In the fall, in the Northern Hemisphere, whitish-yellow flecks commonly appear on the chest, belly and upper front limbs of horses. These flecks are eggs laid by the Bot Fly, a bee-like insect whose life cycle requires the horse as a host. These eggs contain a substance irritating to the skin, and horses often bite at them.

When swallowed, the larvae attach to and live on the lining of a horse's stomach. Vets often see these larvae when performing endoscopy on horses for stomach ulcers. They are orange to red grubs about 3cm in length.

Bot flies themselves are common and can be highly annoying, sometimes causing horses to stampede but they are not harmful otherwise. You will see them repeatedly depositing their eggs on the skin with their long ovipositor.