Sometimes a moult can go wrong. Parts can get stuck in the old skin, legs and fangs are the most common problem. This could be because there was an injury to the old part and it wont separate properly, or because the spider is very old and doesn’t have the required energy to complete the moult, or in the case of spiderlings they are not strong enough. No one can say for sure why it happens, there is a train of thought that low humidity could be to blame, but if the spider is well fed this should not be a factor.

In some cases the spider can manage quite happily if it loses a leg for example.

If the missing part causes a problem it could bring on a quick regenerative moult, where the spider will moult again and regain the missing part. I have read of cases of missing fangs where the spider has survived to moult again, but also of cases where the spider has died. In the case of missing limbs they are usually regenerated at the next moult.

It should be remembered that moulting problems are not very common, but the more spiders you have the more chance there is of having one moult badly.