Expression of th^dsRNA.hs using heat shock in the mid-third larval instar results in some larvae stopping moving as early as 2 hours after heat treatment, with necrotic patches often appearing in their larval tissues. By 6 hours after treatment, 76% of the larvae show visible signs of cell death, from relatively restricted death of larval tissues to widespread necrosis. The remaining animals form morphologically normal prepupae, but they all undergo massive cell death before head eversion and leave only an empty pupal case. Animals in which th^dsRNA.hs has been expressed using heat shock at about 18 hours before puparium formation have larval midguts and salivary glands which resemble those undergoing cell death when analysed 6 hours after heat treatment. The larval midgut in these animals is contracted, with a significantly reduced proventriculus and gastric caeca and an overall degraded morphology (similar to that normally seen 2 hours after puparium formation), in contrast to the healthy morphology seen in the larval midguts of heat treated controls. Similarly, the larval salivary glands of the mutant animals are severely degraded and resemble those seen in wild-type 14.5 hour pupae.