Stomach Bots are the larvae of Gasterophilus flies. The adult horse bot fly emerges a during the summer or fall season. After the fly emerges from the pupa, it quickly finds a mate, lays the eggs on the horse's coat and, on grooming, the eggs make their way to the mouth where they hatch in mouth as larvae. These larvae migrate to the stomach, causing an infestation for 8-10 months.
The larvae cause ulcerations in the lining of the stomach, and it is this which leads to disease. Disease is also caused by the larvae in mouth. Larvae here create borrowing holes, which in turn become infected. The 3rd stage larvae are passed in the faeces and over winters to develop the following summer.