Spiders (in the Arachnida class) are feared by humans for their poisonous bites. However, most spiders are quite harmless – denoting their great diverse family of 38,000 known species. 
Arachnids that closely resemble spiders include mites, ticks, and scorpions. We know that spiders don’t “sleep“, have no bones, plus have a long tubular and masculine heart. But, do spiders have lungs?
The short answer is, YES, spiders have either 1 or 2 pairs of book lungs. The arachnids breathe through their book lungs or trachea – these are the very narrow tubes laid all through the spider’s body.
Details: Do Spiders Have Lungs??
Spiders have either 1 or 2 “book lungs” pairs for respiration. These kind of lungs look like book pages as they have hollow, soft, and thin plates that flap to the air via small openings on abdomen of spider.
The spider’s blood (the Hemolymph) pass across the lung plate’s inner surface to allow exchange of oxygen with carbon dioxide between the spider and its atmosphere.
The spider’s book lungs offers the big surface area to allow appropriate gas exchange. Some tarantulas have surface area of the book lungs being about a square 27.6 inches (70 cm).
The opening slips in the book lungs tend to contract and expand but won’t close completely. However, during intense spider activity, the slits on the book lung will open fully.
How Book Lungs Promote Spider’s Respiration
The book lungs are located in the open-ventral abdominal atrium (the air-filled cavity) and is linked to the environment via the small slits on the side of the abdomen.
The spider’s book lung plates – in their unfolded form – contain hemolymph. The folds work to increase the surface area of the lungs and the air it’ll contact – allowing better gas exchanged. But most spider species won’t require movement of plates to allow effective respiration.
However, some spiders may lack the book lungs but still undertake gaseous exchange through the thin walls in the abdominal cavity and the tracheae tubes.
Definitely, the spider’s trachea seems to have developed from its book lungs since it has super elongated chambers. Thus, other arachnids like harvestmen (Opiliones) amd mites will do gassoue exchange via tracheae or even just the body surfaces.
Arachnids without book lungs are called apulmonate arachnids and they include sun spiders, Ricinulei, pseudoscorpions, and microwhip scorpions.
However, those Arachnids with book lungs are called pulmonate arachnids – which include the spiders, Amblypygi, whip scorpions, Tetrapulmonata, and scorpions.
On book lungs’ preservation – the one from the trigonotarbid was the oldest that recovered in Scotland – Rhynie chert. Further, the Devonian lungs look almost like the book lungs of the spiders / arachnids.
Spider’s Book gills and Respiration
For the horseshoe crabs (in males and females), you’ll notice book gills similar to what is in spiders. Researchers opine that book lungs developed from the book gills – but you’ll find these externally.
The genital operculum is the front flap but it has no gills. Notably, the flap-like book gills seem to have developmned from legs and they allow respiration when the animal is under water.
How Do Spiders Breathe Underwater?
Some spiders like Scuba spider use their web like gills to breather when they’re under water. Others like the Argyroneta aquatica will breathe via fine abdomenal hairs to bubbles floating on the water surface.
Some diving-bell spider use their web like a gill for breathing – particularly when they get under water. For example, Stefan Hetz and Roger Seymour conducted a study using 12 spiders to note that the spiders used their web like their gill when they’re under water.
The web is silk like and will enable oxygen to move from the environment near the ware and get to the spider while also allowing carbon dioxide to get back to the environment.
Seymour reported that the diving-bell spiders could survive for over 24 hours under water. Therefore, these arachnids will easily hide from their prey and predators.
How Sea Spiders Breathe Without Gills or Lungs
Many strange creatures exist in the cold Antarctic depths – particularly under the Antarctic ice. Giant sea spiders are the strangest of them all. Definitely, the Antarctic spider species will grow very big while the normal sea spiders are usually small in size.
In addition, these Antarctic spiders have even more strange internal organs. For example, you’ll find a section of their guts and even all their genitals in their legs. Further, they have a very weak heart and thus it can’t pump its blood (hemolymph) in its body.
Therefore, the spiders will only pump the hemolymph via their gut’s movement as opposed to just relying on their heart for pumping the blood. Definitely, there are additional queries on these spiders but on how the undertake gaseous exchange is a critical issue.
While at it, lobsters, crabs and fish use gills for gaseous exchanges. In this, water will flow over the gills to absorb the available oxygen. However, worms on land or at sea will generally absorb the oxygen via their skin.
However, for sea spiders – they lack lungs or gills while their skin is just the exoskeleton – but no internal bones – which is thick to allow oxygen absorption. However, the sea spiders have many pores what they use for respiration.
