I'm going to have to split hairs on this one... aggressive and defensive have completely different meanings. And in this case, it DOES make a difference. Aggressive: Inclined toward hostile behavior. Defensive: Intended or appropriate for defending; protective. While it can be said that some species of tarantula exhibit hostile behavior, this behavior is only exhibited to defend. Whether defending self, burrow, or egg sac, a tarantula WILL stand it's ground.

Once out of its habitat, most New World species (from the Americas; South, Central, North, etc.) are calm and gentle creatures, content to gently explore whatever surface they are set upon. If startled, they may rear up in a threat posture, run, or flick urticating hairs. I've even been "slapped", a warning delivered by a tarantula standing in threat posture slapping the front two legs on my skin.

Old World species (from Asia, Africa, India, etc.) are generally much more defensive. Their initial response to a threat is to retreat, but if cornered, will bite readily if the threat remains.

As a rule, tarantulas will give you plenty of warning before they actually bite. It's best to learn these warnings and heed them!