| bovis·Fig. 1. Life cycle of Hypoderma bovis (warble fly). 1, 2 Adult females lay several hundred eggs which are singly deposited and become attached to the host's hair (2.1) by an attachment organ. The first-stage larva hatches from the egg in about 4 days, crawls down the hair and penetrates the skin. 3 The exact route taken by migrating larvae in the host is not known, but after several months they reach the final site, the epidural fat of the spinal canal (3.1). 4–6 At the beginning of the year the L1 leave the spinal canal and move to their final site on the back; this is an area of 25 cm on either side of the midline from shoulder to tail and is where the cysts (=warbles) are formed from March to July. The L1 (3.1) measures about 10 mm in length just after arrival; it molts into the second-stage larva soon after reaching the skin and cuts a hole (HO) in it through which it respires by means of paired terminal After the third larval appears and then develops into the 30-mm-long prepupa (6). After several weeks the yellowish-brown prepupa forces its way through the skin's opening, drops to the ground and moves actively seeking shelter. 7 The pupal stage needs 3–10 weeks depending upon external conditions (H. lineatum needs 4 weeks or less). Adults of Hypoderma bovis appear from June to mid September, are unable to feed, and live for only 3–5 days; they emerge early in the day and mate within 1 h. CY, cyst, or warble; H, hair; HK, larval mouth hooks; HO, hole in the skin; OP, opening (for emergence) in the pupal |
