Figure 1.1
Imaginal discs and their cuticular products.
The fly exterior is assembled from separate parts (like an automobile). The epidermis of the head and thorax come from 9 bilateral pairs of discs (one of each kind is shown), and genitalia come from a medial disc, so there are 19 discs total. Abdominal wall comes from histoblast nests (h): tergites from dorsal nests, sternites and pleurae from ventral nests.

Discs are drawn to the same scale and oriented to display their mature shapes and folding. Placements are approximate. Clypeolabral and labial discs are attached to the pharyngeal skeleton (black hooks) [3285], while other discs adhere to other larval organs (not shown) [527, 834, 4565]. 'Humeral' is synonymous with 'dorsal prothoracic' disc. Bristles are omitted for clarity, and flank sclerites are simplified.

An adult fruit fly is ~3 mm long. Full-grown larvae are roughly twice that length [3421]. About half the larval midsection is omitted here. Adapted from Hartenstein, V., 1993 and [4565].

Discs look more alike than the structures they produce. The same is true at the cell level, where discs are nearly indistinguishable by ordinary histology [3165, 4424]. Even at the molecular level, different discs make virtually identical suites of proteins [1459, 1611, 3625, 3756, 3865], though amounts vary. The reason for these common features -- as later chapters will show -- is that all discs use the same basic 'toolkit' of molecules for intercellular signaling [662], though the circuitry (i.e., how those molecules interact) is tailored to the disc-specific patterns [662].