There are three species of horse bots. Their life cycles are very similar, except in where they attach their yellow to gray eggs to the host. Common horse bot eggs most often are attached to hairs on the fore legs but can be found on the outside of the legs, the mane and on the flanks. Throat bot eggs are attached to the long hairs beneath the jaws. Nose bot eggs are stuck to hairs on the upper and lower lips. It is easy to see how horses can be spooked by flies buzzing at these areas and may injure themselves or people working or riding them at the time. Depending on the species, females deposit from a few hundred to 1,000 eggs during their life time.
Eggs of the common horse bot hatch after a 2- to 5-day incubation period, often stimulated by warmth and moisture from the animal's tongue. Eggs of the other species may hatch without stimulation. Newly hatched bot larvae enter or are taken into the mouth. They spend about 3 weeks in soft tissue of the lips, gums, or tongue. The bots then migrate to the stomach or small intestine where they use sharp mouth hooks to attach to the lining of the organ. Bots can damage the lining of the stomach or small intestine, interfere with the passage of food, or cause other gastrointestinal disorders. They spend about 7 months there before passing out in the feces. The mature larvae enter the soil below the dung pile and pupate. In 2 weeks to 2 months, depending upon the season, they emerge as adults.
The adults do not have functional mouthparts so they cannot feed. Females go to horses only to lay their eggs. Most of the egg-laying is done during August and September but may continue until the first hard frost.