The larva of the gall midge Rabdophaga nervorum causes galls in the midrib of the leaves of some species of willow.  A swelling is present in the midrib, with a pale larva or pupa inside. Adjacent leaf tissue darker and appears thickened; midrib may also be bent. 

Photographs of leaf showing position of gall in leaf midrib; evidence of larva or pupa inside gall

Sallows