The swelling is not hard and contains a single , pale yellow larva .
The gall is an approximately 2 -- 3 mm , spindle - shaped swelling on the underside of the midrib of a leaf .
The swelling is not hard and contains a single , pale yellow larva .
The gall is an approximately 2 -- 3 mm , spindle - shaped swelling on the underside of the midrib of a leaf .