Tenebrionid beetles inhabiting the dunes of the Namib Desert spend most of their lives buried below the sand surface. When they surface, which is not necessarily daily, it is often for only a few minutes or hours. Is the decision to bury or emerge determined by thermal benefits or optimization of water vapour pressure relationships or other factors? We evaluated this question by measuring temperature and relative humidity above and below the sand in dunes near Gobabeb continuously over 24 h, and calculating the vapour pressure difference between a hypothetical beetle and its surroundings. When beetles emerged, temperatures were available such that buried beetles could have attained their preferred body temperature, of 36 to 40°C, and avoided solar radiation. Desiccation stress on the beetles was also greater above surface during the day when they emerged, and greater below surface at night, when they were buried. We conclude that it is neither the need to attain preferred body temperature nor the need to prevent desiccation that induces the beetles to emerge or bury. Keywords: Chthonic beetles, Namib Desert, Tenebrionid beetles.