The question of whether Arachnida (Chelicerata) conquered terrestrial habitats only once or several times is controversial. The key group in this respect is the Scorpiones. Several authors claim that they became terrestrial independently of other arachnid lineages. This argumentation uses two lines of evidence. One is that book lungs of scorpions and other arachnids are considered non-homologous because they occur on different segments. The other line is based on fossil evidence which suggests that early scorpions were aquatic, together with a putative sister group relationship between scorpions and the aquatic Eurypterida. To address this problem we undertook a comparative scanning electron microscopical and histological study of the book lungs of scorpions, amblypygids, uropygids, and mesothelid spiders. In addition, we included the book gills of a xiphosuran. We found several detailed similarities in the book lungs shared by all arachnid taxa studied. Based on these findings we conclude that arachnid book lungs are homologous. Furthermore, we suggest that the apomorphic book lungs of arachnids indicate a single terrestrialisation event in the stem lineage leading to Arachnida.