There are 13 short, stout, 1-10mm Byrrhidae in Britain.  They get their common name from their habit, when disturbed, of folding all their appendages into grooves on the underside of the body and playing dead, giving the appearance of a seed or pill.  Generally found in litter and moss or under stones, they are largely a heathland/moorland group.  Both adults and larvae are thought to feed on mosses.