We found a number of viper's bugloss plants with deformed stems and leaves whilst out walking on the Purbeck cliffs and were rather intrigued by them. After some research on the internet I discovered the deformities are caused by a tiny gall mite, Aceraria echii.
The mite is, as the name implies, very small. It is shaped like the horn of a cow and has two hooks by which it attaches itself to the plant to extract nutrients. There can be countless numbers on a single plant and they produce between them what I think is a rather attractive effect! As the viper's bugloss is a perennial this invasion of small mites does not really have a major impact on the plant.
I find it amazing that one species of mite is totally dependant on a single species of plant for its survival. It would not be in the mites interest to severely damage a colony of its host plant yet other species of Aceraria gall mite are used as a biological control on some agricultural weeds, especially bindweed.