Figure 5.1
Geometry of leg development.
a. Mature, left 2nd-leg disc (~150 μm across [3140]). Lines are folds. The central circle is the 'end knob'. Connections to larval skin (stalk) and CNS (nerve) have been cut.

b. Longitudinal section (BM and PM = basement and peripodial membranes; CE = columnar epithelium). Transverse lines are cell boundaries. The black cell is one of several larval neurons that innervate Keilin's organ [834, 2287]. These neurons invaginate with the disc (a piggybacking that is ancient in dipterans [2824]). Their dendrites (not shown) stay attached to larval skin through the stalk [4330], while their axons project to CNS (a route later taken by adult axons). Adepithelial cells are omitted.

c. Idealized fate map of left 2nd-leg disc. Segment primordia are the coxa (Co), trochanter (Tro), femur (Fem), tibia (Tib), basitarsus (Ba), and tarsal segments 2-5 (unmarked), with claws mapping centrally. The sternopleura (Stpl) is part of the body wall (cf. Fig. 4.4). AC and PC (shaded) are A and P compartments. Spokes denote bristle rows, whose spacing is estimated: it may not be so uniform (cf. e). As shown below for the tarsus, the annuli telescope out (toward the viewer) as the covering PM (not shown; cf. b) peels away [1311]. In this schematic the tarsus also rolls clockwise ~90°. Compass directions (D, V, A, P) refer to prospective or actual adult axes (cf. d, e).

d. Anterior face of left 2nd leg (~2 mm long). There are 2 macrochaetes (distal tibia) and numerous chemosensory (curved) bristles on the tibia and tarsus. EB (edge bristle) is at the D midline. Tarsal rows 5-8 are marked on compass below.

e. Basitarsus (whole surface) drawn as if slit along D midline and spread flat. Most bristles have a bract (triangle) and align in rows (numbers at top). Five chemosensory bristles lack bracts and reside between rows, as do 3 sensilla (circles at 3.5, 5.5, & 8.5). Between rows 1 and 8 is a lawn of hairs. Row-1 and -8 bristles are peg-shaped and darker; others are tapered and lighter. Aside from bristle thickness and pigmentation, other useful landmarks include bristle lengths and intervals -- both of which increase linearly from V to D (graph at top). Indeed, the symmetry is so precise [1883] that it was surprising when the A/P compartments were found to be offset from this D-V mirror plane by a few cells ('gaps' below) [1800, 2449]. Row-1 bristles can reside in either AC or PC (cf. Fig. 3.9c).

Panels a and b were traced from [377] and [3426] respectively (see [1774, 2144] for nerve-stalk asymmetry, [1516, 2287] for folding details, and [1311] for region-specific cell shapes); c is adapted from [1807, 3807]; and e is based on data in [1801, 1803].

N.B.: For reasons explained in Fig. 4.4h legend, some authors orient the axes (c) with the D pole pointing into the stalk (at 12 o'clock vs. ~1 o'clock here). Claws (c) actually map more dorsally [1812, 3807, 4140]. Numbering of rows (e) follows original nomenclature [1714]. The numbers are backwards in [837, 1039, 2533, 4159]. Also, bristles do not really sprout from discs (e): they start to differentiate ~1 day after pupariation.