Wikipedia Article About Hiyamugi on Wikipedia Sōmen (Japanese: 素麺, そうめん) are very thin, white Japanese noodles made of wheat flour. The noodles are usually served cold and are less than 1.3 mm in diameter. The next thicker wheat noodles are called hiyamugi and even thicker Japanese wheat noodles are called udon. Thin, fragile, white Japanese noodles made from wheat flour. They are sold in straight strands that are dried and bundled. Occasionally the bundles will contain a couple brown or light pink strands. Hiyamugi is generally served cold with dipping sauces.