Hi Martyn
I might be wrong but the photo looks to me like they have shed their skins as they've grown and what you are seeing is the old skins. Maybe somebody else could comment?
Grant
Your Forum
This forum has now been more or less replaced by the Club's Facebook page at 
The weblog below is for naturalists to use to report interesting sightings, ask questions, report on field meetings and generally post pictures and any information or questions generally relevant in some way to the wildlife and geology of Essex. You will need to register and be logged-on to post to the forum, and you need to upload pictures first, for use in posts. Find out more
The weblog below is for naturalists to use to report interesting sightings, ask questions, report on field meetings and generally post pictures and any information or questions generally relevant in some way to the wildlife and geology of Essex. You will need to register and be logged-on to post to the forum, and you need to upload pictures first, for use in posts. Find out more