(or cattle grub, or heel fly), insect included either in the bot fly family Oestridae or the family Hypodermatidae (order Diptera); warble fly species Hypoderma lineatum and H. bovis deposit their eggs on cattle legs, where larvae penetrate the skin, migrate through the body for several months, and produce a characteristic lump, or warble, on the animal's back; when mature, the cattle grub emerges and drops to the ground to pupate and transform into an adult fly; reduces value of cowhide because it creates breathing holes; widespread in Europe and North America.