Fig. 4. Effect of RNAi gene knock-down on first instar larval cuticles. All larvae are shown anterior towards the left. (A) Wild-type larva with three leg-bearing thoracic segments (T1-T3) and eight abdominal segments. Two additional abdominal segments (A9 and A10) are fused to the telson and bear the urogomphi (u) and pygopods (p). In this lateral view, the mandibles (md) and the labium (lb) cannot be seen because they are covered by the maxilla (mx). (B-D) In all RNAi embryos, maxilla and labium are transformed to T1 and T2, respectively. Intriguingly, the thorax is shifted coordinately such that the mandibula is followed by segments displaying T1, T2 and T3 identity. The following two thoracic segments also have T3 identity. (B) In weak phenocopies, the transformation is not accompanied by segmentation defects. (C) Most giant phenocopies also display segmentation defects. In this specimen, five thoracic and four residual abdominal segments are formed, and the urogomphy are missing. Together, five segments are deleted. Because abdominal segments have identical cuticle pattern, it is not possible to determine which segments are missing. Often, the penultimate pair of legs is less well patterned or homeotically specified than the most posterior one (white arrowhead). (D) In this strong phenocopy, nine segments are deleted. Three thoracic segments are left and the presence of a pair of stomata (white arrowhead) indicates the presence of one abdominal segment. Even in such severely disturbed larvae, the terminal pygopods are usually present. (E,F) The gnathal transformation in a ventral view: antenna (at), labrum (lr) and mandibles (md) are not affected, but maxilla (mx) and labium (lb) are completely transformed to thorax. (F) Schematic representation of E; transformed maxillary appendages highlighted in grey. (G,H) In a few larvae, the transformation of the maxillary segment was not complete. Here, the lower appendage is transformed to leg, while the other appendage adopts an intermediate identity. Note that partial transformations are rarely observed in the maxillary but never in the labial segment. In no case were thoracic identities shifted only one segment towards anterior.