Since their origin, 350 million years ago, spiders have used silk in multiple aspects of their biology. This has contributed not only to its permanence on Earth, but also to its spread in practically all environments.
Maybe once while walking through a forest, a park or a garden, or in your own house, a spider web got stuck on you without you realizing it. How was it possible that you hadn’t seen her? What is it made of to be almost invisible?
A spider web is one of the structures that spiders make with the silk they produce themselves. Silk is a fibrous material that some arthropods (insects, arachnids and mites) secrete through special glands.
Unlike the silkworm (Bombyx mori, a butterfly larva) that only produces one type of silk, a spider is capable of producing different kinds with different elasticity, resistance, flexibility, thickness, adhesiveness, affinity or repellency to water, among other characteristics. In addition, they can mix various kinds of silk and produce new materials. The wide variety of uses of silk is a key fact in the diversity of spiders (more than 40,000 species) and their colonization of numerous terrestrial habitats.
Construction material
Like other animals, spiders build shelters to avoid direct exposure to rain, wind, sun and the attack of their natural enemies, but in their case silk is the main, and in many species the only, material. used. Those who live underground use silk to cover the inside of their homes and prevent tunnels from collapsing. There are spiders that even make a door with silk at the entrance to the shelter. Many of those that live above the ground build a shelter made up mainly of silk threads, under stones or trunks, in rock fissures or on the bark of trees, or occupying air spaces inaccessible to other animals: between two trees, between the branches of a tree, between the leaves or hanging below them.
The Argyroneta aquaticaa species of spider that lives in streams in Europe, weaves a shelter among aquatic vegetation under which it deposits air bubbles until forming a “diving bell” where it can eat, molt, mate and even lay its eggs.
safety rope
With silk, spiders also do “mountaineering”. From their birth and throughout their lives, they produce a thread with the same function as a mountaineer’s safety rope. As they move, the spiders produce this highly resistant thread, and they fix it section by section in the substrate where they are. This allows them to chase prey or flee from an attacker without risk of injury from a fall; If necessary, they return along that thread to the place from which they disconnected. It also helps them descend to the most suitable position to weave a web, position themselves waiting for prey or reach where a potential mate is.
Like any arthropod, spiders shed their skin to grow, and when doing so they are very vulnerable; In order to protect themselves, they use silk to form a completely enclosed shelter, or they molt while hanging from their safety thread.
Spiders in “flight”
Safety thread allows spiders to move short distances, and other types of very light silk make it possible for them to “fly” long distances. This is done by young spiders of many species, but also by adults of small species. To begin the “flight” the spiders climb to a high place; They face the wind, raise their rear and emit one or more threads, which are pulled by the air current like a kite or kite. From time to time they add silk tassels to their thread to increase its resistance, and when it is the right size, the spiders let go and let themselves go. The direction and distance of the movement depend on the wind, but by changing the length of the thread they can control the speed or the moment of descending or stopping.
Spiders travel so well on silk that they have reached almost all terrestrial environments, including very distant or rare sites. Spiders have been found landing on ships at sea, on airplanes in flight, and also on the tops of the highest mountains. Among the first colonizers of the island that emerged near the Krakatoa volcano, was a type of spider (the linifid). With silk, spiders also build bridges, for example between two trees, which allow those that are too heavy to “fly” to move. These bridges are also used as baselines to build spider webs: the line, carried by the wind, gets stuck when it hits a branch or leaf, and the spiders pull it from time to time to see if it is stuck and once it is That’s right, they hold her on her side to be able to move on the bridge.
hunting net
Many spiders make some silk device to capture the insects they feed on. They range from those made up of one or two threads to two-dimensional or three-dimensional networks.
The characteristics of the nets are closely related to the habitat and the type of prey captured. It has been discovered that some nets have parts that reflect ultraviolet light just like flower petals, and that with them they attract pollinating insects.
The web also serves the spiders to perceive and intercept possible prey beyond the extension of their legs. When it hits the web, the prey makes it vibrate and alerts the spider; These vibrations tell it the position of the prey in the web, its size, weight and, in some cases, even the type of animal it is. Some species include adhesive threads in the web that hold prey long enough for the spider to complete its attack.
There are two types of adhesive silk: viscose and cribellate. The first has a sticky substance distributed in droplets along the thread. In the second, the adhesiveness is due to the presence of a tangle of threads of an extremely small caliber (with a diameter of about 0.015 millionths of a meter), which in contact with a surface, become entangled in the slightest of its asperities or roughness ( something similar to velcro, but on a microscopic scale). Spiders do not stick to their web because they put their legs on the non-sticky threads.
There are spiders that use silk to completely envelop their prey. This can have several functions: immobilize the captured animal; inject poison into dangerous prey or feed on it without harm (wasps or ants, for example); reducing the size of voluminous animals—such as a butterfly with large wings—to transport them to the place where the spider is going to eat them and, finally, it also allows them to store prey to eat later.
Some species of net-weaving families, such as those commonly called “boleadoras”, have very peculiar habits. These spiders reduce the web to a single thread, which they hold with one leg, which has a ball of highly adhesive, viscous silk at one end. Apparently, a volatile compound is produced in the body of these spiders that imitates the sex pheromone of some female moths and attracts the males of those species. When a boleadora detects the presence of a moth, it begins to swing or spin its thread (in the manner of a gaucho with his boleadoras), until the moth collides with the slimy ball, and then the spider pulls it toward itself to devour it.
silk courtship
Spiders have a complicated sperm transmission mechanism. In both sexes the genital opening is located in the posterior region, but in the adult male the second pair of appendages in the anterior region, the pedipalps, carry out the transfer of sperm to the genital opening of the female. To bring the sperm to the pedipalps, the male previously weaves a small network (“sperm network”) and deposits on it a drop of semen that will be “sucked” by the pedipalps in the manner of a syringe loaded with a medicine. Once the entire drop has been aspirated, the male is ready to look for a mate.
The male locates adult females by the scent of their sexual pheromones, deposited on some of their silk threads. In wandering spiders (those that do not weave hunting webs), pheromones are frequently found embedded in the security thread. When a male wanders in search of a mate and stumbles upon the thread of a “marriageable” female, he is guided by it to reach her. In other species, sexual pheromones are found in the threads of the shelter or capture net, and are diffused by evaporation. There are cases in which the male is able to locate an almost adult female in her last molting chamber guided by the pheromones impregnated in her threads. When this happens, he waits for the adult and virgin female to come out to court her.
In several species of web-weaving spiders in which both sexes are of similar size, the first thing a male who has located a female does is eat most of the web. This reduces the possibility that the pheromone scent continues to attract other males. On the other hand, in species that build networks and whose males are usually smaller than the females, it is common that during courtship they make the threads of their network vibrate. The vibrations must be produced in such a way that they are not confused with those of a dam. In certain species, the male weaves a “mating thread” next to the female’s web, which serves especially for vibratory courtship, for copulation and probably as a safer place for him than the web of his partner. .
It is interesting to mention that there are males that conquer females by offering them a “nuptial gift”: a prey wrapped in silk. While they are busy eating their treat, they engage in copulation.
Soft protection
Silk is very useful when it comes to providing protection to the offspring, especially in the egg stage. The females make a silk “sack” or ovisac, where they deposit their eggs so that they are protected against rain, wind, sun, desiccation, predators or parasites. Some only entangle the egg mass with a few threads, while others build ovisacs with several layers of silk, and still others even “decorate” them with materials found in their environment, providing them with protective camouflage. The degree of complexity of an ovisac is related to the dangers to which the eggs are exposed.
Care extends beyond the egg stage. Spiders of several families carry their ovisac until shortly before the emergence of the young; Then they weave a structure around it, and when the babies emerge they stay for several days inside their “nursery” or “corralito”, under the supervision of the mother.
Artificial silk?
Studies on the mechanical properties of spider silk have…