Spiders: Our Eight Legged Friends

Author: Beverly Eschberger
Published on: Jul 23, 1999

I have been recovering from a nasty spider bite that caused my left hand to swell up to about one and a half times its normal size. Unlike my husband, who is quite arachnophobic (afraid of spiders), I actually like these creepy, crawly beasties. So, I decided that I was destined to write this week's article about our eight-legged friends.

Spiders belong to the Family of Arthropods, a group of invertebrate animals with jointed legs and an external skeleton made of chitin (pronounced "KEYE-tin"). This is different from the vertebrate animals, which have internal skeletons usually made of cartilage and bone.

The earliest arthropods arose in the oceans and were already highly developed in the early Cambrian Period (570 million years ago) with the marine trilobites. Modern aquatic arthropods include lobsters, shrimp (also known as prawns), and crayfish.

The arthropods left the oceans by the Carboniferous Period (345 million years ago) and quickly began to colonize the land. Land-dwelling arthropods include millipedes, insects, spiders, scorpions, and mites. As the only predators on land at that time were some early land-dwelling vertebrates, the millipedes, insects, and scorpions were all able to develop some giant species.

Spiders, scorpions, and mites all belong to the Arachnidae. The arachnids get their name from a Greek myth in which a weaver named Arachne was so proud of her weaving ability that she challenged the goddess Athena to a contest. Athena destroyed Arachne's weaving during the contest, and Arachne hanged herself. Athena then changed Arachne into a spider and gave her name to the Arachnida.

The first true spiders appeared in the Carboniferous Period and are grouped into the Ricinulei, a group of plump and primitive spiders. A few species of the Ricinulei survive today as "living fossils" in tropical forests.

Spiders are often mistakenly grouped with the insects. Insects, however, all have three pairs of legs (six total), and their bodies are divided into three main parts: head, thorax, and abdomen.

In contrast, spiders have bodies divided into two main parts, a cephalothorax (a combined "cephalic" (head) and thorax region) and an abdomen, and have five pairs of legs (ten total).

"Wait! Wait!" I hear you say, "I thought that spiders had EIGHT legs?" Well, they do. They have four pairs (eight) of walking legs, and they also have a pair of graspers or chelicerae (pronounced "kil-lis-er-ree") on their cephalothorax. Spiders use these chelicerae to maneuver prey to their mouths.

Since the arthropods are invertebrates, their bodies do not fossilize well, and most of the insect fossils found are only isolated wings. However, if the conditions were right, some extraordinary fossils were produced. Florissant Fossil Beds National Monument, a U. S. National Park, is an excellent place to see invertebrate fossils, including spiders. Florissant is best known for its petrified trees, which conservators are trying to save from further deterioration. Arachnids and other arthropods can also sometimes be found preserved in amber.

Remember, when you visit Florissant or any other National Park, it is illegal to take home any fossils that you might find. It is always a good idea to get permission from the owner before you go looking for fossils on private property, too. Fossils are a non-renewable resource -- they are something that we cannot make more of -- and if we remove them from the ground, paleontologists cannot use them to learn more about the many animals that once roamed the earth.

Visit my web site for links to U.S. National Parks and Monuments that feature paleontology.

So, the next time you see a spider, don't squash her. Instead, take some time to study her. Note her two body parts, her eight walking legs, and try to see her chelicerae. Then, carefully use a jar or piece of paper to put her outside where she can provide you with some free, all-natural, organic pest control.