Continuation of comment # 39:The aliens in AVP had a smooth dome over its skull to protect it from the cold.Theres a worm that lives in -10*F temps but as with all life on Earth if it gets too cold then it will freaze to death.so in hotter enviroments the alien does'nt need that extra chiton to keep it warm,therefor it can move faster(To me the aliens in AVP seemed a tiny bit slow compared to the ones from A2). As i said befor,please dont get mad at me if my theory does'nt sound good or if i left something out by mistake,i'm just trying to come up with a theory that makes everyone happy.