Calponia harrisi is a fascinating relict spider species belonging to the family Caponiidae, endemic to parts of California. While most modern araneomorph spiders have evolved to possess a single pair of book lungs alongside a network of tubular tracheae, Calponia harrisi stands out as a striking evolutionary anomaly. It retains two pairs of functional book lungs—a respiratory arrangement typical of primitive mesothele and mygalomorph spiders, such as tarantulas, but largely lost in derived araneomorph lineages.
Evolutionary biologists consider C. harrisi to be among the most basal living araneomorphs. Its primitive respiratory morphology has been preserved due to specialized ecological niches in microclimates characterized by stable, high-humidity leaf litter environments where metabolic demands and respiratory water loss were kept low.
Retaining four book lungs provided sufficient gas exchange without forcing the respiratory vascular system to undergo the evolutionary transition toward tracheal consolidation seen in cursorial or desert-dwelling spiders. Studying Calponia harrisi provides crucial morphological evidence for understanding the transition from primitive four-lunged arachnid ancestors to advanced two-lunged modern spiders.