If you drag and drop rendering nodes
on top of each other, Maya automatically connects them in a default
way. The default depends on the type of node. For example:
Middle-mouse button drag a 2D or 3D texture
swatch over a material to automatically connect the texture’s output
color to the material’s output color.
Middle-mouse button drag a Bump2d
General utility node over a material node to automatically
connect the bump map to the material’s Bump Mapping attribute.