Question: Why Does My Tarantula Have a Bald Spot on Its Abdomen?
There are a couple of reasons a tarantula might have a bald spot on its abdomen, depending on the species.
Answer: A common cause of a bald spot on the abdomen of a new world tarantulas (from the Americas; for example the popular Chilean rose tarantula) is that the tarantula has kicked off a patch of its urticating hair. These urticating hairs are the extremely irritating and itchy hairs that certain tarantulas will actively kick off their abdomens as a first line of defence if they are disturbed. Once these hairs are shed, they will not come back until the next molt.
Speaking of molting, a spider about to molt will sometimes develop a bald spot as well. This spot may get progressively darker as molting nears, but once the molt is complete the abdomen will be hairy again.