Differences in head pattern of (A) Natrix natrix (photo: Henrik Bringsøe) and (B) N. helvetica (photo: Wolfgang Wüster). (a) Anterior dark occipital spot, (c) light occipital crescent/collar, (p) posterior dark occipital spot. Note the presence of (a) in N. natrix and its absence in N. helvetica and that (p) is narrow in N. natrix and wide in N. helvetica. This figure was inspired by a similar one in Meyer (2020).

 
 
  Part of: Fritz U, Grismer LL, Asztalos M (2023) Hybrid zones of Natrix helvetica and N. natrix: Phenotype data from iNaturalist and genetics reveal concordant clines and the value of species-diagnostic morphological traits. Vertebrate Zoology 73: 383-395. https://doi.org/10.3897/vz.73.e103319