Spiders: Web-Builders and Wanderers – Background

Hanging under eaves, tucked in the cracks on tree bark, hidden in tall grass, spiders and webs can be found nearly everywhere you look in late summer. Not all spiders spin webs, however. Some actively hunt for prey, scurrying over dirt in the garden or ambushing pollinators visiting flowers. Whether wanderers or web weavers, spiders abound in nearly every habitat on Earth, with estimates of one million individuals living in each acre of grassy field. There are about 2,500 spider species in North America, all different and each well adapted to its role as a small but effective predator.

Spiders are arthropods, which means that they have jointed legs and hard exoskeletons, as insects do. However, spiders are not insects. Spiders are arachnids, with eight legs, and so are more closely related to ticks or mites than to dragonflies or bees. Where insects have six legs and three body parts, spiders have just two body parts: a cephalothorax and an abdomen. Their eyes, jaws, specialized appendages called pedipalps, and four pairs of legs are all located on the cephalothorax. The abdomen, generally much larger than the cephalothorax, contains the heart, the respiration, digestive and reproductive parts, and the spinnerets.

Look at a spider up close, and you may be surprised to find it looking back at you. Most spiders have eight simple eyes, each with a single lens (not like the multiple lenses in the compound eyes of insects). The spider’s four pairs of eyes are often arranged in two rows and may vary quite a bit in size. For active hunters, like the wolf spiders and jumping spiders, good eyesight is key, and these groups generally have large forward-facing eyes. Web-weaving spiders have poor eyesight. They sit and wait for prey to be ensnared and sense trapped prey through the vibration of their web caused by the creature’s struggles.

Located below the eyes, a spider’s fang-tipped jaws (chelicerae) are used to grasp and pierce prey. As small predators, spiders face many challenges, not the least of which is having a very small mouth. When an insect is captured, a spider grabs it and injects paralyzing and fatal venom from its fangs. Some kinds of spider begin dinner by regurgitating digestive juices on the prey and sucking up the resulting soup. Other spiders pierce and inject those enzymes into their prey and then slurp out the remains, leaving behind an empty insect exoskeleton.

Unlike insects, spiders do not have antennae. Instead, they have a pair of pedipalps just in front of their first legs. Pedipalps help the spider feel and sense objects it encounters. They also aid in prey capture, web weaving, and food manipulation. In males, the palps are modified, with small bulbous tips that are used for mating.

Most people readily recognize spiders by their eight legs, which are often much longer than the creature’s body length. These appendages vary quite a bit from species to species. Jumping spiders tend to have thick, short legs, while cellar spiders’ legs are long and thin, stretching many times their body length. Feeling, digging, jumping, spinning, grabbing, courting, spiders’ legs have many functions beyond just helping the spider get from here to there.

A spider’s abdomen, with its different colors and patterns, is useful in identifying the species. Some are brightly colored, some are spotted or striped, some are well camouflaged, and some, like the crab spiders, can even change color to match the flower they are sitting on. The abdomen of many spiders is covered with hairs, and these have a variety of functions. In fishing spiders, dense water-repelling hairs help it float on the water surface. In other spiders, the hairs play a role in web weaving, locomotion, prey detection, parenting, and protection. From the hair pegs upon which wolf spiderlings cling onto their mother’s back to the sharp spines on some jumping spiders that prevent injury during a skirmish with prey, spiders’ hairy body and legs help in survival.

Underneath the tip of the abdomen are the spinnerets. All spiders make silk, even those that don’t spin webs. Most spiders have six separate spinnerets, basically spigots connected to silk-producing glands that can produce different types of silk. Inside the spider, silk is a liquid protein. As the liquid silk passes through ducts and is pulled out into the air, it forms a strong, elastic thread.

Spiders use their silk for a variety of purposes. They may wrap their prey in silk to subdue it. Some sew leaves together to form a shelter or weave a silken retreat. Others trap air in a silk bubble to use as an underwater diving bell. All leave a silk trail, a dragline, behind them when they travel about that they can quickly release to drop down or to climb back up if they are disturbed. A female spider wraps her eggs in silk. She may attach this egg sac to the web, hide it under bark or in a folded leaf, or she may carry it with her to guard and protect. Very young spiders, known as spiderlings, let out lengths of silken threads that are caught by the wind, dispersing the young to new habitats in a process known as ballooning. These thin threads are called gossamer. And, of course, as evident on a dewy summer morning, many spiders weave webs of silk.

Webs are snares spiders use for capturing a meal, quite an assist for a small, flightless predator hoping to trap an insect on the wing. Once an insect is trapped, the spider, often hiding close by, feels the vibrations and gets to work, quickly pouncing on the struggling insect and subduing it. If the spider has recently eaten, it may wrap silk around the insect and save it for a later meal. Web-weaving spiders eat their old webs, routinely recycling the material into new webs.

Webs are commonly sorted into four different categories: cobwebs, sheet webs, funnel webs, and orb webs. Each species of web-weaving spider makes a characteristic web.

We often notice cobwebs, those irregular messy constructions, hanging in a hard-to-reach corner or dark basement, though they certainly can be found outside as well. Cellar spiders and common house spiders are cobweb weavers.

Sheet webs may be flat platforms or more complicated three-dimensional structures, and the spider is often perched inverted beneath the sheet. Bowl-and-doily and filmy dome spiders are common sheet web weavers.

Funnel webs are frequently found tucked into shrubs, woven into tall grasses in a field, or hidden in the corner of a window. These webs have a sheet-like platform with a funnel-like tunnel in one corner in which the spider lurks.

Best known are the orb webs with their beautiful circular snares strung vertically between weed stalks, stretched across window frames, glistening in the forest. The beautiful black and yellow garden spider is a familiar example of an orb web weaver, as is the very common barn spider, the heroine of Charlotte’s Web.

Some spiders, like barn spiders, live just one year. Others can live more than one season, even  upwards of fifteen years, and many of the tarantulas can live for thirty years. Spiders begin life as eggs. They emerge from the egg sac as tiny spiderlings and begin to grow. Because spiders have hard exoskeletons, they must molt, shedding their old skin and expanding to fill the new one. Spiders may molt anywhere from four to twelve times before reaching adulthood. Each molt provides an opportunity to regenerate a lost appendage. Once they’re mature, however, true spiders will not shed their skin again.

Many spiders will eat their own kind, which can make courtship and mating rather risky, especially for the generally much smaller males. As a result, the males of many species stage a variety of courtship displays to alert the female to his intentions; he may pluck her web, wave his arms, drum for attention, or perform other courting rituals. During mating, the male transfers sperm with his bulbous pedipalps to a slit on the underside of the female’s abdomen, and a new generation of spiders begins.

Though many people fear or simply dislike spiders, the vast majority of spiders are harmless to humans. In fact, as a major predator of insects, spiders are extremely beneficial, keeping populations of many pests under control. Take a moment to appreciate the great diversity in color and shape of spiders in your yard, the artistic skill of the web weavers, or the perseverance with which the wanderers search for food and shelter. In both form and function, spiders are worthy subjects for study.

Suggested Reading

Discovery Books.  Insects and Spiders: An Explore Your World ™ Handbook. New York: Discovery Communications, 2000.

Herbert W. Levi, Herbert Spencer Zim, Lorna Rose Levi. Spiders and Their Kin: A Golden Guide. St. Martin’s Press, 2001.

Weber, Larry. Spiders of the North Woods. Duluth, MN:  Kollath-Stensaas, 2003.

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