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It's normal. They do this when they are disturbed. They rub the setae off their abdomen as a deterrent to anything that may be bothering it. A breeze, prey or even handling can cause them to do it. Also, right before they molt, they will lay a mat of this setae down on top of their webbing....I'm not sure if it's for extra "cushion" or what but I've witnessed it quite a few times, myself. The bald spot isn't dark, yet so it may not be too close to a molt. Time will tell. No need for alarm.