Figure 7.8
'Fickle Bristle Mystery' of the fly's eye.
The entire left eye of a wild-type female specimen is schematized (cf. key). Bristles (black symbols) occupy vertices in the lattice of facets (hexagons), but their pattern is less perfect. Directions (compass) are A (anterior), P (posterior), D (dorsal), V (ventral). The general rule is: 'Bristles reside at the A end of each horizontal interface between ommatidia.' Bristles that obey this rule are indicated by solid black circles. Wherever this rule is obeyed, eye bristles occupy alternating vertices around each facet, with tertiary pigment cells at the remaining vertices (cf. Fig. 7.2). The rule is violated at the A and P margins, where bristles are missing (unfilled circles). It is also broken by bristles that lie at P ends (triangles). These sporadic 'errors' occur most frequently near the A margin. They exhibit their own sort of rule (lower right): 'Wherever a bristle arises at the P end of an interface, none forms at the A end.' This correlation implies that (1) one bristle SOP arises per interface, and (2) this SOP is free to move to one end or the other [1804, 4715]. In other words, the errors are actually displacements (cf. Fickle Sensilla Mystery; Fig. 5.12g). Redrawn from [3539].

N.B.: The fly eye is nearly circular: there are 32-34 columns in a typical wild-type female, with ~32 ommatidia in the tallest column [2571, 3539, 4715]. The absence of bristles from the front of the eye is probably due to the remnant of wg-ON tissue that is left after the D and V margins of wg-ON cells have shrunk as the MF encroaches anteriorly [2554]. Wg's role as a diffusible 'bristle suppressor' is shown by (1) wg^GOF clones which scar the eye and -- in so doing -- delete bristles from a swath of facets on either side of the scar [4390], (2) sevE-wg eyes which lack all bristles due to expression of wg in every facet under the control of the sev enhancer [518, 597, 3659], and (3) naked cuticle^LOF eyes whose bristleless margin widens due to an increased cellular responsiveness to Wg [3659]. We do not know why bristles are normally absent from the rear of the eye. In other insects the inter-ommatidial bristles obey similar rules but vary in density [1853]. The number of bristles per vertex increases in E(Elp)24D^LOF [4034].