Hypoderma lineatum (Villers), the common cattle grub, is an insect parasite that resides in a warble in the subcutaneous tissues of the backs of cattle during a portion of their life cycle. Inside the warble, the larva undergoes 2 molts to the 3rd instar. In this study, the development of the posterior spiracular plates of the 3rd instar of H. lineatum was observed in situ. Larvae were observed to molt to the 3rd-instar phase 1 stage of development 28.6 +/- 3.9 d (+/- SD) after digesting a breathing hole in the backs of previously uninfested calves. Development of the spiracular plates through each of the various recognizable stages occurred on a 5-6-d interval. It took 54.2 +/- 5.1 d in the back for larvae to develop to the phase 3 stage, the stage reached before larvae exit the host. The average elapsed time from the 3rd-instar phase 3 stage to exit from the host was 5.5 +/- 2.9 d. Of 22 larvae that were followed from arrival in the back to pupariation, the elapsed time was 59.4 +/- 6.1 d. Most larval mortality occurred in the back during the 1st and 2nd instar. Of larvae surviving to the 3rd instar, 86.7% successfully exited from the host. Of 3rd instars surviving to the phase 2 goldplate or phase 3 stage, 93.3% exited successfully from the host.