During pupation, the larva contracts within its own integument so that the integument becomes a cylindrical puparium about 6.3 mm long.

Pupal stage of the house fly. A) Puparium ("pupal case") showing remains of the posterior and anterior spiracles of the larval stage. B) Pupa which develops inside the puparium. C) Puparium after adult fly has emerged through the anterior split

The puparium gradually darkens to a rich, dark brown color. Since the pupal case is formed by the larval skin, the pupa within is said to be coarctate.

The pseudocephalon is completely withdrawn, resulting in the anterior spiracular processes being very near the anterior end of the puparium.

Locomotor pads persist on the ventral surface although the puparium is immobile.

An obscure pair of pupal spiracles are in the conjunctiva between the fifth and sixth visible segments on the dorsal side; these are the only source of air for the pupa.

Pupa Development

Most of the basic features of an adult fly develop within the puparium in 48 hours.Full development requires 3–21 days depending on temperature. The threshold for pupal development is about 11 °C (52 °F).

Typical times for pupal development at different temperatures are as follows:

Upon completion of adult development, the adult pushes off the anterior end of the puparium. A circular slit appears in segment six (fifth visible segment) of the puparium and the detached cap splits into two parts. This is done with the ptilinum, an inflated sac that protrudes from the frontal region of the head just dorsal to the base of the antennae of the adult fly (Figure 14).

Eversion of the ptilinum is by changes in blood pressure and contraction is by muscles. Once its head is free, the fly crawls out of the puparium. It crawls about while the wings unfold and the exoskeleton hardens and dries. When completely withdrawn, the ptilinum leaves only the crescent-shaped suture (frontal lunule) above the antennae.