Adult Hypoderma, known also as heel flies or gad flies, are ~15 mm. long, hairy, and bee-like in appearance. In late spring or early summer, they attach their eggs on the hair of cattle, particularly on the legs and lower body regions. The eggs hatch in 3–7 days, and first-stage larvae travel to the base of the hair shaft and penetrate the skin. Normally, the first-stage larvae travel through the fascial planes between muscles, along connective tissue, or along nerve pathways. They secrete proteolytic enzymes that facilitate their movement. During fall and winter, larvae migrate toward two different regions, depending on the species. H. lineatum larvae migrate to the submucosal connective tissue of the esophageal wall, where they accumulate for 2–4 months. H. bovis larvae migrate to the region of the spinal canal, where they are found in the epidural fat between the dura mater and the periosteum for a similar period.
Beginning in early winter, the larvae arrive in the subdermal tissue of the back of the host, where they make breathing holes (central punctum) through the skin. Cysts or warbles form around the larvae, which undergo two molts (second and third stage). The warble stage lasts 4–8 week. Finally, third-stage larvae emerge through the breathing holes, drop to the ground, and pupate. Flies emerge from the pupae in 1–3 months, depending on weather conditions. Adult flies, which do not feed, live