what are book lungs and book gills?
A book lung is a type of respiration organ used for atmospheric gas exchange and is found in arachnids, such as scorpions and spiders. Each of these organs is found inside a ventral abdominal cavity and connects with the surroundings through a small opening. Book lungs are not related to the lungs of modern land-dwelling vertebrates. Their name describes their structure. Stacks of alternating air pockets and hemolymph-filled tissue give them an appearance similar to a "folded" book. Their number varies from just one pair in most spiders to four pairs in scorpions. Sometimes the book lungs can be absent and the gas exchange is performed by the thin walls inside the cavity instead, with its surface area increased by branching into the body as thin tubes called tracheae. It is possible that the tracheae have evolved directly from the book lungs, because in some spiders the tracheae have a small number of greatly elongated chambers. Many arachnids, like mites and harvestmen (Opiliones), have no traces of book lungs and breathe through tracheae or through their body surface only.
Book lungs evolved from book gills. Although they have a similar book-like structure, they are found in different locations. Book gills are found externally while book lungs are found internally. Book gills are still found in horseshoe crabs which have five pairs of them, the flap in front of them being the genital operculum which lacks gills. Book gills are flap-like appendages that effect gas exchange within water and seem to have their origin as modified legs. On the inside of each appendage there are attached over 100 thin leaf-like membranes called lamellae which appear as pages in a book, and are the areas of the gill where gas exchange takes place. These appendages move with rhythmic movements to drive blood in and out of the lamellae and to circulate water over them. Respiration being their main purpose, they can also be used for swimming in young individuals. If they are kept moist, the horseshoe crab can live on land for many hours.
BOOK GILLS: Among the chelicerate (possessing fanglike front appendages) arthropods (for example, scorpions, spiders, ticks, and mites), the horseshoe crab, Limulus, has a series of book gills (gills arranged in membranous folds) on either side of the body into which blood from the ventral sinus passes for oxygenation prior to return to the heart. The largely terrestrial arachnids may have book lungs that occupy a similar position in the circulatory pathway, a tracheal system comparable to that of insects, or, in the case of smaller species, reduced tracheal and vascular systems in which contractions of the body muscles cause blood circulation through the sinus network
In book gills we have arthropods like limulus( also known as living fossil) and in book lungs we have organisms like scorpions and spiders.. ( btw scorpions and spiders ain't insects)
Hope it helps