Grainworms are the dried larvae of the Calliphoridae insect, Big-headed Golden Fly, or other closely related insects. Grain insects are generally flat cylindrical in shape with a pointed head, 1-1.5 cm long and 2-3 mm wide. Grainworm is a kind of traditional Chinese medicinal material, mainly produced in Guangdong, Zhejiang, Anhui and other places. Among them, the ones produced in Longmen County and Boluo County of Guangdong are of high quality.

Grain insects are usually collected between July and September. The larvae are generally rinsed in running water and then dried in the sun.

The texture of grain insects is crisp and brittle, and the cross-sections are mostly vacuoles. Better quality insects are light yellowish white in color, light in weight, clean and odorless.