Leg formation
As the imaginal disc is laid down it begins the transition of becoming the future leg.
The imaginal disc first takes on a cone like shape, with rings of epidermal cells, each giving rise to the segmented structures of the leg.
As Drosophila goes through metamorphosis the legs are essentially already formed but lie tucked inside the pupa, as the fly completes the transistion the legs are pushed our in a kind of inside out fashion.
As the picture illustrates above the most distal structure is the Tarsus, which wound be analogous to the tarsals in humans, if you can try to imagine that the small ring in the centre of the disc is positioned deepest inside the pupa and then during the final stages of metamorphosis this cone is pushed out by changes in epithelial cells.
Further information can be found at Wolpert online resources.