Complex sentence: His best-known work, however, was on insect development, demonstrating that the various phases during the life of an insect--egg, larva, pupa, and adult--are different forms of the same animal.
Split into simpler sentences: His best-known work, however, was on insect development. He demonstrated that the various phases during the life of an insect--egg, larva, pupa, and adult--are different forms of the same animal.