• The paper by Mile Gu and colleagues on emergent properties of Ising lattices is at www.arxiv.org/abs/0809.0151 (4 October, p 12).
• We said ambiguously that “a molecule released by bacteria called lipopolysaccharide seems to initiate tolerance” to bacteria (27 September, p 16). To clarify: lipopolysaccharides are a class of molecules.
• We seemed to say both that pterosaurs weighed up to a quarter of a tonne and that they were four times heavier than a 22-kilogram albatross (4 October, p 10). We meant to say that members of the extinct teratorn family of birds weighed up to 88 kilograms – which is more than the proposed 40 kilogram limit for flight.
