In areas which are covered like under the sofa, I keep finding small white soft things which look and are the size of grains of white rice. I kept hoovering them up but then decided to collect some and put them in a jar to see if they were larvae which would transform into some sort of bug. I could see that there were tiny caterpillar-like creatures in the white 'cocoon' but then they stopped wriggling and eventually died after a few days. The caterpillers had black heads. Have googled 'carpet bugs' and images do not match up to anything I have seen. Any ideas?