Caerostris (Araneidae: Araneae) cryptic diversity highlights the need for taxonomic expertise in the genomic era

Bark spiders (Caerostris Thorell, 1868, Araneidae Clerck, 1757) are large spiders distributed across the Old-World tropics. This genus was understudied until recently, but received more attention following the discovery of unique web evolution and biomaterial properties of its species. Caerostris are characterized by web gigantism, reaching extremes in Darwin’s Bark spider (C. darwini Kuntner & Agnarsson, 2010), which employs the toughest known silk. Due to the exceptional silk of C. darwini, recent research provided whole-genome sequencing and silk-gene mapping for C. darwini and a sympatric congener misassigned, due to cryptic diversity, to C. extrusa Butler, 1882. We describe a case of convergent evolution in somatic morphology that further hinders species identification in the field. Conducting a morphological and molecular investigation of exemplars that share the “C. extrusa morphotype”, we conclude they belong to four distinct species. We redescribe the two valid ones, C. extrusa and C. hirsuta (Simon, 1895), elevate the previously synonymized C. bankana Strand, 1915 to species level, and newly describe C. kuntneri sp. nov., the species used in the assembly of its genome. We argue this to be an example of how the global taxonomic impediment can lead to errors in rapidly advancing fields such as genomics.