1. New Silk Materials Laboratory, Insect Biotechnology and Sericology Department, National Institute of Agrobiological Sciences, 1-4-8 Gohda, Okaya, Nagano 394-0021, Japan, and Biomaterial Development Laboratory, Insect Biomaterial and Technology Department, National Institute of Agrobiological Sciences, 1-2 Ohwashi, Tsukuba, Ibaraki 305-8634, Japan