What does this behavior mean?
 

I know it means don't mess with me when they raise their front legs in the air but what does it mean when a rosehair sticks it's butt up in the air with its head and front legs down when you approach it?
It looks like it is trying to hide its head and because of that its butt is stiking up.

The butt in the air is another defence posture, ya see the abdomen has the urticating hairs on it and when disturbed they will sometimes raise that. So in a situation where the spider is getting molested whatever is bothering them will come in contact with the urticating hairs on the abdomen and if a mouse or whatever will most likely get a face full and leave it alone.

Or they put their front legs over their eyes to sheild from light, but in your case John is 100% right.

Poor bitsy baby is scared of big SPJ guy. :)

Avicularia are well known for showing thier ___ as well. The bad thing about the Avics is if that doesn't work they can shoot poo :ack2: a pretty good distance. So imagine this , your a rat, heh heh. and you see this yummy spider, you sniff him and get some hair rubbed on your nose...but man your friggin hungry, I mean hungry...so you persist and something hits your eye...you rub it off and well like we rats do...ya sniff it and it's poo :ack2: !!
the spider laughing slowly saunters off.
Thank god I haven't seen rosies do this :D