What Are Spiders?
Spiders are invertebrate animals. They belong to a large group of invertebrates known as arthropods. All arthropods have jointed legs and a hard, outer covering or exoskeleton.
Spiders are classified in a special group of arthropods known as the arachnids. Spiders, scorpions, harvestmen and ticks are arachnids. They have eight legs. Some of them have book lungs. Spiders are the only arachnids, though, that have spinnerets.
More than 630 kinds of spiders have been found in Illinois so far.
What Do They Look Like?
A spider’s body has two main sections: the prosoma, also known as the cephalothorax, and the abdomen. The prosoma is made of the combined head and thorax and is covered by a carapace. Eyes, legs, pedipalps, mouthparts, brain and stomach are attached to or in the prosoma. The abdomen contains the heart, most of the digestive tract, the reproductive organs, book lungs and the silk glands. Most spiders have eight eyes. The eyes of many spider species only detect movement.
How Big Are They?
Spiders are measured from a stretched-out position. Although there are other ways to measure them, leg span is the one we will use here. The measurement is taken from the tip of a front leg to the tip of a back leg and often occurs diagonally.
The largest native spiders in Illinois are the rabid wolf spider,
Rabidoa rabida, and the dark fishing spider,
Dolomedes tenebrosus. Both species have a leg span of 2 ½ inches to 3 inches.
Many spider species in Illinois, possibly hundreds of them, have a leg span of less than 0.08 inches. They include members of the sheetweb spiders, dwarf spiders, cobweb spiders, goblin spiders and other groups.
Where Do They Live?
Spiders live in nearly all Illinois habitats, even on and sometimes in water.
How Do They Reproduce?
When a male spider finds a female of his species that is ready to mate, he communicates with her through courtship actions so that she knows he is not a potential prey item or predator. He may send vibrations as signals by pulling on web threads or drumming his legs on the substrate. He can also wave his legs and pedipalps. When he receives a proper reply from the female, he transfers sperm to her. The female makes a silken sac into which she deposits hundreds of fertilized eggs. The egg sac may be carried with her, deposited in a safe place, or in some species, guarded. If conditions are favorable, she may make several egg sacs per season.
What Do They Eat?
All spiders are predators. They catch and eat animals for food. Spiders eat insects and other arthropods, including spiders. Some spiders can capture and eat tadpoles, small fishes and other types of prey.
Does Anything Eat Them?
Spiders are prey items for many birds. They are also eaten by frogs, toads, lizards, fishes, parasitic wasps, bats, shrews, other spiders, ticks, insects, centipedes, scorpions and other animals.
Some spiders build webs to catch prey. Spiders use silk that they produce to build webs. Silk is liquid protein that becomes solid when it is drawn out of the openings of the spinnerets on the spider’s body. There are many different types of webs. Orb webs are those most often thought of as typical spider webs. Cob web spiders build irregularly shaped webs. Funnel webs are built as a triangle that ends in a funnel where the spider hides. Sheet webs are a flat sheet with other web threads above and below it. Webs are used for reproductive activities and other purposes, too.
Most spiders do not bite humans. Some spiders are too small to do so. Others could puncture human skin but tend to bite only when picked up or threatened. There are two spiders in Illinois that are considered to have a dangerous bite: the brown recluse (Loxosceles laeta) and the black widow (Latrodectus mactans).
brown recluse Photo © Illinois Department of Natural Resources
The brown recluse may live in houses. Bites generally happen when clothing, bedding or towels containing the spider come into contact with human skin. Some bites show no effects. Others may have a red welt followed by a crust that falls off and may take several months to heal. The brown recluse has a dark-brown marking shaped like a violin on its prosoma.
black widow Photo © Hank Guarisco
The black widow builds a web under objects in sheds, barns, other outbuildings, trash piles and similar locations. Male black widows roam away from the web and do not bite. The female stays at the web and will bite if provoked. Severe abdominal pain, sweating, swollen eyelids, muscle pain and other symptoms may occur. Recovery usually takes a few days.
If bitten by any spider, try to catch the spider and preserve it in rubbing alcohol so that it can be identified. Do not pick up the spider. Put a container over it to entrap it. Once it is settled inside the container, turn the container over and add rubbing alcohol to kill and preserve the spider for later identification. Call your local poison control center or physician should you have any reaction to the bite.What Else Should I Know About Spiders?
Spiders spit digestive enzymes on or in prey to digest tissues. They suck in the resulting liquid and the prey’s body fluids.
When a web is taken down by a spider, the old web material is eaten. Studies have shown that the protein from the old, eaten web can reappear in newly produced silk in about 30 minutes.
Spiders are very beneficial to humans, particularly through insect control. Silk threads of cob web spiders have been used in optical instruments. Spider venom is used in research for drug development. Spiders have also been used to study the effects of weightlessness in space and the hazards of drug use.
Different types of silk are made for different reasons: webs; snare; retreat; egg sac; dragline.
The exoskeleton does not grow as the spider grows and must be shed, a process called molting. A spider sheds eight to 12 times before becoming an adult.
Spiderlings can build webs and hunt as soon as they exit their egg.
Most adult spiders die after reproducing. The natural life span is one to two years.
Learn more about spiders at
Wild About Illinois Spiders!