During pupal stage many larval structures are lysed and new structures are formed from the imaginal disks. Imaginal disks are developed from larval undifferentiated cells and give rise to multiple adult structures such as the adult head, legs, wings, thorax and reproductive apparatus. Some larval structures like the nervous system or gonads are preserved during the pupal stage [1]

Eclosion is the process by which an adult fly emerges from the pupal case. In this video can be observed drosophila pupae eclosion. As it can be observed in the video, the puparium turns black before eclosion. Eclosion rate and pupa lethality are two parameters often used as readouts for developmental defects in genetic screens.

Metamorphosis and pupal eclosion are highly energy-demanding processes. Prepupal and late pupal  (right before eclosion) steps have high energetic rates. These patterns follow metabolic activity of pupal tissues, which becomes very low after larval tissue lysis and increases again during adult tissue differentiation. These metabolic rates are also temperature dependent (becoming higher at high temperatures) [4].

Gene expression changes drive the metamorphosis process, with the steroid hormone ecdysone playing a central role, together with other hormones and neuropeptides [2]. Ecdysone is first synthesized in the larval prothoracic gland and plays also a role in the moulting of larval stages. Pulses of ecdysone during the pupal stage allow for imaginal disc differentiation and adult structure formation through different transcription factors.