Abstract: Obvious pathological changes in the midgut of the orient armyworm, Mythimna separata and the diamondback moth, Plutella xylostella were observed following treatment with cantharidin. The microvilli were sloughed off, goblet cell cavities enlarged, their projections sloughed off and the plasma membrane disappeared. Mitochondria swelled irregularly and their cristae fractured and become fuzzy. The cisternae of rough endoplasmic reticulum swelled obviously and the ribosomes denuded. The cell nucleus swelled and chromatin became concentrated. All the above observations show that cantharidin acts on the midgut membrane system.

Key words: Cantharidin, Mythimna separata, Plutella xylostella, midgut, toxic mechanism