what kind of grub is this ?

vin sent me this pile of photos of grubs asking if i could help him identify them. he knew i wouldn’t have a clue but wondered if you did.
He writes:

I wonder if you’d be kind enough to post these pics of a tiny grub I
found today in the hope that some knowledgeable reader can identify
it. I’ve had no luck so far on Google.

It was in this kind of discus-shaped sac-thing, just lying in a
flower -bed. It measured a quarter of an inch (6mm) across, and when
the grub wriggled (more like a convulsive jerk), the whole thing
moved. When I first saw the movement in the dirt, I had assumed an
ant was tugging at a leaf disc (as they do), but it wasn’t really
going anywhere, just turning around and jerking about. Thanks Vin