White larvae/grubs falling from sycamore

Hi all,

I noticed this last year as well but forgot to ask about it.

We have a very healthy and vigorous sycamore in the garden 20m plus in height. Was out in the garden this morning and noticed a steady shower of white larvae falling from the tree that the hover flies were frantically trying to catch before they hit the ground.

They are white, 3mm ish, no black.brown head and perhaps a small spot of darker at the bottom front (could be the top back but I think it was the front).

Anyone have any ideas what these are...? Doesnt look like they are doing any harm to anything and the hoverflies like em but just for interests sake Id like to know.

Cheers
Rolf