The Orb-Web

Teacher's Story

F all the structures made by the lower creatures, the orb-web of the spider is, beyond question, the most intricate and beautiful in design, and the most exquisite in workmanship. The watching of the construction of one of these webs is an experience that brings us close to those mysteries which seem to be as fundamental as they are inexplicable in the plan of the universe. It is akin to watching the growth of a crystal, or the stars wheeling across the heavens in their appointed courses.

The orb-web of the large, black and yellow garden spider is, perhaps, the best subject for this study, although many of the smaller orbs are far more delicate in structure. These orb-webs are most often placed vertically, since they are thus more likely to be in the path of flying insects. The number of radii, or spokes, differs with the different species of spiders, and they are usually fastened to a silken framework, which in turn is fastened by guy-lines to surrounding objects. These radii or spokes are connected by a continuous spiral line, spaced regularly except at the center or hub; this hub or center is of more solid silk, and is usually surrounded by an open space; and it may be merely an irregular network, or it may have wide bands of silk laid across it.

The radii or spokes, the guy-lines, the framework and the center of the web are all made of inelastic silk, which does not adhere to an object that touches it. The spiral line, on the contrary, is very elastic, and adheres to any object brought in contact with it. An insect which touches one of these spirals and tries to escape, becomes entangled in the neighboring lines and is thus held fast until the spider can reach it. If one of these elastic lines be examined with a microscope, it is a most beautiful object. There are strung upon it, like pearls, little drops of sticky fluid, which render it not only elastic but adhesive.

Some species of orb-weavers remain at the center of the web, while others hide in some little retreat near at hand. If in the middle, the spider always keeps watchful claws upon the radii of the web so that if there is any jarring of the structure by an entrapped insect, it is at once apprised of the fact; if the spider is in a den at one side, it keeps a claw upon a trap line which is stretched tightly from the hub of the web to the den, and thus communicates any vibration of the web to the hidden sentinel. When the insect becomes entangled, the spider rushes out and envelops it in a band of silk, which feat it accomplishes, by turning the insect over and over rapidly, meanwhile spinning a broad, silken band which swathes it. It may bite the insect before it begins to swathe it in silk, or afterwards. It usually hangs the swathed insect to the web near where it was caught, until ready to eat it; it then takes the prey to the center of the web, if there is where the spider usually sits, or to its den at one side, if it is a den-making species, and there sucks the insect's blood, carefully throwing away the hard parts.

A dewy morning. Insect Life, Comstock.

The spider does not become entangled in the web, because, when it runs it steps upon the dry radii and not upon the sticky spiral lines. During the busy season, the spider is likely to make a new web every twenty-four hours, but this depends largely upon whether the web has meanwhile been destroyed by large insects.

The spider's method of making its first bridge is to place itself upon some high point and, lifting its abdomen in the air, to spin out on the breeze a thread of silk. When this touches any object, it adheres, and the spider draws in the slack until the line is "taut;" it then travels across this bridge, which is to support its web, and makes it stronger by doubling the line. From this line, it stretches other lines by fastening a thread to one point, and then walking along to some other point, spinning the thread as it goes and holding the line clear of the object on which it is walking by means of one of its hind legs. When the right point is reached, it pulls the line tight, fastens it, and then, in a similar fashion, proceeds to make another. It may make its first radius by dropping from its bridge to some point below; then climbing back to the center, it fastens the line for another radius, and spinning as it goes, walks down and out to some other point, holding the thread clear and then pulling it tight before fastening it. Having thus selected the center of the web, it goes back and forth to and from it, spinning lines until all of the radii are completed and fastened at one center. It then starts at the center and spins a spiral, laying it onto the radii to hold them firm. However, the lines of this spiral are farther apart and much more irregular than the final spiral. Thus far, all of the threads the spider has spun are inelastic and not sticky; and this first, or temporary spiral is used by the spider to walk upon when spinning the final spiral. It begins the latter at the outer edge instead of at the center, and works toward the middle. As the second spiral progresses, the spider with its jaws cuts away the spiral which it first made, and which it has used as a scaffolding. A careful observer may often see remnants of this first spiral on the radii between the lines of the permanent spiral. The spider works very rapidly and will complete a web in a very short time. The final spiral is made of the elastic and adhesive silk.

References—Comstock's Manual; Common Spiders, Emerton; The Spider Book, Comstock; Nature's Craftsmen, McCook.

Method—There should be an orb-web where the pupils can observe it, preferably with the spider in attendance.

3. Observe the silken thread laid around the spokes. Is it a spiral line or is each circle complete? Are the lines the same distance apart on the outer part of the web as at the center? How many of the circling lines are there?

4. Is the center of the web merely an irregular net, or are there bands of silk put on in zigzag shape?

5. Touch any of the "spokes" lightly with the point of a pencil. Does it adhere to the pencil and stretch out as you pull the pencil away? Touch one of the circling lines with a pencil point, and see if it adheres to the point and is elastic. What is the reason for this difference in the stickiness and elasticity of the different kinds of silk in the orb-web?

6. If an insect touches the web, how does it become more entangled by seeking to get away?

7. Where does the spider stay, at the center of the web or in a little retreat at one side?

8. If an insect becomes entangled in the web, how does the spider discover the fact and act?

9. If the spider sits at the middle of the orb, it has a different method for discovering when an insert strikes the web than does the spider that hides in a den at one side. Describe the methods of each.

10. How does the spider make fast an insect? Does it bite the insect before it envelops it in silk? Where does it carry the insect to feed upon it?

11. How does the spider manage to run about its web without becoming entangled in the sticky thread? How often does the orb-weaver make a new web?

A partially completed orb-web. a. the temporary spiral stay line; b. the sticky spiral line; c. the fragments of the temporary spiral hanging in a radius. Comstock's Manual.

2. How does it make the framework for holding the web in place?

3. How does it make the first radius?

The zigzag strengthening band at center of an orb-web.

4. How does it make the other radii and select the point which is to be the center of the web?

5. How does it keep the line which it is spinning clear of the line it walks upon?

6. After the radii are all made, are they fastened at the center?

7. How and where does the spider first begin to spin a spiral? Are the lines of this spiral close together or far apart? For what is the first spiral used?

8. Where does it begin to spin the permanent spiral? Where does it walk when spinning it? By the way it walks on the first spiral, do you think it is sticky and elastic? What does it do with the first spiral while the second one is being finished?

9. If the center of the web has a zigzag ribbon of silk, when was it put on?

10. How many minutes did it take the spider to complete the web?

Supplementary reading—"Argiope of the Silver Shield," Insect Stories, Kellogg.