The entomological literature,1977,states that the Figwort beetle,Cionus scrophulariae,'s larva spins it's cocoon taking fine threads of silk from it's anal segment with it's mouth(!?):
It does no such thing!It's initial cocoon skin forms on the body of the larva - then it curls into ring & re-inforces this from the inside with milky globular lumps taken from the anus with it's mouth & bunged in place.
They've known about Cionus cocoons for 150 years!
At present it seems as if I'm the only one to have viewed Cionus cocooning under a microscope! Only 10 & 20 times magnification needed.