On the MD80 the items you are referingg to are spray deflectors. They stop water spray from the nose wheels getting thrown up into the engines.
On the Beech 1900 I think they are just to keep the thing flying in a straight line!. I remember seeing pictures of the proto type and it didn't have them for the first flight. Looks a complete mess to me, mind you they have made more of them than I have so it must work!!
Rgds Dr I