In the first-instar larva, the two posterior spiracles each consist of small slit-like apertures in a slight prominence.
In the second-instar larva the slits become larger and more conspicuous.
The posterior spiracles of the third-instar larva each have three distinctly sinuous slits surrounded by a heavily sclerotized ring with a conspicuous perforated button that extends inward from the mesal side (Figure 6A). The button is the extraction site of the spiracle of the previous instar during the moulting process. The second- and third- instar larvae also have anterior spiracles on the third segment (apparent second segment). The anterior spiracles are stalked projections with 6–8 branches.
The first-instar larva has no spines on the first 4 segments. On the fifth to twelfth segments there is a transverse, fusiform, swollen area provided with spines ventrally occupying the anterior third of each segment.
On second-instar larva, segments 2 through 5 each have a complete spinose ring. Segments 6 through 12 have ventral swollen spinose areas, and spines continue dorsally to form a complete ring.
In the third-instar larva, the ventral swollen spinose areas become more prominent and crescent-shaped on the sixth to twelfth segments. These ventral spinose areas are called locomotor pads.
On the tapered anterior end, the larva has a mouth surrounded laterally and ventrally by two fleshy, grooved oral lobes. Extending downward in front of the mouth is a dark, heavily sclerotized mouth hook. The visible single mouth hook is the proximal process of the internal cephalopharyngeal skeleton (Figure 7A) and is attached to the mandibular sclerite. The mouth hook can be drawn inward.
Internally and posterior to the mandibular sclerite is the hypostomal sclerite. This attaches to the pharyngeal sclerite, which in turn branches into two lateral sets of prominent posterior ventral and dorsal lobes or “wings”. These lobes (cornuae) are sites of internal muscle attachments.
The shape of the cephalopharyngeal skeleton is used as one of the characteristics for identification of fly larvae.
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