NatureSmart Column

Spiders and their Webs

by Stan Tekiela
© 2003 NatureSmart
May 30, 2003

Thanks to some well publicized Hollywood movies, just about everyone knows that spiders are not insects. They are (now say it with me) an arachnid. This may seem like a technicality to some but upon a closer look there a many difference between spiders and insects.

Insects have three body parts; a head, thorax and the abdomen. Spiders have only two body parts; a cephalothorax (head and thorax combined into one part) and the abdomen. Insects have three pairs of legs and two pairs of wings compared to spiders which don't have any wings and four pairs of legs. Insects have antennae while spiders do not. Insects have large compound eyes (many small lens combined to make one large eye) while spiders have eight small simple eyes.

Even their mouth parts are different. Insects have mouths that bit or pierce, but they don't have fangs like spiders. Nor do insects have poison glands like spiders that they inject into their prey.

The rear end of insects is also very different from spiders. Insects tend to have stingers and or ovipositors, a long hollow tube-like structure, for laying eggs and lack the spinnerets and silk glands that most spiders have. And it's the spinnerets and the spider's ability to spin a web that really sets them apart from the insect.

In fact the name spider came from the word "spinder" and it may be the single feature that spiders are known for. Nearly all spiders have six spinnerets arranged in pairs at the end of its abdomen. Usually they are not visible. Each spinneret is connected to an internal silk gland. Inside the gland the silk is liquid, but upon being forced out and exposed to the air, it quickly solidifies into silk.

A spider can produce seven kinds of silk, each used for different jobs; web creation, wrapping up prey, egg sac formation, sperm web construction, pulling leaves together to construct a shelter, dragline formation and traveling on the wind.

Spider silk is a remarkable substance that is unmatched in human-made chemicals. A spiders silk has a tensile strength greater than bone per weight. Yet it is so elastic that wet threads can stretch more then 300 percent of their length before breaking.

Nearly every species of spider uses silk in one way or another. The stereotypical spider web is known as the orb web (see image). Although everyone knows how these webs look they are not the most common web that spiders can make. In general there are four types of webs—cobwebs, sheet webs, funnel webs and orb webs. In the north country only three families of spiders use the orb web type. The rest use the other variety of webs.

Construction of the orb web is fascinating stuff. First of all, the spiders need to take down and remake their webs each night, or at least every other day. All the proteins used in making the silk are recovered by eating the old web. Radioactive tagging has shown that 80-90 percent of the initial web material shows up in the new web, even though it may be only a half hour between eating the old web and spinning the new one.

It takes the average orb web weaver about one hour to eat the old web and spin the new web. Most are constructed a couple feet off the ground and are designed for one reason—to capture insects (of which they are not) for food. All orb webs are constructed basically the same way. The main threads are called spokes and anchor the web in place. The spokes of the web are not sticky. That is how the resident spider travels around the web without getting stuck themselves. The web that connects the spokes and spirals around the web are sticky and hold the insects in place until the spider can get there.

Once constructed the spider will either position himself in the center of the web or off to the side. It uses is legs to feel any vibration. Even though a spider may have eight eyes, their eyesight is poor. Once they feel a struggling insect caught in its web it rushes out and delivers a single poisonous bite to immobilize the prey before either devouring or wraping it up in silk to feed upon later.

Some spiders produce a zigzag formation in the center of their web called a stabilimentum. This is most common in the genus Argiope. It's not well known what this structure is used for, but some believe that these special silk threads stand out in ultraviolet light (insects see in ultraviolet light) and may attract insects like a lure. It may also help to conceal the spider who waits in the middle of the stabilimentum.

Until next time...