Continued from All about spiders — Part 1
On Part 1: History, Physical description, Senses, Communication
Spinnerets and silk
Spinnerets are the organs through which the spider produces silk threads. They’re located at the rear of the abdomen, on the underside. Spiders normally have three pairs, but some species have only one pair and others as many as four. Seven types of silk glands have been identified, but no spider has all seven. The spinnerets move independently, but in a coordinated way.
Silk glands produce proteins and other organic molecules which flow in liquid form through microscopic tubes to the spinnerets. As the mixture moves along, a process occurs which removes water from the proteins and adds hydrogen. The result is an acidic fiber which solidifies as it exits into the air through the spinnerets.
The glands each produce silks for different purposes: attachment silk, dragline and web frame silk, silk for wrapping, sperm web and egg cocoons, and sticky silk. Spiders can use the threads singly or in combination. Silk threads are about oone-millionth of an inch thick, but spiders can use them singly or in combination to thicken them, using muscles and valves in the necks of the spinnerets.
Spiders use their silk in many ways, and different spiders use silk for different purposes. Not all spiders build webs, but all spiders do produce silk and use it for draglines. All males use silk to spin a sperm web before mating, and all females spin a silk cocoon around their eggs. Some spiders use silk to build shelters. Spiders are thrifty with their silk. Webs begin to degrade after a few days, so often spiders eat them to re-use the protein.
Spider silk is stronger than steel on a weight basis. If you made a steel rod and compared it to a rod of silk of the same weight, the silk rod would be stronger. Silk is amazingly elastic, too, able to stretch up to many times its original length. The thread of the orb-weaver spider Stegodyphus sarasinorum, an Asian species, is said to stretch up to 20 times its original length.
Not all spiders build webs, but those that do know instinctively how to. Webs may be relatively simple or very elaborate. Different species create differently patterned webs. Some spiders attach a formless tangle of silk threads to something (like the corner of a ceiling — the cobwebs we see up there are tangled webs that have accumulated dust and dirt). Sheet web spiders create sheets of silk between tree branches, on shrubs and other suitable places — you may have noticed some on your lawn, where they last until they’re mowed away. Some webs are triangular, some funnel-shaped. The webs of bola spiders (in the genus Mastophora) are a single strand of silk, and there are other shapes, too. There are some exceptions, but a web can often help in identifying a spider.
Webs are used for capturing prey. When an insect gets caught in the web’s sticky strands the spider rushes to it and uses its fangs to inject venom to kill it. Some spiders wrap their prey completely in silk to immobilize it while it dies. Webs are also used for courtship rituals and for securing egg sacs.
The most noticeable webs in our yards are probably the circular ones made by orb-weaving spiders (families Araneidae, Tetragnathidae, Uloboridae). Like other spiders, they’re methodical about spinning their webs and do it using only the tips of their legs.
How an orb-weaver builds a web
Orb-weavers spin their webs at night. From a suitable starting point, such as the branch of a tree, they release a thread of strong silk from their spinnerets into the wind. Once it catches on something, the spider pulls the thread tight and anchors it. Then, it walks back and forth, reinforcing the line by adding more threads. This is called the bridge line; it will support the weight of the entire web. If the initial thread doesn’t catch, the spider may eat it to recycle the protein before trying again. Bridge thread isn’t sticky.
Next, the spider strings a loose thread, sort of U-shaped, from the bridge, then moves to the bottom of the U and drops a thread from it, forming a Y-shape. After that, the spider forms a bottom line to attach the tail of the Y thread to, and this completes the frame. The completed Y forms the first three radial threads of the web. More radial threads are then strung. Following that, threads are added in an “auxiliary spiral” from the center outward.
So far, the threads aren’t sticky. But, now, a sticky thread is strung between the radial lines, with the spider eating up the auxiliary spiral as it lays the sticky spiral.
The final construction consists of a non-sticky framework and radial lines, plus sticky circular lines. The spider then sits motionless in the center, waiting to feel vibrations on sticky silk — an announcement that prey is trapped. The spider moves on the web by stepping on non-sticky radial threads and the framework, while avoiding the sticky circular threads. Come daylight, the spider slips away to a resting spot. If the web doesn’t get too damaged, the spider may use it again and again, even patching it up a bit. Some spiders eat their web, except for the framework, each morning and re-spin it each night.
Orb-weavers don’t generally get stuck to their webs. That’s because they know where to step; they move their hind legs at an angle that somehow minimizes the effect of the stickiness; and they have tiny barbs on the bristles of their feet that keep them from sliding into sticky silk. In spite of those advantages, accidents do happen. When spiders get stuck, they seem to have a special oily coating that makes them less likely to stick. They don’t get trapped permanently.
Males actively search for females. Courtship differs, depending on the species. Males of some species “dance” or wave their pedipalps. Male gift-giving spiders (Paratrechalea ornata) catch prey and wrap it up as a gift of food for females (but, if they’re hungry they may cheat and eat the prey, then present its wrapped carcass as a gift!) Mating may take only seconds or many hours, depending on the species. It’s often said that females eat males after mating, however, that doesn’t commonly happen. Males wander off to die a few days later, having accomplished their life’s purpose.
Spiders reproduce in an unusual way. The female’s organs for producing eggs and receiving sperm are in her abdomen. The male’s sperm-producing organs, his two testes, are in his abdomen, but he lacks a penis. Not to worry, though, as males have a workaround for that: They extrude sperm through a furrow in the underside of their abdomen onto a small web they construct, called a “sperm web.” The sperm is then drawn into an organ called an embolus, located at the tip of the pedipalps, probably by capillary action. Only then does mating proceed — the male inserts the tip of his pedipalp into the female’s external genitalia, called the epigyne (EP-ee-jyne). The epigyne is a small area of hardened tissue that’s located on the underside of her abdomen, near the book lung.
In most groups, the male inserts his left pedipalp into the left opening of the female’s epigyne and his right pedipalp into the right opening. In more advanced species, the female’s epigyne and the male’s pedipalp are so species-specific that, like a key and a lock, spiders of different species don’t match up and can’t mate. This makes the epigyne the most distinguishing characteristic for identifying species. (Some spiders, including more primitive groups, are less complicated.)
Sperm moves from the epigyne into storage receptacles called spermathecae, where it will stay until the female is ready to use it. As eggs pass from her ovaries to leave her body, she releases some or all of the stored sperm to fertilize them. Females can store enough sperm from a single copulation to last their entire life. The expelled eggs, which often look like drops of liquid, are deposited onto a silk pad and then wrapped in multiple layers of silk. The wrapped sacs are round or disk-shaped.
Females of some species stick their egg sacs to a surface and abandon them. Others guard them, lay them on their web, or carry them around between their chelicerae or attached to their spinnerets.
A single egg sac may contain hundreds or thousands of eggs. If a female constructs several sacs, each one will contain fewer eggs. In many species, the female dies after her last egg sac. Others, however, live one or two years and take care of their offspring for a period of time after they hatch.
Most eggs hatch when the weather is warm. In spring and summer, eggs may hatch in two to four weeks. If eggs hatch in the fall or winter, the spiderlings stay in the sac until spring.
Spiderlings are usually on their own from the time they leave their egg sac, but mothers of some species feed their young. Wolf spiderlings ride around on their mother’s back for about 10 days. Most spiderlings leave their birthplace by “ballooning” away: They climb up a tall object, such as a branch, tree or fence post, and let the breeze pull silk from their spinnerets. This becomes a “dragline.” The newly hatched spider is so tiny and weighs so little that as the dragline lengthens it eventually plucks the spider away. At some point the airborne spider catches on something and from there begins its independent life. As they grow, baby spiders outgrow their skin and must shed (molt) it several times. The bigger the spider grows, the more times it will molt.
A full life span for most spiders is a year or two, depending on the species. But, most become food for predators or die in some other way before they reach maturity. There are some exceptions. Spiders in the infraorder Mygalomorphae, which tend to be big and hairy (tarantulas, for instance), can live years longer. Some take a decade or more just to reach maturity. Captive tarantula males live only one or two years, but females can live 25 years!
Whether they’re captive or not, male spiders have shorter lives than females. One reason is because they move about in search of mates, facing more attention from predators, as well as other hazards. Males that don’t come to a tragic ending tend to die soon after mating.
Spiders feed mostly on insects, but also other spiders. Very large spiders may catch small animals, such as lizards, frogs, baby snakes and baby rodents. Large orb-weavers will feed on small birds or bats caught in their web. One exception to an all-meat diet is the male crab spider Misumena vatia, known to feed on pollen while sitting on flowers as they lay in wait for insects. There are four typical ways that spiders catch prey. Depending on the species, there are spiders that: 1. live in silk-lined burrows and leap out to catch prey; 2. lay in wait to ambush prey on plants, flowers, the ground, under stones, on tree bark and other suitable places; 3. actively search for prey; 4. spin webs to entrap prey.
Predators and defenses
Spiders are preyed on by birds, frogs, toads, snakes, lizards, ticks and wasps. In some countries, monkeys prey on them. Daddy Longlegs (which look like spiders, but are not) prey on spiders. Spiders have to watch out for their own kind, too, as spiders will prey on other spiders. Roundworms and mites parasitize spiders. Humans accidentally or intentionally kill them.
A spider’s best defense is to hide. And because many are colored to resemble the background they’re usually on, they often hide in plain sight. Or, when threatened, they can run into a crevice, under bark, under a rock or anywhere else the predator may not be able to enter. Some trapdoor spiders construct burrows with hinged doors made of silk, soil and vegetation. The burrow serves other purposes, too, but when the spider is threatened it can run inside and tightly close the door. Golden orb-weavers of Asia coat their webs in a chemical that repels many insect predators. Wolf spiders will bite in defense. Tarantulas and others in the family Theraphosidae have urticating (stinging) hairs they flick at predators.