Lime nail galls

I'm on a roll here! I find these red nodules on my lime trees every year and have always wondered what they were but never bothered to look into it.
The hollow structures are produced in response to the mites feeding on the underside of the leaf. The mites subsequently enter the galls to shelter, feed and breed.
They are not harmful to the tree and in fact I think they add an extra splash of colour to the garden!