Plutella xylostella is an arthropod, and its body is covered with a hard exoskeleton, which will not grow with its body, so it has molting phenomenon.

Plutellaxylostella(Linnaeus). Diamondbackmoth (alias: small caterpillar, two-headed tip) is an insect belonging to the genus Lepidoptera, which is a worldwide migratory pest, mainly harming cruciferous plants such as cabbage, purple cabbage, broccoli, shepherd's purse, mustard, cauliflower, Chinese cabbage, rape and radish. Damage characteristics: the first instar larvae only eat mesophyll, leaving epidermis, forming transparent spots on the vegetable leaves, opening the skylight, 3 ~ 4 instar larvae can eat the vegetable leaves into holes and nicks, and in severe cases, the whole leaves are eaten into a net. At seedling stage, the central leaves are often damaged, which affects the core. It harms young stems, young pods and seeds on seed-reserved plants.