natural preposition.

 

[emsr2d2]

I have freckles on my face, moles on my body and a scar on my back. None of them are removable. They don't have to be removable to be on the surface.

 

[Skrej]

Diamondcutter, I'm curious where you got this notion of 'on' having to be removable. Why do you think it must?

 

[diamondcutter]

Hi, Skrej.
My misunderstanding came from these two examples.

1. There’s a bird on the picture.
It’s a live bird and it may fly away any time.
2. There’s a bird in the picture.
The bird was drawn by the painter, which is part of the content of the picture.

Now, I know I made a mistake of overgeneralization.