Gasterophilus intestinalis – Horse Bot Fly Female adult bot flies glue their eggs to the hair on the fore legs of horses. The horse licks the eggs while scratching and the heat of the tongue causes the eggs to rapidly hatch and the larvae to attach to the inside of the mouth. The larvae then travel down the esophagus to the stomach where they attach to the stomach wall. They feed on the horse’s food for the next 7-9 months before dropping off and traveling through the rest of the gastrointestinal tract. They pupate in manure and emerge as adults. Larvae have rows of heavy spines that may cause irritation to the mouth or stomach and secondary infection may result.