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You are watching a movie of a fly pupa, showing normal leg development.

This video was made by Tina Fortier and Craig Woodard at Mount Holyoke College. Early in development small sets of cells, called imaginal discs, are set aside for later differentiation. Under the influence of hormones, these discs undergo tremendous shape changes to emerge as legs or eyes or wings. Tina and Craig genetically engineered a line of flies carrying a specific gene for green fluorescent protein. In these flies, this protein is only expressed in the cells of the leg imaginal discs. In this fluorescent time-lapse video, you are watching a living fly pupa, and only the cells of the leg discs are brightly fluorescent. With time, you see the short, squat discs elongate into the elegant legs that will serve the adult fly after metamorphosis is complete.

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