Late instar warble fly larva
Life cycle of the horse stomach bot fly.
There is also a human bot fly and you don't want any. Avoid collecting adults by hand. Incidentally, Dr. L. H. Dunn in the 1930`s successfully reared several larvae of the human bot fly in his arm! This really requires more dedication than the author has; however, if you are interested in reading about this, it occurs in Psyche, vol 28, pages 327-342.