According to the causes,the surface defects can be divided into blister containing non-metallic inclusion,blister without non-metallic inclusion and “raindrop”,“gills”,“mountained”.
"blister beetle:any of various soft-bodied beetles of the family Meloidae, such as the cantharis, that secrete a substance capable of blistering the skin."
Bullae are rounded or irregularly shaped blisters containing serous or seropurulent fluid.They differ from vesicles only in size, being larger than 1 cm.
The midgut gland cell differentiation occurs at Z2, There are 4 types of cells, i. e., embryonic cell (E-cell), fibrillar cell (F-cell), blister cell (B-cell) and resorptive cell(R-cell).