WHy does my spider have a fupa?

[tarcan]

This is for all of you FUPA lovers! I am gald I now know how to call that lump...

 

[Spider_Monkey]

Just a thought, maybe it's a weak part of the abdomen membrane. After molting, since it is even more flexable while drying, it stretches out bigger; hence the reson that it looks bigger now.
Again, this is just my hypothesis.
Any thoughts? Does this logic make sense?