There first stage at which significant lethality of rig^k07839/Df(2R)exu1 or rig^k07917/rig^k07839 animals occurs is the 2nd instar. Of those mutants that die as second instar larvae approximately half survive two or more days past the time when they should have molted to the third instar and obtain the size of a late third instar, but retain the anterior spiracles of a second instar larva. These larvae usually become stationary and die around the same time as their third instar siblings. Some mutants enter the 2nd to 3rd instar molt, but die during this process. Such larvae have two sets of mouth parts and, in some cases, two sets of anterior spiracles and/or two cuticles. Of those mutant animals that do survive the molt, approximately half stop moving during the first day of the third instar, becoming extended and stiff in an apparent attempt to pupariate. They survive in this immobile state for 2-3 days, after which patches of necrotic cells form throughout the animal. Most of the remaining mutant third instar larvae become stationary over the next few days, while some stop eating and shrink in size until they finally die. Some of the mutants pupariate normally, although 1-5 days later than control animals, and then die as prepupae. A few rig^k07839/Df(2R)exu1 animals (but no rig^k07917/rig^k07839 animals) make it to the pupal stage before dying. rig^k07839/Df(2R)exu1 animals that die as prolonged second instar larvae have normal second instar mouth hooks, but those that become third instars have severely malformed mouth hooks. The lethality of rig^k07839/Df(2R)exu1 is not rescued by feeding with 0.5 mg/ml 20-hydroxyecdysone. In fact, as in controls, lethality 2nd-3rd instar larval moult is increased.