"Book lung"

I was recently reading about mantidfly larvae that hitch rides on spiders, often inside the spider's egg sac.  This statement startled me:

If they end up on a juvenile, they can crawl inside the host’s lungs and wait there until the spider moults into an adult. If they find a host at the wrong time of the year, they can spend the entire winter on board, delaying their own development until the next year.

I have seen scanning EMs of mites inside I think primate lungs, but "spider lungs" had me baffled, because I didn't view insects and arachnids as having lungs.  Turns out, spiders do have lungs - in the form of "books":

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.

The unfolded "pages" (plates) of the book lung are filled with hemolymph (the arthropod blood). The folds maximize the surface exposed to air, and thereby maximize the amount of gas exchanged with the environment. In most species, no motion of the plates is required to facilitate this kind of respiration.

Scanning EM above from PhysOrg.  Other relevant diagrams here and here.  You learn something every day.