Feared around the world for their venomous bites, spiders represent a fascinatingly diverse family and most are harmless. Members of the class Arachnida, spiders breathe through book lungs or trachea, which are extremely narrow tubes running through their bodies. Spiders are similar to insects but have eight legs and no antennae. Their closest relatives include scorpions, ticks and mites. About 38,000 spider species are known, but there are probably many more waiting to be discovered.
Some spider species breathe using one or two pairs of "book lungs." Named for their resemblance to the pages of a book, book lungs contain layers of thin, soft, hollow plates open to the air through slits on the spider's abdomen. Hemolymph, which is the spider equivalent to blood, passes across the inner surface of the plates and exchanges oxygen and carbon dioxide with the atmosphere. Book lungs provide a large surface area for gas exchange. In large tarantulas the surface area is up to 70 cm (27.6 inches) square. The slit openings of book lungs can expand and contract but never fully close. During periods of intense activity spiders open their book lung slits wide.
Two of a Kind
Tarantulas breathe using two pairs of book lungs, but daddy longlegs and other spiders use only one pair. Members of the spider groups Mesothelae and Mygalomorphae, which includes tarantulas, have two pairs of book lungs, and this is considered a feature of primitive spiders. More recent species, such as daddy longlegs, orbweavers and wolf spiders possess only a single pair of book lungs. Orbweavers and wolf spiders also breathe through "trachea" that branch out from their book lungs throughout their bodies. Scientists agree that trachea are a later development in the evolutionary history of spiders.
Trachea are breathing structures that spiders and insects have in common. A network of narrow tubes lined with a hard substance called "chitin," trachea extend the passage of air from book lungs in some spiders, and open directly to the surface through tiny holes called "spiracles" in others. Spiders that don't have book lungs and breathe through trachea include members of Caponiidae and Symphytognathidae. Most spiders that breathe using only trachea have a single spiracle on the underside of their abdomen. Scientists have identified a specialized form of trachea in spiders called "sieve trachea," which are numerous fine trachea extending from larger main trunks.
Spiders transport oxygen around their bodies in "hemolymph," a blue, blood-like substance. Oxygen diffuses across thin membranes in book lungs and trachea into hemolymph, which is blue because it contains a copper-based substance called "hemocyanin." Hemocyanin works in a similar way to red blood cells, binding to oxygen and releasing it in areas with low oxygen concentrations, and transporting waste carbon dioxide to areas where it can diffuse out into the atmosphere. Spiders have a single-chambered, tubular heart, arteries and veins but no capillaries. When spiders are excessively active, muscle contractions cause hemolymph to move around the body, increasing the transportation of gases.