Cuticular proteins (CPs) are crucial components of the insect cuticle.Although numerous genes encoding cuticular proteins have been identified in known insect genomes to date,their functions in maintaining insect body shape and adaptability remain largely unknown.In the current study,positional cloning led to the identification of a gene encoding an evolutionarily conserved RR1-type cuticular protein,BmorCPR2,highly expressed in larval chitin-rich tissues and at the mulberry leaf-eating stages,which is responsible for the silkworm stony mutant.In the Dazao-stony strain,the BmorCPR2 allele is a null mutation with significantly lower expression,compared to the wild-type Dazao strain.Dysfunctional BmorCPR2 in the stony mutant lost chitin binding ability,leading to reduced chitin content in larval cuticle,limitation of cuticle extension,abatement of cuticle tensile properties,and aberrant ratio between internodes and intersegmental folds.These variations induce a significant decrease in cuticle capacity to hold the growing internal organs in the larval development process,resulting in whole body stiffness,tightness and hardness,bulging intersegmental folds,and serious defects in larval adaptability.To our knowledge,this is the first study to report the corresponding phenotype of stony in insects caused by mutation of RR1-type cuticular protein.Our findings collectively shed light on the specific role of cuticular proteins in maintaining normal larval body shape,and will aid in the development of pest control strategies for the management of Lepidoptera.