We have found about 20 cocoons on my son's wall - the biggest is about 1cm long, the smallest about 1mm. They are white and cottony in appearance with open holes at both ends. The larvae inside are alive and pop their heads out from time to time, they also move the cocoons around by reaching out and pulling themselves along.
What are they? are they harmful? should we have them removed?
The only thing I know of that fits this description is a lacewing larva. Some do live inside a cotton-like protective shield. But indoors? What would they be feeding on? Do you have lots of potted plants covered in greenfly? Could they have come in from outside?
If it is a lacewing larva the head should posses a fairly decent set of jaws.
I may be on complerely the wrong track. If so, sorry to have misled you.
These are likely to be Case-bearing Clothes moth caterpillars, which live inside a protective case spun from silk and the fibres that they are feeding on. They feed on natural fibres including woollen carpets, blankets, felts etc. etc. In nature they are associated with birds nests and such nests in loft spaces or chimneys are often the initial source of adult moth.
See image here.
The concept of 'nest' does not apply here. An adult female moth lays a batch (or batches) of eggs on, or near, suitable larval food sources - so items manufactured from organic fibres, woollens are a particular favourite. Once hatched the larvae feed and develop on this resource and will wander of in search of other suitable food - they camouflage and protect themselves by spinning a silken case incorporating fibres of their chosen food item into this. Where there is one there are usually several others of the same cohort - but not a nest.
Treatment should be the prolific use of the vacuum cleaner in that area, perhaps the use of a propriety insecticide spray and freezing of affected fabrics/items if small enough to be practicable.