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3614 Administrative Services Building
Ames, Iowa 50011-3614
(515) 294-9915

6/9/03

Contacts:
Betsy Matos, Entomology, (515) 294-1999, bmatos@iastate.edu
Jean McGuire, Continuing Education and Communication Services, (515) 294-7033, jmcguire@iastate.edu

Yard and Garden Column for the Week Beginning June 13, 2003

Invite a Spider Into Your Garden

By Betsy Matos
Entomology Graduate Student
Iowa State University

If you have been working in your garden during the past couple of weeks you might have encountered spiders. A very peculiar chain of events starts when we see a spider. The first reaction is to move as far away as possible from the creature. Then, we move closer to look at the spider and its web, and finally we try to squish it with whatever we have within our reach. But stop! Before you take a swing at a spider, there are some things you should know about this remarkable creature. Contrary to what most movies depict, spiders are very important animals in our lives.

Even though the average spider is about the size of a quarter, their contributions to humans are enormous. There are about 30,000 described species of spiders in the world, with 3,000 species found in North America. Spiders are distant relatives of insects; however, several physical characteristics separate them. Spiders have eight legs, two body parts (cephalothorax and abdomen), no wings, no antennae, and no compound eyes. Insects have six legs, three body parts, most are winged, and they have antennae and compound eyes. However, both spiders and insects have a hard outer skeleton (exoskeleton) and undergo metamorphosis, or change of form, during their life cycle.

All spiders start out as an egg. The female spider, which is usually bigger than the male, lays one or more eggs in a silk case. After a few weeks (some may take up to a year to emerge) a young spider, known as a spiderling, emerges. Spiderlings look like miniature versions of the adults. A spiderling may molt or shed its exoskeleton from four to 12 times before becoming an adult. Most spiders have a short life span, especially the males, which usually die after mating.

Spiders are predators and eat many insects, other spiders and small arthropods. There are even a few spiders that feed on small fish and small birds.

Spiders use different strategies to capture and consume their prey. Most spiders we see in our gardens, such as the garden and cobweb spiders, use a web to capture their prey. The prey is trapped and entangled in the silk strings. Other spiders, such as the wolf spider, actively hunt for their prey, thus do not build a web. Passive hunters, such as the crab spider, do not roam or build a web, but instead sit and wait for the prey to come to them and then catch them.

Once the prey is entangled on the web or is captured, the spider's fangs inject venom to subdue the prey. The fangs pierce the exoskeleton of the prey and enzymes are injected to start digesting tissues. Strong muscles in the mouth help suck the digested juices. When the spider is done feeding, the only part left is the prey's exoskeleton. Some spiders may take minutes, but others may take a few hours, to finish eating. The feeding scene is not very appealing.

The most captivating wonder of spiders is the art of producing silk and spinning a web. The spider's spinning organs are found at the end of the abdomen and they are called spinnerets. The ability to spin webs starts after the first molt or the first stage of the spiderlings. From then on the spider is never found without a silk strand unless the silk cord is broken for some reason. Even the spiders that do not spin webs have the ability to produce silk. Spiders have several uses for silk, including protection of eggs and spiderlings, web structures associated with mating, structures for stopping and entangling prey and bands for binding the prey.

You may ask "How is it that the web can capture insects and other small animals but the spider itself does not get stuck to it?" There are several reasons why the spider does not stick to its web. First, the spider knows the type of silk it is producing, thus it knows which silk lines to sit and walk on. For example, the garden spider is usually sitting in the center of the web. This area only has dry silk threads, therefore, the spider will not get stuck but have the ability to move around.

Second, the body is never in contact with the sticky portion of the web, especially when the spider is moving. The spider has minimal (but vital) body contact with its web via the claws and bristles at the tip of each leg. Because the prey comes in contact with the web without expecting it, the prey's legs and wings become entangled in the sticky silk threads, making escape impossible. When the spider is ready to consume its prey, it just needs to make a quick charge toward the prey and it will not get stuck. Finally, spiders also spend a lot of time grooming their legs, thus any silky material is cleaned off. Some secretions from the mouthparts may help make the leg tips less susceptible to sticking.

Next time you encounter a spider, think of the grasshopper that was trapped on the spider's web before it attacked your sweet corn. Spiders are very important friends, not enemies.

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ml: isugarden

Editors: A color photo, suitable for publication, is available at right. Click on the thumbnail photo to go to the fullsized photo. The fullsize photo is 304K.

Caption: Garden spider


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