Fig. 1

Research organisms and their embryos. A Chelicerate phylogeny. ‘True spiders’ (Haplogynae and Entelegynae—separated by the morphology of their female mating apparatus, represented by Pholcus and Parasteatoda, respectively). Mygalomorpha is represented by Acanthoscurria expanding the study towards spiders sensu lato. True spiders possess a pair of book lungs on the second opisthosomal segment (O2), a pair of tracheal tubes on O3, and spinnerets on O4 and O5. In tarantulas, book lungs develop on both, O2 and O3, and the spinnerets on O4 are rudimentary. A whole genome duplication (WGD) in the lineage leading to Arachnopulmonata is indicated. The harvestman Phalangium represents a chelicerate outside Arachnopulmonata, and thus a species that has not undergone a WGD. In comparison to spiders, harvestmen only have one pair of tracheal tubules on their opisthosoma (O2), and do not possess book lungs and spinnerets. B Adult female of the common house spider Parasteatoda tepidariorum. C Adult female of the cellar spider Pholcus phalangioides holding a cocoon with her chelicerae. D Adult female of the tarantula Acanthoscurria geniculata. E Adult female of the harvestman Phalangium opilio on a house wall. E Size comparison of the embryos of the investigated chelicerate species. The embryos are stained with the nuclear dye SYBR-green