The nose strakes were introduced on the DC-9-50 to improve the directional stability and aerodynamic airflow at high angles of attack and certain flap positions.The DC-9-40 had an internal mechanical solution to this problem,but this was deemed uneccessesary with the arrival of the strakes.On the DC-9-50 and MD
-80 series the strakes are anti-iced by bleed air via the airfoil anti-ice switches,on the MD
-90 they are electrically heated.
The engine nacelle strakes were introduced on the MD
-80 series for another reason,namely stall recovery.They add area which in turn adds drag and in a stall condition,this will help the nose drop faster than the tail.On the MD
-90,with it's much heavier engines,there are small "flaps" at the trailing edge of the engine pylons in addition to the nacelle strakes.These operate only in the air and when the control column has reached it's fully forward position.The nacelle strakes are not heated in any way.