Like pseudoscorpions and harvestmen, Solifugae lack book lungs, having instead a well-developed tracheal system that inhales and exhales air through three pairs of slits on the animal's underside.
22.
Sometimes, book lungs can be absent, and gas exchange is performed by the thin walls inside the cavity instead, with their surface area increased by branching into the body as thin tubes called sunspiders ).
23.
Trigonotarbids share many superficial characteristics with spiders, including a terrestrial lifestyle, respiration through book lungs, and walked on eight legs, with a pair of leg-like pedipalps near the mouth and mouth parts.
24.
One of the long-running controversies in arachnid evolution is whether the book lung evolved from book gills just once in a common arachnid ancestor, or whether it evolved in multiple groups of arachnids in parallel as they came onto land.
25.
The abdominal and tracheae in more advanced spiders ( most of them still have a pair of book lungs intact as well ), it is possible the trachea of insects was formed in a similar way, modifying gills at the base of their appendages.
