Question

How do spiders breathe?

Answer 1

The process of moving air inside the lungs and expelling it out from the body after the gases are exchanged with the internal environment is called breathing. Animals take in oxygen and remove waste products like carbon dioxide through the process of breathing.

Complete answer:
Araneae or spiders are a diverse group of organisms and form the largest order of arachnids. They are air-breathing arthropods and are carnivorous in nature. They belong to the subphylum- Chelicerata and thus have chelicerae with fangs that have the ability to inject venom. Chelicerae are a pair of appendages present in front of the mouth. Their body is divided into two segments- Cephalothorax and the abdomen and they have 8 jointed legs. They lack wings or antennae and have specialized appendages called chelicerae and pedipalps. Their body is made up of an exoskeleton that is shed periodically. They have simple eyes and spines threads of silks to form a web-like structure, which is used for capturing prey. They are cosmopolitan and have a worldwide distribution.
The breathing in spiders is done by two specialized organs- book lungs and tracheae.

Book Lungs- They are the main respiratory organ of the spiders and are present in the abdominal portion. One pair of book lungs are present in spiders. They are present within an air-filled cavity and are connected with the surrounding for an atmospheric gas exchange through small openings. Their structure includes hemolymph tissues alternating with a stack or pile of air cavities to form a folded book structure. This structure increases the surface area for the exchange of gases. The air enters these book lungs by the slits present on the belly through the thin membranes. The air is pushed in and out by the movement of spiders through the process of diffusion.

Tracheae- It is a complex network of narrow tubules that run throughout the body of spiders. They are connected to the atmosphere by external openings or pores called spiracles. They open through the stigma and the stigmata. Through the openings or spiracles, the air enters and carbon dioxide moves out of the small tubules through the process of diffusion. These gases are then passed to the internal organs. They provide flexibility to the spiders by providing an extra method to perform respiration.
The spiders have the potential to simultaneously use- book lungs and the tracheae- for breathing. It benefits the spiders to have a good gaseous exchange in their body.

Note: Book lungs are assumed to have evolved from book gills. The book gills are present in scorpions and are an external structure that performs the function of gaseous exchange.