Milwaukee Public Museum

by Joan P. Jass

EDITOR'S NOTE: This article is reprinted from LORE magazine, a benefit of museum membership. ©1996 Milwaukee Public Museum, Inc.

If you have learned to fear spiders, or have never been impressed by their interesting behavior or beautiful colors, you might wonder, what good are spiders, anyway? For one thing, they consume a lot of insects, a characteristic which benefits agriculture. In turn, other animals use spiders for food. Some kinds of birds - chickadees, for instance - utilize spiders in their diet. For us, spiders present a fascinating world worth getting to know.

Because most spiders are very small, they are more difficult to get to know than some other animal groups. However, by learning where to look for them, you can become visually attuned to their presence and eventually find yourself wanting to know "What kind of spider is that?" Because of the large number of spider species and because color patterns can vary greatly, you may become frustrated if you are unable to find a picture which matches the spider you would like to identify.

The key to success is to learn to use features other than the spider's color pattern to assist you in identification. Spiders' identities are usually more accurately revealed by aspects of their life style, which are reliable and easy to observe once you have a little practice in knowing what to look for. All spiders belonging to a certain family will behave in a characteristic manner. Recognizing there key family characteristics is the first step in pinning down the identity of the spider you've been observing.

Probably one of the first things that comes to mind when thinking about spiders is their webs. What we usually picture is the dew-covered circular web, with spoke-like lines of silk radiating out from the center. This web represents the life style of some of our most well-known spiders, the web-builders. The family building the orb shaped webs are simply enough called the orb-weavers. These may be large-bodied spiders that sit and wait for a prey item to be caught in the web. They don't need good eyesight to survive; the vibrations of the prey struggling in the web are sufficient for the spider to locate it.

In picturing this sort of classic orb-weaver's web, we can distinguish the three basic zones present in such webs: 1) the center, 2) the catching zone and 3) the frame. The center is composed of the hub, which is the exact center and, in certain species, where the resident spider sits and waits. Also considered part of the center is the free zone, an area immediately adjacent to the hub where there are no spiral threads. From the point where the spiral threads do start up to the perimeter of the circle is the catching zone. Beyond that is the frame. The framing threads are those which attach the web to nearby supports and provide the framework within which the actual orb itself is built and supported.

Not every species that is a member of this family builds the exact same kind of orb web; there are variations. Some are more densely woven, with spaces between the threads being much smaller. An earlier assumption about the more densely woven webs was that they were for catching smaller insects. But recent observations have shown that webs like this actually catch bigger insects than do the more loosely constructed ones. The way the structure of such webs is suited for bigger insects is that the density of the mesh makes the web strong enough to be able to trap and hold the large, more strongly flying type of insects.

A spider's life style affects principally two things: 1) how the present generation accrues the necessities of life and 2) how the next generation is produced. Because of their predatory way of life, spiders generally have to signal to each other that a particular encounter is to be reproductive rather than predatory in nature. Logically, the life style that they have for food gathering will also show up in their courtship behavior. So the male of a web building species usually has a way of vibrating the web to signal the resident female that a courting male is approaching.

If we place the web building spiders into one group, we can then divide the non-web-builders into short-sighted and long-sighted types. First we'll discuss the short-sighted types. Because they are not web builders, they capture prey by hunting it down. But because their eyesight is no good, they rely mainly on their sense of touch to detect a prey item.

An example of this kind of spider is the tarantula. "Tarantula" is a confusing name because there are several kinds of spiders that have been called that. First, there are the ones that are large and hairy. In the United States, they live in the southwestern part of the country, extending northeast as far as Arkansas. Recently it has been something of a fad to keep them as pets.

A second "tarentula" is a European member of the wolf spider family with the scientific name Lycosa tarentula. This is a fairly large spider but not nearly the size of the hairy tarantulas already mentioned. This species is the one that got the reputation of being dangerous to humans and that if you were bitten, dancing the tarantella was the only cure. However, now people think that Lycosa tarentula is not particularly poisonous and the spider responsible for the story was probably a black widow type spider.

While we are on the subject of spiders harmful to humans, in Wisconsin we do have the black widow. The are not really that uncommon but people usually do not see them. There are other spiders that are black with red markings, one of them being a jumping spider that is very often seen in houses. Some people have also heard of the brown recluse spider. This one has not been reported from Wisconsin; we are not within its natural range, which is further south. That does not mean that it could not be brought into the area accidentally but it is quite unlikely that you would run into it.

The thing to do if bitten by a questionable spider is to go to a doctor for treatment and try to save the spider responsible for identification. Many people call the museum to ask whether a particular spider is poisonous. Almost all spiders use venom to catch their prey, but only a few have a venom that could be harmful to humans. A small percentage of people have an allergic reaction, just as some people do to bee stings. The best attitude to take toward spiders, or for that matter any wild creature, is respect - not fear, not carelessness, but respect.

Before we got sidetracked on venom we were discussing tarantulas. There is at least one other creature that has been called that. It is not actually a spider but a close relative. Another name for it is tailless whipscorpion. "Whipscorpion" because the first pair of legs is long, thin and whiplike. And "tailless" because there is another group call whipscorpions whose last abdominal segment is drawn out into a long tail. There is a single family of tailless whipscorpions and their scientific name it Tarantulidae.

The original reasons for bringing up tarantulas, the hairy kind, was that they are a good example of a type of spider that does not build a web to catch its prey but also does not have very food eyesight. Tarantulas hunt at night, detecting their prey by touch. And if we look at the courtship behavior of short-sighted hunters, we can see their reliance on non-visual types of signals there too, such as touch and their chemical sense.

The third and last of our life style groupings is the long-sighted hunters, to which the jumping spiders belong. They have excellent vision, some of the best of all invertebrates, in fact. So they can spot a prey item, follow it up and leap upon it quite accurately. This is something you can watch for yourself fairly easily since the zebra spider - it has black and white markings - is frequently seen around buildings. You can observe it looking over its surroundings and its swift, running and jumping way of moving. It should be easy to guess what kind of courtship signals spiders like this are going to have - visual ones. The male often has an entire series of display motions, including waving its front legs in the direction of a prospective mate, as part of the courtship procedure.

One of the most interesting things about any kind of spider is its silk. The orb-weavers are the most conspicuous weavers of silk. But there are other families among the web builders, some of them making really complex webs consisting of domes and snares. And just because they don't build prey-catching webs doesn't mean the short- and long-sighted hunters don't use silk. Many of them make little silken retreats for themselves - sort of a sleeping bag arrangement. And they set down draglines. You can observe this by watching a spider as it falls from some surface. It will pay out a line of silk, braking its fall and allowing it to find its way back to the spot from which it fell.

Another interesting thing spiders do with their silk is called ballooning, a means of dispersal for young spiders. They climb to the top of some vertical object in their surroundings, such as a plant or a fencepost, and pay out some lines of silk. If the wind is strong enough and the spider light enough, it is picked up and carried along by the wind, a varying distance depending on circumstances. Spiders are often carried long distances this way. For example, 100 years ago when Java Sea island Krakatoa exploded, wiping out all life there, the nearest life was on another island 25 miles away. The first re-colonizer of Krakatoa was a spider, which must have gotten there by ballooning on the wind.

An even longer distance record for dispersal over great ocean distances was reported in 1981 by a Japanese weather boat which had been stationed for two years over 240 miles offshore from the mainland. During this time they collected over 100 living immature spiders, in three one-meter diameter nets they had mounted on a mast about 60 feet up in the air.

For purposes of identifying some of the spiders you see frequently in you house or yard, you might like to have in mind a sort of "Rogues Gallery" of common spiders to look for. To actually identify most spiders definitively, you need to put them under a microscope and look at fine details like the number of claws it has. But there are some that have pretty unmistakable characteristics that will allow you to identify them at least as being in a certain family.

One characteristic to look for is egg sacs. Ray spiders are tiny and leave egg sacs looking sort of like miniature punching bags suspended from twigs. Nursery web spiders, so-called because the mother constructs a silken nursery for the young after they hatch, carry their egg sacs around with them in their jaws. Wolf spiders, fairly large spiders like the nursery webs, also carry the egg sac around, but they attach it to the spinnerets at the end of the abdomen. Other kinds of spiders place their egg sacs into curled up leaves.

Wolf spiders are unique in that, after the young leave the egg sac the mother has been carrying around with her, they remain with the mother and ride on her back. The parent who has playfully allowed his child to jump on his back for a "horsey ride" around the living room floor would no doubt collapse in laughing protestation should more than one individual at a time climb on board. Contemplate with admiration therefore, if not the patience, at least the strength of the mother wolf spider, who carries her numerous offspring on her back for a period of time after they leave the egg sac.

Within the last 10 years, biologists have been able to investigate the mechanics that make this feat of biological engineering possible. How do the hundreds of young wolves manage to cling to their mother's back without falling off? Recently experimenters have shown just how this can work. Since wolf spiders are rather hairy, an initial idea was that the mother's hairy back might somehow assist the young in clinging on. The first step in the experiment was to cover the mother's back with cloth, which prevented dislodged young from clinging to the mother; any that were lucky enough to get back on board did so only by finding a spot that the cloth did not cover. Fearing that the cloth might be disrupting the behavior of the young by some other unknown means, the researchers then decided to actually remove the hairs in a delicate shaving procedure. And sure enough, initially the young spiders did not return to their position on the mother's back.

In looking at the hair they had removed under the scanning electron microscope, the scientists were able to verify just why the hairs had been so important: in contrast to the structure of hair from other kinds of spiders, these were covered with curved spines and ended in a large knob. Comparison to the hair from an adult male wolf spider revealed only smooth hair on the male with none of the curved spines or knobbed ends. Don't fear the fate of the dislodged young, though. Kept with their mother, they were able to resume their old piggy-back position after a few days. They accomplished this by the accumulation of silk draglines laid down each time they had climbed up over her back. So, the special hairs probably serve their main function in the first day after the spiderlings emerge from their sac. They are easily able to cling to these special hairs with their claws. Later, the young maintain their position by the meshwork of silk they have built up in clambering around on the mother's dorsal surface.

Mother spiders generally use a large amount of silk in making the egg sac. This gives it a dense cushiony appearance, protecting the eggs inside, and the young when they hatch out of the eggs. Usually the young stay inside the egg sac until after they have completed their first molt. Then they make an opening in the wall and emerge to the outside world.

For a number of different species of spiders, the egg sac is the only link between one year's generation of spiders and the next. For spiders like this, all adults, die off as freezing weather signals the approach of winter.

Other species, however, have their generations timed so that the young emerge from the egg sac by late summer. In a still-immature state during the autumn these spiders must seek a sheltered spot in order to survive the winter, for example, under the bark of dead trees and in the leaf litter on the ground. Even species that normally do no frequent such habitats during the rest of the year will be found there by late fall, as you may discover by bringing some leaf litter into a warm area and sorting through it over a light colored surface.

One trick to becoming a spider watcher is to get used to looking for particular kinds in certain habitats, the way a bird watcher entering a cattail marsh expects to see a red-winged blackbird. If you are at the edge of a lake, for example, look for the fishing spider, a member of the nursery web family. It is actually capable of running across the water's surface and diving beneath it, perhaps catching a small fish for food. But usually you see them as fairly large individuals sitting quietly by the water's edge. Another type of spider you can expect to see near water is the long-jawed spider. It has a long thin body and rests holding its long legs out in front of and out behind its body, rather than at an angle to it. So it looks like a long thin stick-like creature.

One family of spiders whose members live on lawns and around homes is the funnel weaving grass spiders. You can see their flat webs most easily when there is dew on the ground and they show up especially well. If you look closely, you should be able to see a funnel-like opening going off from the web. That is where the grass spider waits for its prey. Neither the sheet web itself nor the threads above it catch the prey because they are non-adhesive. When a flying insect hits the threads above, it falls onto the sheet below, signaling the spider to rush out, catch it and take it back to the funnel. Also belonging to this family is the European house spider, one of the most common species in such places as basement corners and barns. Despite its name, "European" house spiders are found all over the world. Individuals may live t be several years old, and males and females can be found living peaceably together, sharing the same sheet web.

Another type of spider you might see in your garden is the huge black and yellow garden spider. It is an orb-weaver and is the largest bodied species in Wisconsin. One thing you'll notice about its web is that there is a sort of zig-zag line of extra heavy silk running through the center. An early biologist named the special area of the web the stabilimentum, no doubt thinking that its purpose was one of stabilization of the structure through some reinforcing effect. But one of the more recent hypothesis as to its function has to do with a camouflaging purpose. That is, that a predator, such as a bird, might easily spot the garden spider sitting smack dab in the middle of this web, but might have a much more confusing time figuring out where to grab its intended prey if its outline were broken up by the passage of this zig-zag line through the hub. A closely related, though somewhat less common sister species which we also see here in Wisconsin is the banded garden spider.

Supporting this camouflage hypothesis for the functional purpose of the stabilimentum is the situation that we find with another member of the orb-weaver family, this one named Cyclosa. Cyclosa creates a similar effect, obscuring the outline of itself in the center of its web, but not by adding zig-zag lines of extra silk. Instead, it creates a disrupting line with debris, the spider's egg sac and the spider itself. It's hard to tell visually where one element of this line ends and the other begins.

Thus, the anatomy, behavior and life styles of spiders present an almost infinite variety for both the scientist and the casual observer. Often misunderstood, spiders are one of the most interesting animal groups you can get to know.

Suggested Reading

Gertsch, Willis J. 1979. American Spiders, Second Edition. Van Nostrand Reinhold Co., New York.

Levi, Herbert W. & Lorna R. 1968. Spiders and Their Kin. Western Publishing Co., Inc. Racine.