Bright something on underside of nettle

[Dillybythesea]

This caught my eye and I wondered if it's a gall of some kind, do you think?

[PMJ]

It looks like a caterpillar to me.

[Dillybythesea]

Yes, understand why you say that, but was definitely part of the central rib of leaf, and just a caterpillar-lookalike, promise!

[Paul mabbott]

It's a rust, whose name I forget. We've had them before so try nettle rust in the Gallery ... It will come to me as soon as I send this.
PS: told you so, it's a Puccinia but there has been debate about which species in the past:
ID for Nettle Rust / Mold required please

[Dillybythesea]

Many thanks indeed Paul! That explains a lot! Such a good thread and kind of you to unearth it! ATB, Rhona