Aposematism is the combination of a conspicuous primary signal with a secondary defence. In order for aposematism to be successful, predators must learn to associate the aposematic signal with unpalatability or toxicity. However, the level of defence may not be uniform across all members of a species. Automimicry occurs when some individuals possess lower levels of defence, but retain the same aposematic signal. This is especially likely to develop if the defence if costly. The levels of automimicry in a population may dramatically affect predator learning and avoidance. It is thought that without some mechanism selection against automimics, keeping signals ”honest”, cheaters may invade the population and break down the relationship between signal and defence. Here we investigate variation in chemical defence in an aposematic insect, the wood tiger moth. This species represents an interesting case study as it displays both within and between-sex variation in colour. Female colouration varies continuously from yellow to red, with red providing the stronger anti-predator signal. We show that the level of chemical defence in this species varies considerably between individuals, despite not being sequestered from diet, and is highly heritable. Additionally, our data suggest that honest signalling is operating within male moths, and potentially also across the sexes, as females appear to have both the strongest primary signal and chemical defence. Despite this, variation in female colour was not correlated with defence level.  This has important implications for the maintenance of aposematic signalling, and the occurrence of signal polymorphism, in aposematic species.