Can anyone help in identifying what these grubs might have turned into please? I opened a window and found that these "nests" had been built between the opening part of the window and the frame. Opening the window broke the tops off. The broken off bits were reasonably hard, and looked as though bits of soil had been welded together, possibly with saliver, to form quite a strong casing. It felt like sandstone. Inside each one was a very bright, hardish interior (again felt like sandstone), and from each little nest the body of the grub was protruding (obviously because I had broken their nests). The top part of the nests was adhering to the opened window.
Would be interested to know what these grubs would have turned into, as I have never seen anything like this before.