Photoactivated insecticides show great potential as new resources of biorational insecticides.The discovery of phototoxins in plants provides new thinking in understanding the relationship between insects and their host plants.It is suggested in this paper that the avoiding light habit and the spe-cial body characteristics of Noctuidae larvae are due to feeding plants con-taining phototoxins for long period.This is the result of coevolution between insects and their host plants.