Spiders are an order of arthropods belonging to the class of arachnids. Other arachnids are also referred to by the term ‘spider’, such as the sea spiders and whip spiders. The representatives of the order Araneae are therefore also called “true spiders” to distinguish them from the other groups.
Nearly 50,000 spider species have been described and named, and that number is still increasing every year. Spiders are found throughout the world and show great variation in physique, behavior and food specialization. Population densities can be high. British spider expert William Syer Bristowe once described spiders as a giant carpet that spans the earth.
Spiders are terrestrial; they are typical land-dwelling predators that eat live prey, which are generally captured using spider webs. Many spiders make a capture web and are passive hunters; they wait for a prey animal to become entangled in the web after which the prey is captured. Other spiders actively hunt prey or wait from ambush. Spiders dispose of large numbers of insects, especially flies and mosquitoes. A number of spiders have specialized in catching other animals, such as terrestrial crustaceans, right-winged insects or ants; the spider-eaters even eat other spiders.
Spiders come in a variety of shapes, colors and sizes. A number of tropical species have a rather large size, and sometimes variegated colors, a striking body shape or characteristic protrusions. Most spiders, however, have good camouflage. A number of spiders are so strongly camouflaged that they cannot be recognized as such. Examples are spiders that resemble animal droppings or plant parts such as leaves and twigs. There are also species that imitate other animals such as wasps or ants.
Most spiders remain small and have a body length -excluding legs- of about an inch. The smallest spider species do not grow longer than one millimeter. The largest species can have a leg span of more than 25 centimeters.
Distribution and habitat
Spiders have spread throughout the world and are absent only in permanently cold regions. Different species are found on all continents and as a group are common in all countries of the world, unlike many other groups of arthropods.
The distribution of spiders is often related to a particular family; some groups are found only in a limited part of the world. However, representatives of a number of spider families are cosmopolitan and can be found all over the world. Examples include the tarantulas, which are found only in northern Eurasia and central and northern North America. The barrel spiders (Corinnidae) are found almost worldwide and have one of the largest ranges of all spider families.
Families that have a limited distribution are the Holarchaeidae that live in New Zealand and Tasmania and the Liphistiidae that occur only in southeast Asia and Japan.
Spiders occupy all possible niches and there are few areas where spiders cannot be found. Most spiders live in tropical, subtropical and temperate regions but they can also be found in very dry, cool or just hot biotopes. Spiders live in several deserts and even on Mount Everest spiders have been found by British spider expert Thomas Savory. These lived at an altitude of 6600 meters above sea level and almost 1200 meters above vegetation. Only in permanently cold areas such as the poles and high mountain peaks can spiders not survive.
Spiders are also found in highly isolated places, such as the remote rocky islets of St. Peter and St. Paul rocks, located between Africa and South America. Spiders never live in the sea but there are some species that are adapted to a life in water.
Human buildings are very suitable habitats for a number of spiders, for example, gardens, parks and roadsides are good habitats for many species. Spiders that live in caves can appreciate subterranean structures and can be found in railroad tunnels, water wells, sewers and mines. An example of such a species is Meta menardi.
Spiders always have eight legs and a body consisting of two parts, the front part of which carries the legs. This front part is clearly distinguishable from the sac-like abdomen. Spiders usually possess four pairs of eyes, thus eight in total, and they generally do not have body protrusions transformed into prehensile claws. These characteristics distinguish spiders from all other arachnids such as whip spiders, whip-tailed scorpions and “true” scorpions, which always have grasping claws. Scorpions additionally have an extended part of the abdomen with a venom sting and whip-tailed scorpions have a long single appendage here that spiders always lack. From harvestmen and ticks and mites, spiders can be distinguished by their body divided into two parts, in the aforementioned groups these parts are fused into one whole.
Most spiders reach a body length -excluding protrusions and legs- of no more than one centimeter. A few species grow to a length of several centimeters, and there are species that grow even larger. The largest species can reach a body length of about 12 cm, such as the goliath bird spider. The spider Heteropoda maxima from Laos is one of the largest spiders in the world. This hunting crab spider can reach a leg span of up to 30 centimeters.
The smallest spiders do not exceed 0.7 millimeters in length. The smallest known species is Anapistula caecula from Ivory Coast, whose females reach a body length of up to 0.55 millimeters. The males of the species are probably even smaller but these have not yet been described scientifically. The males of the spider Patu digua from Colombia grow to 0.37 millimeters. Females, however, grow longer than those of the previously mentioned Anapistula caecula.
Males often remain smaller than females in spiders, sometimes significantly smaller. In some spiders, the differences are so great, in fact, that the two sexes seem rather like two separate species, as occurs in the wasp spider. In some spiders, however, males are about the same size as females.
Almost all spiders have a camouflage color such as brown or black so they are less visible to enemies. They use their colors as camouflage on the surface on which they live. Among spiders, however, all colors of the rainbow have been described, such as blue, red, pink, yellow and even green with a red spot. The species in the genus Gasteracantha are undoubtedly some of the most colorful and strangely built spiders. This applies only to females; males are unsightly small and less attractive. In fact, males of many species have not yet been discovered. Gasteracantha species, in addition to bright colors, often have spine-like structures on the abdomen, which are sometimes twice as long as the spider itself. The spines are usually angled backward or upward and probably serve to warn enemies that the spider is not easy to swallow.
Spiders that have a highly venomous bite often warn their enemies with warning colors. They use this to indicate that they are better left alone, an example being the black widow. Many larger tarantulas also have bright colors, however, these are mainly caused by the iridescent body hair, such as the multicolored tree spider (Avicularia versicolor). Males of the highly venomous red-headed mouse spider (Missulena occatoria) have a predominantly deep black body but a contrasting blue abdomen and a blood-red upper surface of the head and jaws (chelicerae). The autumn fire spider has a bright red abdomen with four black spots reminiscent of a ladybug. Finally, the spider Paradictyna rufoflava also has unusual coloration; the entire body is green in color but the back of the abdomen is actually red in color.
Spiders are sometimes very well camouflaged and barely noticeable because they blend in against the background. This can be caused by body color or body shape and combinations are also common. Spiders where both shape and color are matched to the background are very difficult to see when sitting still. Many species have good camouflage to avoid being noticed by enemies; others use their camouflage color to ambush prey.
A number of spiders can change color. A well-known example is crab spiders, which can change their body color from yellow to white by redistributing body fluids in the skin. There are also species that have a “summer color” and an “autumn color,” such as the four-spotted web spider (Araneus quadratus). This species normally has a greenish-brown color, but turns reddish-brown in autumn.
The gravel spider gets its name from its ground-dwelling lifestyle on a substrate of gravel. Because of its colors and spotting, the spider is difficult to see in its natural habitat. Spiders belonging to the genus Hypochilus live on moss-covered tree trunks and their body color mimics these very well, making them stand out against the substrate. The genus Pandercetes has representatives that live on tree trunks and hardly stand out on them. Not only the body color with jagged spots, but also the strong hairs all over the body make the spider very difficult to see on natural surfaces. The hair is placed so that the spider’s body casts no shadow under any angle of light.
Spiders are mainly eaten by birds. Some species of spiders have a body shape and color that resembles bird droppings. This is no coincidence since birds will never eat bird poop. There are even spiders that emit a body odor similar to the smell of bird poop. This serves not only to repel birds, but also to attract insects that live on bird poop. Other spiders look tellingly like twigs or other plant parts such as the triangle web spider (Hyptiotes paradoxus). This spider lives in coniferous forests and externally resembles a dry bud of spruce (Picea). Species belonging to the genus Arachnura look strikingly like a leaf thanks to the petiole-like fusion of the abdomen.
Species from the genus Poltys live on branches and have a specific camouflage where they resemble a spur of a twig. Not only the color but also the body shape are adapted to this, as well as the posture of the legs.
The Brazilian crab spider Epicadus heterogaster is one of the most remarkably shaped spiders. This species has seven very tall “humps” on the abdomen that are spaced apart, making the whole thing look like an orchid-like flower. The spider’s colors reinforce this resemblance; in addition, the spider has several widely different color forms. The most common body color is all white, but purple, bright yellow and red colors also occur.
A number of spiders have a very similar appearance to ants. Such species often live primarily from ants or their larvae and pupae. These spiders belong to the genus Myrmarachne and the family jumping spiders. Different species of spiders sometimes even resemble different species of ants, depending on which species they eat. For example, the species Myrmarachne plataleoides imitates the weaving ant Oecophylla smaragdina.
The head thorax is the front part of a spider and consists of one piece. The head is fused with the potted thorax. The head thorax is called cephalothorax in arthropods, and in spiders this part is specifically referred to as prosoma.
Spiders do not have antennae as found in insects. Spiders do have tactile antennae but these arise from mouthparts and are called the pedipalps. These articulated and highly mobile appendages have a similar tactile sensory function, similar to the antennae of insects. In addition, in the male spider, the pedipalps perform a specialized role as a sexual organ. The last segment of a male’s palp consists of a distinctly enlarged storage organ called a bulbus that has a function of spermatophore. They are storages in which the spider stores sperm and mating organs with which he inserts it into the female’s orifice or epigyne. The bulbus bears at its end a needle-like appendage, the embolus. The whole has a pipette-like function; sperm is sucked through the narrow opening into the bulbus where it is stored, see also under reproduction.
The palp of a female is thread-like and is used as a tactile organ. In a number of species, they are used by the females to carry the egg cocoon with them. The shape of the male’s palp is one of the most important characteristics in determining spiders.
Spiders have two chelicerae or venom jaws, which are hollow so that venom can be injected into the prey animal. The spider’s venom gland is located internally at the front of the head thorax and is operated by two twisted muscles that deliver the venom with great force into the jaws and then into the prey.
Spiders are divided into two groups based on the construction of the chelicerae. The original jaw shape is orthognathic; here the chelicerae are parallel and the venom sting is folded down. The chelicerae – seen from the body axis – can only move from top to bottom. Modern spiders have a jaw construction called labidognate; in these species the chelicerae can be moved from left to right and this allows them to be used as forceps.
In many such spiders, the chelicerae are also used for chewing. The jaws are hinged and the ends are rubbed against a thorn-covered base, grinding the food. There are also spiders that suck out their prey and leave behind an externally perfectly intact skin of their victim.
Not all spiders inject venom, in some species the venom gland has degenerated and the chelicerae have no opening at the tip through which venom is administered in other spiders. An example is the triangle web spider. Such spiders often spend more time packing a prey animal to compensate for the lack of paralyzing venom.
On the upper part of the thorax are located the eyes and often this part is somewhat curved so that the eyes are located a little higher and thus have a better view. Such an elevation is called the eye hillock and is also found in other arachnids. In some species the eye hillock is extremely elongated, reminiscent of a “giraffe’s neck,” an example being the species in the genus Afrarchaea found in South Africa and Madagascar. Spiders always have single eyes, which contain a single lens and not compound eyes as occurs in insects and crustaceans.
Most spiders have four pairs of eyes, making eight in total, and a few species have three pairs of eyes, making six in total. Some species of spiders have two or only one pair of eyes, and in a few species of spiders the eyes are functionally completely lost. Such species live exclusively in caves since visual senses are useless here because of the permanent darkness. An example is the species Neoleptoneta myopica: it still has six eyes, but these contain no pigments and thus cannot perceive light. This is not true of all cave dwellers; the spider Spelungula cavernicola also lives in caves but still has developed eyes.
The eyes are always located in pairs, often one pair is somewhat better developed than the other three and these are usually the front-middle eyes. Especially hunting wandering spiders such as the jumping spiders have at least one pair of eyes that is very well developed. Such spiders can see depth, perceive different colors and see polarized light. The retina or retina is movable so that the field of vision is expanded. Most spiders, however, see quite poorly and can perceive only coarse shifts in light. They can thus perceive whether it is day or night and form images of objects that are very close. Vision varies greatly among spiders, species belonging to the wolf spiders have about 5,000 retinal cells in the eye while the gray house spider has about 400 retinal cells. By comparison, a human’s eye has about 80 million of them.
The jumping spiders, unlike most spiders, have fairly good eyesight. The representatives of this family can see depth and thus estimate distances and thus have stereoscopic vision, unlike the other spiders. This is because two of their eyes are enlarged and additionally placed next to each other at the front of the head. This allows them to jump a distance of 40 times their own body length far, which is not possible without being able to see depth. Good eyesight has also led males to give a visual representation during courtship by rhythmically waving their legs. Even in jumping spiders, however, vision is limited; although they can perceive objects sharply, they can only see up to a distance of about 30 centimeters.
Cell spiders, for example, have six small eyes located close together at the front of the head. Species in the genus Oxyopes have two enlarged front-middle eyes with two smaller ones below them. One pair of eyes is located on the sides of the head and the other on the top, allowing the spider to see in all directions.
The eyes of spiders, both their number, shape and exact position, are one of the body characteristics in which spiders can differ greatly. Often in a spider, this configuration of the eyes shows which family the species belongs to. Species in the family Dysderidae, for example, have six small eyes located in a group on the center of the head. In wolf spiders, two eyes are enlarged and located at the front of the head, an enlarged eye is present on either side of the head, and four small eyes are located on the underside of the head in a horizontal row. In the web spiders, four vertical rows of eyes of two often occur side by side where the eyes are small and equal in size. Finally, in the jumping spiders, two greatly enlarged eyes are located at the front of the head, and the six other eyes are positioned on the sides and back of the large eyes. A special genus of spiders is Walckenaeria, whose males have a periscope-like protrusion at the front of the head thorax. Located at the top of this stalk-like structure are the eyes. The protrusion plays a role in the spider’s mating process.
Like all arachnids spiders have eight legs and not six like insects or ten and more like crustaceans, centipedes or millipedes. This is also evident in most species but in a few spiders the palps are greatly enlarged so that it looks like they have an extra pair of legs. There are also species that have greatly elongated spider nipples on the abdomen reminiscent of legs.
The eight walking legs sprout from the cephalothorax. The anterior pair of legs is denoted by a roman I, the posterior pair of legs by IV. The dorsal side of the cephalothorax is called the carapace; the ventral side is called the sternum. The legs have flexor muscles (musculus flexor) but no extensor muscles (musculus extensor) in some of the largest joints. The extension of the paws here occurs hydraulically by increasing blood pressure in the head thorax, see also under circulatory system.
The legs are often covered with various types of sense hairs, which can sense touch, humidity, vibrations and odors. Species that live in burrows often have short, velvety body hair that provides no resistance when digging. Other species, on the contrary, have long and spiny body hair, such as species belonging to the family lynx spiders. The paw surface of the tarses is very well developed in some spiders. Many spiders can self amputate (autotomy) a paw at the border between coxa and trochanter as a protective mechanism if it is grabbed by an enemy.
The end of the paw is different in spiders, species that make webs have two comb-like structures on each paw, with a hook-like structure in between. By clamping these together, a spider can hold on to the spider thread. However, species that do not make webs but are soil-dwelling or live in trees have a completely different structure on the paw surface, in these species the underside of the paw bears many fine hairs that themselves have outliers. The hairs are so small that they stick to all kinds of surfaces. Such spiders can even run vertically against glass, this mechanism also occurs in entirely different animals such as lizards from the anoles family.
The abdomen of a spider is usually spherical to egg-shaped and contains most of the organs. The abdomen contains the heart, the book lungs, most of the digestive tract and the reproductive organs. The abdomen is called abdomen in arthropods but in spiders the term opisthosoma is used for this purpose. In spiders, the abdomen can always be distinguished from the carapace by the petiolus, which is the strong constriction of the spider’s body. The petiolus is a close connection between the front and back of the body of spiders. A spider’s body becomes less “rigid” because of the petiolus, a similar structure is also found in some insects such as the wasps (“wasp waist”) and in many ants.
Nervous system and senses
Spiders have a nervous system that differs from that of other invertebrates and even other arachnids. In insects and centipedes, there is a so-called rope ladder system that consists of a thickened part (nerve node or ganglion) in the abdomen and two nerve strands that run throughout the body and always have branches to the limbs. This makes it resemble a rope ladder where each cross connection can be considered a kind of “partial brain. This allows insects, crustaceans and centipedes to move their legs independently without using the ‘main brain’ to do so. A double nerve is also common in other arachnids; in scorpions, the different partial brains are connected so that better coordination is possible.
In spiders, however, the nervous system is located entirely in the cephalothorax. There are nerve pathways that lead through the petiolus to the spider’s tepals and to the hairs on the skin of the abdomen. During embryonic development, the nerve canals appear as segmented sections, as in insects and other arthropods. However, they are relocated to the carapace early in development. The brain is centralized into a single “node of nerve nodes. Such centralization of the brain can greatly enhance an animal’s intelligence; for example, centralization of the ganglia is the reason squids are highly intelligent compared to other mollusks.
The nervous system responds to signals from the spider’s sensory cells. Spiders can perceive several variables; first, they can perceive colors and movements through the retina cells in the eyes. The eyes are connected directly to the ganglion or nerve node through a nerve strand. The main senses of spiders, however, are the hairs on the body. The skin of spiders contains hairs that can sense vibrations in the air, detecting approaching enemies or prey. Spiders probably cannot distinguish sound tones but can only detect their presence. Such hairs are called cup hairs because of the cup-like shape of the skin opening from which they protrude. The cup hairs can detect air vibrations from millimeters to several centimeters and are effective only at a short distance.
The olfactory hairs play an important role in social spiders, which recognize each other as family members. Social spiders chase away conspecifics that do not belong to their own family and they are able to do so because of the highly specialized olfactory hairs. The spiders can thus distinguish the body odor of conspecifics.
Spiders are able to determine their geotaxy; that is, they are able to perceive how high up they are. Positively geotactic spiders are arboreal and seek out higher plant parts. Negatively geotactic spiders are terrestrial and will continue to prefer the soil. In addition, spiders can identify various environmental variables, such as temperature, atmospheric pressure and humidity.
Spiders belong to the arthropods and thus do not have lungs like mammals. Many spiders have so-called book lungs that get the name from the many leaf-like plates that absorb oxygen. Many species also have branched tubes that supply oxygen to the body, called tracheae. A combination occurs in many spiders. The respiratory apparatus is always located in the abdomen, and the breathing openings are also positioned here. The breathing openings consist of a slit-like opening on the abdominal side of the abdomen where the number of openings may vary among families. Usually the slits are hairless to facilitate breathing. Many species have a pair at about the middle of the underside of the abdomen and a pair just in front of the spinnerets.
Book lungs are not originally internal organs; in fact, book lungs arose from legs transformed into a kind of gills. Such legs are still found in other terrestrial invertebrates, such as terrestrial woodlice where they are called pleopods. In spiders, the “legs” are no longer recognizable as such and were incorporated into the abdomen during the development of the ancestors of spiders. These lived in the sea and had gills that had to be in contact with water and therefore consisted of external organs. This can also be seen in the embryo of a spider, where the respiratory organs initially develop as bulges but later sink into the abdomen.
The book lung is a highly developed organ composed of thin slices of tissue in contact with atmospheric air. The name book lung is derived from its construction, the lungs of spiders consist of structure similar to a page from a book. The “pages” consist of thin sheets of tissue. These plates are hollow so that blood fluid can flow through them; blood fluid is the carrier of oxygen and contains the CO2 to be excreted. Gas exchange takes place between the platelets in which carbon dioxide is released and oxygen is taken in. To prevent the platelets from sticking together, they have small protrusions. Oxygen is transported to the various organs via a blood fluid.
A spider’s tubular heart is located approximately in the middle of the abdomen at the top. Blood enters the heart through four valves (ostia). Spiders that breathe through a tracheal system that supplies oxygen to the entire body have a smaller and often less powerful heart than spiders that are primarily supplied with oxygen by the book lungs. Spiders that breathe through book lungs must pump all the oxygen to the organs via blood fluid, whereas spiders with a developed tracheal system partially oxygenate their bodies directly.
Through a system of vein-like tubes, blood is directed to the various tissues. Spiders have an open circulatory system; their blood fluid flows only partially through tubes similar to veins, but flows freely through the organs. The blood fluid is returned to the ducts so that it can be passed along the book lungs to be oxygenated and cleared of CO2.
The blood of spiders contains substances that bind oxygen in order to distribute it throughout the body. In spiders, this does not involve an iron compound such as hemoglobin but a copper compound called hemocyanin. When hemocyanin carries oxygen molecules, it does not turn red as iron compounds do, but blue.
When walking, the spider uses blood pressure to stretch its legs, bringing blood into the leg every time the spider places its leg forward. In fact, spiders have flexor muscles but no extensor muscles in their legs, see also under the heading legs. These blood pressure differences are not influenced by the heart in the abdomen but by muscles in the head thorax.
Reminiscent of a hydraulic system, this has the disadvantage that if a spider is injured and loses blood, the blood pressure drops and the legs cannot be moved. This mechanism also explains why spiders fold the legs after death; the loss of blood pressure causes the legs to fold.
Digestion and excretion
Spiders digest their prey largely outside their own bodies. They do this by grasping the prey with their jaws and injecting the venom into a prey through the venom canal of the venom gland, where the venom paralyzes the prey but also has a strong digestive effect. The body of a prey is thus digested from the inside after which the spider sucks up the liquid. In the stomach a negative pressure is created so that the liquid components are sucked up. Located at the top of the stomach are muscles attached to the dorsal side. These muscles provide the pumping action of the stomach. These muscles can not only suck up food but are also involved in excreting the digestive juices into prey.
Located behind the stomach are two ducts that continue into the base of the legs. These supply the limbs with food. The spider’s intestines run through the close connection between the head thorax and the abdomen, this is called the petiolus. The spider’s actual intestines are located in the abdomen, they are greatly widened here and end in a rectum. This has a bladder-like structure, in the rectum the waste products are collected. Fluid is extracted from it so that the waste is thickened.
Spiders do not have kidneys or a liver that can completely disintegrate difficult-to-degrade substances into easily excretable ones. Therefore, spiders partially convert their wastes into water-insoluble compounds that are stored in the body.
The spider’s wastes are surrounded by cells that tie off after which the wastes are carried to the rectum. Spiders, like other carnivores, produce mainly ammonium as waste and convert it into guanine. This substance crystallizes easily and is stored in the intestinal endings located against the skin. The degradation product guanine is carried to the terminal intestine through Malphigi’s tubes. In many spiders, some of the guanine is stored in the skin. Because the guanine crystals reflect all light, they have a white color. In some spiders-such as the cross spider-the substance creates a white back pattern.
Like other arachnids, spiders have coxal glands; these are located on the head thorax near the attachment of the first and third pairs of walking legs. In some spiders, one of the openings has disappeared. The coxal glands mainly secrete fluid.
The intestines of spiders can be infected by single-celled parasites. Spiders have several methods to counteract an infestation, which begins when they ingest food. Liquid food is passed through a fine filter of very small hairs so that only dissolved substances are digested and many pathogens are kept out. Single-celled parasites that do get through this barrier are killed by the digestive juices and if they get into the gut they soon run out of food; the food particles are absorbed almost immediately by the intestinal cells.
The abdomen appendages of spiders are called the spider tepals. They are paired, cone-shaped structures covered with small glandular openings. Spider spines originally arose from limbs on the fourth and fifth abdominal segments. The spider nipple itself does not produce spider web; it is a carrier for many hundreds of so-called spinnerets that secrete the spider web.
Spiders have different types of spinnerets, the smaller ones serving to produce fine spindle threads and the larger structures serving to make sturdier threads. In spiders, several glands are known to make spider thread, each of these glands has its own name. The glands Ampulleceae major and A. minor are used to make running threads for the web, the gland Coronatae is used to produce adhesive threads and the gland Aggregata provides the adhesive on these. The Aciniformes gland serves to make fine thread to wrap prey, the Tubiliformes gland is specific to make cocoon threads. Finally, the Pyriformes gland produces sturdy spinning thread to make attachment threads.
Most spiders have three pairs of spinnerets, so six in total, some spiders have only one pair and in a few species two to four pairs of spinnerets occur. Each spinneret contains small glandular openings whose number can vary widely, from two to thousands. In spiders possessing a sieve plate or cribellum, the number of spinnerets can exceed 50,000.
The ability to produce spinel is very characteristic of spiders; they all can do it. There are some insects and mites that also produce spinel but not like the spiders with spinnerets located at the abdomen. Caterpillars that produce spinel do so, for example, with glands near the head.
The spinnerets of spiders secrete a liquid that hardens into a thread once the liquid is stretched by the legs. Both males and females have developed spinnerets, although the spinnerets of a male and female are often specialized in different ways.
Chemically, the spindle is composed primarily of protein and includes glycine, alanine, serine, valine and leucine. There are different thicknesses that the spider can make and the spindle can be processed in different ways. The carders, for example, are a family of spiders that features a cribellum or sieve plate through which very fine, woolly threads can be made. In some spiders, the cribellum has degenerated and only a cone-shaped structure is present, this is called the colulus.
Spiders have not just different spider glands but different types of glands, according to the purpose the spinning is secreted. In wheelweb spiders, two types of spinning are used in making a web. A thicker type of spindle serves as an anchor thread and is not sticky, on this the spider can walk without its body sticking. In addition, thinner threads that are sticky are used, which allow prey to stick to the web.
Spiders use their cobwebs for the following purposes:
- Catching prey by sticking them (multiple methods)
- Packing prey for consumption
- Coating the living tunnel with a layer of spider webbing
- Anchoring the body when walking or jumping
- Spreading out as a young spider by hovering away
- Packing the sperm in a spermatophore and making a shell (cocoon) for the eggs
All spiders can produce spin and most species use this to capture their prey, however, not all of them do this.
Sticking to prey
Some spiders shoot the spider web, or even the entire web, at the prey to capture it. The spider web occurs as proteins coagulate as soon as they emerge from the spider’s nipple; they change structure as the spider pulls on the thread. So the transition from liquid to solid is not due to the action of air as is often thought.
Spiderweb is stronger than the best synthetic materials humans can make at the same thickness. However, it is not rigid so that, for example, use in bulletproof vests is not possible. The bullet does not pass the vest but enters the body with vest and all. The web of a cross spider weighs only about 0.1 to 0.5 milligrams.
Of all the species that capture their prey using spider web, the vast majority make a spider web, but again there are exceptions. Examples of species that produce a spindle thread to capture prey but do not make a web are those from the genera Mastophora and the family glue spiders. Mastophora species make a single thin thread whose end is coated with a sticky droplet. This droplet contains scent from female moths that attract the males. The spider lowers this thread and when a moth approaches, the thread with the droplet is moved back and forth like a bola until it hits the moth and the spider lifts its thread -with prey- back up.
The spiders belonging to the glue sprayers (family Scytodidae) go even further; they spray the spinel like a sticky double thread directly onto the prey to fix it. During spraying, the chelicerae are moved back and forth to create a zigzag thread which increases the effective surface area. Glue sprayers not only spray spider web, but also shoot the prey with venom so that it becomes paralyzed.
The spider web
Many but certainly not all spiders use a web in their hunt. Many spiders exhibit a strong specialization on a particular prey group or way of hunting. The webs that spiders use can be divided into four types:
- Round webs stretched perpendicularly between plants, such as the web of the cross spider (Araneus diadematus). Such webs are used to capture flying insects.
- Irregular webs, these are stretched across the soil or horizontally on plants to catch crawling insects, such as the web of the gray house spider (Tegenaria domestica).
- Funnel-shaped webs, these are distinctly cone-shaped where the spider sits in the small opening waiting for prey to pass by. Many funnel spiders make such a web.
- Tunnel-shaped webs, these are used by subterranean spiders. The spinel is used to line the burrow, often the tunnel is closed by a suitable door as in trap door spiders.
Some spiders can make extreme webs. For example, species in the genus Nephila can spin a web strong enough to catch bats and birds. Large insects that can fly through other webs, such as dragonflies, are also no problem for such species. The web is so strong that it is used by locals as a fishing net. Spiders are also known to make very large webs, such as the species Caerostris darwini from Madagascar. In 2010, a web was found with an area of nearly 3 square meters stretched above the water. There are spiders that make a small web but do not hang it up but hold it with their legs. They hold themselves above the bottom and when a prey animal passes under the spider, the web is shot down. Spiders that have a cribellum make spider webs that are so fine that any potential prey will be entangled in them. The cribellum is a comb-like structure on a spider’s abdomen that makes threads up to a hundred thousandth of a millimeter in diameter.
Many spiders not only capture a prey with a web, but immediately wrap it with spinning threads so that the prey cannot escape. The threads used for this purpose are not sticky, otherwise the spider would stick to them.
Some insects have adaptations to escape from a spider’s web. The wings of lacewings, for example, are covered with fine hairs that stick to the web and release so that the wing surface itself does not stick. The lacewing then frees itself from the web by gnawing through the threads.
The trapdoor spiders and many tarantulas make a living web, which is not usually used as a capture web. The tunnel-digging species coat the entire tunnel with spider web. When they need to molt or carry eggs where the spider is more vulnerable, the tunnel has a number of “walls” made of spider web. So these species use it to protect themselves.
Some spiders living in residential tunnels make a pocket at the bottom of the tunnel under which they can hide should an enemy get into the tunnel. The spider can thus shield itself through a layer of spider webbing.
A number of groups of spiders do not use spinning at all to capture their prey, nor to hide. Examples are jumping spiders and crab spiders. Jumping spiders have very good vision and pounce on their prey; crab spiders are often well camouflaged and lie in wait in ambush. Incidentally, both groups do use the spinneret to anchor themselves to the substrate. Anchoring the body to the substrate is common among spiders. When a spider is disturbed, it often lowers itself down on this “safety thread.
Most spiders, including the web-making species, stick a spindle thread to the substrate when walking so that they are always assured of a safety thread. this thread is much thinner than the threads used to make a web.
A major reason for the occurrence of spiders in the most isolated places is their ability to hover. Spiders do not have wings and cannot fly themselves but can float away on a spindle thread with the wind, traveling great distances. This behavior is particularly well known in young or small spiders; larger spiders are soon too heavy for this. As a result, spiders have been found at altitudes of several kilometers.
Some species have been described as being able to move through the air even as adults. An example are a number of hammock spiders that float through the air over great distances and kilometers high by using the spider web as a kite.
Almost all spiders deposit their eggs in a spun cocoon or egg sac. See the Egg section for a description of the eggs of spiders.
The species that do not make a cocoon often wrap the spindle loosely around the eggs to hold them together. An example is the large vibratory spider, whose female frequently clips her eggs between her chelicerae and walks around with them that way. Most spiders, however, spend a lot of time hiding the eggs, and spiders make a wide variety of cocoons. The variation is very great; based on the shape and size of the cocoon, the group of associated spiders can often be assigned. The cocoons of species that are closely related resemble each other and are more difficult to identify.
Many species deposit the eggs on the substrate and spin a dense web over this, and other species store the eggs in the shelter near the web. There are also spiders, such as wolf spiders, that carry the egg sac with them permanently, the cocoon here is often very solid and round in shape. Some spiders make remarkable shapes from the cocoons, such as the large lantern spider (Agroeca brunnea). This species produces a cocoon that looks strikingly like a silk lantern, and the cocoon is reinforced with soil material. The water spider first makes a fine-mesh web and fills it with air to create an underwater bubble that acts as a nursery. The eggs are deposited in this “diving bubble” and guarded by the female. The labyrinth spider makes a cocoon composed of disks and suspended on a thread. Each disc contains several eggs and consists of two parts. As the clutch grows and increases in volume, part of the disc falls off so that the hatching juvenile spiders have more space.
The spindle used for the cocoon is usually white to yellow. The glue spider’s spinel, however, turns green in color after some time which serves as camouflage.
Male spiders primarily use their spider web to create a sperm web. Spiders do not have direct fertilization; the males first introduce the sperm into their palps, which are then introduced into the sex orifice (epigyne) of the female.
Before the male fills his palp with sperm, a so-called sperm web is first spun. This is a kind of “rug” on which a drop of sperm is deposited. Many other arachnids, such as scorpions a whip spiders, then wrap the sperm in the web creating a sperm packet or spermatophore. Spiders, however, do not do this but instead suck the drop into the bulbus. A male with a “loaded” bulbus then seeks out a female. The sperm web is immediately eaten again by a number of species.
Reproduction and development
Spiders are born from an egg, and many species have a so-called post-embryonic stage where they have an almost round body shape. After the first molt, the tiny spiders already resemble their parents although they are much smaller and often have a different body color than their parents. Juvenile spiders live on smaller prey. Juvenile spiders often have similar lifestyles and food preferences to the adults. With the larger spiders this is very different, they have no food competition because of this. The animals that serve as prey for the adult spider are often actually enemies of the juvenile spiders because they grow much larger.
From the eggs emerge small larval spiders that resemble a miniature rendition of their parents only after the first molt. The tiny spiders often disperse by spinning a thread of free-hanging cobweb in the air when the breeze rises and allowing themselves to be carried along on the wind. In the Dutch language, this spiderweb is also called autumn thread. A young spider, like all other animals with an exoskeleton, must undergo several molts before it becomes an adult. Small spiders become adults after about five – some tarantulas only after 10 moltings. Molting is often an intense activity for the spider; the animal applies pressure in the abdomen until the skin tears and then works its way backward out of the old skin. During molting, the spider is completely defenseless and the armor is somewhat softer at first.
Many spiders have a strong sexual dimorphism which means that the male and female look different. The male is often much smaller than the female, although this is not the case in all species. In the water spider (Argyroneta aquatica), for example, the male is sometimes actually larger than the female. In many jumping spiders, the males are often the same size as the females. In species from the genus Nephila, however, the males are unsightly small compared to the female. There are even spiders in which the male is so small that it amputates one of its palps to save body weight and thus can move more easily.
In most spiders, the males have a smaller body but proportionately longer legs. In the spider world, it is the males that seek out females, and because spiders cannot detect each other over long distances, males often lead a wandering existence in search of a mate. Males can be recognized in many species by the thickened ends of the palps.
The sexual organs of spiders are positioned on the underside of the abdomen in both males and females. They are located at the front, that is, close to the carapace.
In spiders, the male seeks out a female and thus it is the males that are commonly seen during the reproductive season. The males often travel great distances in search of a mate.
Mating is sometimes a predicament for the male because there is a chance that he will be eaten. This form of cannibalism often does not seem to provide much benefit because males are relatively low in nutritional value. Probably the female only attacks the male when she is in danger of dying of hunger, not out of ordinary hunger. Eating the male after mating has been described in several spiders and also occurs in other animals such as praying mantises. In most species, however, the male is in no danger or usually manages to escape. If a male dies after mating, it is often due to exhaustion and malnutrition during the search for a female rather than the voraciousness of the females.
When a male approaches a female he will first try to eliminate her hunting instinct. Spiders have developed a wide range of methods to accomplish this. The male cross spider quiets the female by strumming against her web in a certain rhythm so she knows he is not prey. In other species, the males simply pounce on the female and try to impregnate her as quickly as possible. Spiders in the genus Pisaura bring the female a present in the form of spun-in prey. In doing so, the male performs rhythmic movements to attract her attention. While the female eats, she is fertilized by the male.
In spiders, a behavior is known in which the male guards the female after mating. This has the advantage for the female that she is protected by the male while her partner thus tries to prevent her from mating with other males.
Some spiders lure the opposite sex by vibrating the body and a number of species are able to produce sounds by rubbing hardened body parts along each other, which is also called stridulation. The most highly developed form of courtship in spiders involves the courtship of jumping spiders. The males use their legs to transmit complex, species-dependent visual signals to the female. This also involves complex vibrations. Imaginative videos of the male seducing the female circulate on the Internet.
The male mates with the female by inserting a pedipalp “loaded” with sperm into her orifice. At the end of the palp is a balloon-like structure called the bulbus. At the end of this is the embolus, a hollow structure through which the sperm is delivered into the female orifice and fertilizes the eggs.
The sperm is stored in a body cavity called the spermatheque, which means something like “sperm chamber. Here it can stay for weeks to months-in exceptional cases up to a year and a half-until the female’s eggs have matured. A female’s two ovaries contain the eggs, which are still soft before fertilization to allow sperm to pass through. Only after the egg is deposited and dries to the outside air does the shell harden.
A spider’s egg is usually deposited in a group in a nest, which is often surrounded by a cocoon. A spider’s eggs are soft and fragile. The eggs are usually pale in color and somewhat oval in shape. The eggs of spiders do not have a hardened shell as found in other invertebrates and are therefore susceptible to desiccation. Spider eggs have a relatively large yolk which allows the embryos to develop completely inside the egg. When the eggs hatch, the young spiders already look somewhat like adults. This is in contrast to other arachnids such as mites that sometimes have a worm-like larval stage.
The number of eggs laid by different spiders varies by species; on average, spiders deposit several hundred eggs. The size of the eggs can vary considerably, as well as their relative size to the female. Females of the species Uroecobius ecribellatus are themselves about 1.8 millimeters long but manage to develop an egg 0.5 millimeters in diameter.
The number of clutches, clutch size and intervals between each clutch can also vary. Species in the genus Nephila can produce more than 1,000 eggs; the spider Idioctis intertidalis, on the other hand, deposits only a single egg. The fishnet spider (Cyrtophora citricola) produces about 1,000 eggs that are deposited in eight to 10 clutches. In this, the first clutches are larger than the last. The previously mentioned species Uroecobius ecribellatus produces up to twenty nests that always contain three or four eggs, and each clutch has an interval of 1 to 2 weeks.
Some spiders develop an egg tooth, a pointed structure that serves to open the egg. This is located on the chelicerae of the spider. In other species, the mother spider helps her young out of the egg, which can be seen as a form of brood care. When the young spider leaves the egg it often molts immediately, leaving the old skin with the egg teeth inside.
Brood care is common in spiders; several forms have been described. In most species, the female guards her eggs. The eggs are placed in a protective cocoon, are hidden in the immediate area or are carried on the body. Other species stick the eggs together and take the globular clump of eggs between their chelicerae. Such species never release their eggs, so they cannot eat until the offspring have hatched. Examples of spiders that take their eggs into their mouthparts are the house-dwelling glue spider and the vibrating spider.
Other species take the egg cocoon under their abdomen, as is known from many wolf spiders. When the eggs hatch, the females carry the young spiders along on the abdomen for some time.
Some spiders, such as the small wedge-winged spider (Phylloneta sisyphia), give up food to their young, indicating their desire to be fed by waving their legs. The juvenile spiders of this species grow much faster as a result.
A far-reaching form of brood care has been described for the species Stegodyphus pacificus, in which the female deposits her eggs in an underground burrow and resides in it herself to guard them. When the spiders crawl out of their eggs she feeds them by giving up food. After a while, the female dies and the young spiders suck her bodily fluids. Because they live in a burrow all this time, the juvenile spiders do not have to hunt for prey and are protected from danger. This increases the chances for the female’s offspring which explains her self-sacrifice.
Juvenile spiders are called nymphs and at first glance are miniature versions of their parents. Spiders have no larval stage but molt, like all other animals with an exoskeleton. Many species molt 5 or 6 times but this is not true of all spiders. Some spiders are biennial and other species take five years to become adults and then live for many years as an imago.
Differences from adults are that the latter are much larger, can reproduce and usually have different colors and patterns. The ratio of head to leg length is also often different.
In venomous spiders, younger animals are often venomous but not yet as strong as the parent animals.
Also, young spiders may not produce spinel until they first molt. In some species of spiders – such as tarantulas – males do not develop a developed bulbus until after the first molt. The young spiders eat their skin after each molt, so it is recycled.
Most young spiders give birth en masse. In some species, the young spiders stay close together in a spherical configuration, for example the young of the cross spider. When disturbed, these young spiders run in all directions but when peace returns they crawl back together. The tiny spiders disperse when the breeze comes up, for example, by spinning a thread of free-hanging cobweb in the air and letting themselves be carried along on the wind by hanging from it.
When disturbed, the ball dissolves and the little spiders run in all directions, but they soon reunite. At first they eat each other until a certain size is reached, usually the first or second molt. Then there are often only a few specimens left; however, these are the strongest of the nest. In some species, the first hatched spiders eat the eggs that have not yet hatched.
When the juvinal spider has grown enough it leaves the nest by letting itself float away on a spindle thread, this is also called Autumn thread. The young spider is not yet safe from conspecifics at this time; many spiders are cannibalistic and eat their own young. In females, the cannibalistic traits are probably “turned off” for a while after the eggs hatch. In many nest-keeping species, they defend the brood with their lives.
Spiders belong to the arthropods, they have an external skeleton of chitin called the exoskeleton. Therefore, spiders cannot grow and must molt regularly. Only just after a molt is the spider’s body soft so it can expand. Molting is also called ecdysis.
Spiders that molt soon become less vigorous and behave lethargically. The spider hides when molt is near and during molt the spider is very vulnerable to enemies as the old armor is shed. After each molt, the spider’s exoskeleton is soft which makes the animal vulnerable. A newly molted spider is additionally pale in color; the pigments in the exoskeleton have yet to color out. Juvenile spiders are often less hairy than adult spiders and usually have different colors.
Juvenile spiders become adults after about five – some tarantulas only after 10 moltings. Molting is often an intensive activity for the spider; the animal applies pressure in the abdomen until the skin tears and then works its way backward out of the old skin. The legs are pulled out of the old skin last. Right after a molt, the spider moves its legs frequently to keep the joints from becoming stiff.
Many spiders are active during the day but there are also many nocturnal species. Some spiders live in permanently dark caves and have no day or night rhythm at all.
Spiders are known to be strictly solitary animals; most species exhibit aggressive behavior toward conspecifics, other spiders and larger animals. However, some more or less social spiders are known, which do not form tight colonies like wasps but tolerate each other in large numbers.
Food and hunting.
Spiders live off living prey but have a wide variety of food preferences and methods of capturing prey. Best known are the wheelweb spiders (Araneidae), which make a round, wheel-like web of a spiraling main thread and many “spokes” that serves to capture small flying prey such as flies and mosquitoes. They make their web in bushes and among leaves at some height. However, there are also spiders that focus on somewhat larger and heavier prey, an example is the wasp spider that makes a spider web that is mainly grasshoppers strictly. The wasp spider therefore makes the web between grass stalks just above the ground. Other spiders make a system of tripwires on the soil and grab a prey as soon as it becomes entangled in it. Trap door spiders get their name from the underground spider-covered dwelling entrance that is closed by a flap. As soon as potential prey walks past the flap, the spider shoots out of its burrow to capture the prey.
Species in the genus Dolomedes are sometimes called “fishing spiders” because of their remarkable hunting methods. These spiders live along the shore and keep their front legs on the surface of the water. When an insect falls into the water, the spider feels the ripples and runs across the surface of the water to the insect that has fallen into the water. The spider can also hunt underwater by grabbing fish that are below the water’s surface. The spider has even been described moving its legs underwater to lure fish that are then grabbed with its chelicerae. Other spiders also hunt in the water but they must always eat their prey on dry land. Indeed, under water their digestive juices would dilute greatly in the water and spiders cannot eat.
A few species of spiders can walk on water and the bank spiders sometimes grab prey such as tadpoles that live in the water. The water spider (Argyroneta aquatica) lives almost permanently underwater but is a rare exception. The spider Arctosa littoralis lives on the beach in the surf which is a dangerous place for a spider. The spider cannot swim and cannot use shelters in its barren habitat. However, the spider is so well camouflaged on a background of sand grains that the animal is virtually invisible.
Bird spiders are often claimed to eat birds, and while this has indeed been observed on occasion, tarantulas never hunt birds; they live off anything they can get their hands on. Usually the prey consists of invertebrates but they can also capture and eat mice, amphibians and even small snakes. Bird spiders are not the only family of large spiders and other spiders do grab smaller vertebrates. The large edged bank spider has been known to capture and eat small frogs.
Spiders are one of the most efficient collectors of insects. Exactly how many they eat is difficult to calculate but that they play an important role in any terrestrial ecosystem is widely recognized. The arachnologist W. S. Bristowe estimated the spider population of a meadow in Sussex at 2.25 million and the total number of spiders in all of England and Wales at 2,200 billion. Since spiders eat more than 100 prey in a year, they would be responsible for clearing more than 200,000 billion insects in the southern part of Britain alone.
A number of species of spiders are known to sometimes eat plant food as well. This often involves pollen rich in protein but nectar, plant sap, leaves and seeds are also consumed. Eating plant parts has been observed in representatives of at least ten different families.
There is only one known exception of a spider that mainly eats plant parts, this is the species Bagheera kiplingi from Central America. This spider lives largely on sweet, ant-like plant secretions of certain species of acacias. Such acacias live in symbiosis with ants, giving the ants nectar and sweet, seed-like secretions in exchange for protection from herbivores. When these attack, the ants fight them fiercely. However, the spider manages to take away the ants’ food. It also drinks nectar from the flowers when it is actually meant for the ants. An additional peculiarity is that the spider eats its food “raw,” all other spiders inject enzymes into their prey after which the half-digested goodness is aspirated. Although the spider’s diet can consist of up to 90% plant parts, it is not a herbivore, sometimes eating ants or their larvae.
Enemies and defenses.
Spiders have a wide range of enemies, ranging from predators, parasites, parasitoids and single-celled pathogens. The main enemies of spiders are a variety of insectivores. Almost all animals called “insectivores” see no difference between prey with six or eight legs and eat spiders just as easily. Examples include reptiles such as lizards, amphibians such as frogs and various birds.
The most imaginative enemies of spiders are spider killers. Spider killers are a family of wasps that belong to the parasitic wasps. However, spider wasps do not eat the spider; like other wasps, they live on nectar themselves. The spider is hidden in a nest after which eggs are deposited in it. The hole is then closed and some time later a new wasp emerges. Its larva has eaten the spider in the nest.
Some species of spiders have specialized in detecting and killing other spiders and other prey are even ignored. These species are called the spider-eaters (Mimetidae), an example being those in the genus Ero. The venom of such species acts specifically on other spiders, often a bite to the victim’s leg is enough to kill it. The spider is then eaten in its own web. Spider-eaters can walk through a spider web of another species without being noticed. Some species even eat the victim’s egg cocoon and then place their own eggs in the cocoon. One group that has similar behavior are the thief spiders. These do not hunt the web-building spiders but specialize in stealing the prey from the web. An example are species from the genus Argyrodes.
Web-building spiders often hide in plants and when disturbed they “drop” where the spider, however, anchors itself to the substrate and lowers itself on a wire. During the descent, the spider retracts its legs to be less conspicuous. Once the danger has passed, the spider can climb back up through the wire. The sand-dune-dwelling spider Carparachne aureoflava is known to fold up like a ball and roll down at relatively high speeds. The spider exhibits this behavior when confronted by a spider killer.
Spiders often exhibit threatening behavior consisting of raising their legs and showing chelicerae. A number of species have brightly colored undersides of the legs ditto cheliceren so they stand out well. Larger spiders are often capable of hissing. A number of spiders have an active form of defense when attacked. Larger tarantulas brush the hairs of their abdomen with their legs after which they become airborne. The hairs are fire hairs that strongly excite the mucous membranes. Species have also been known to point their abdomen at an enemy and shoot a jet of body fluid at it.
Spiders and humans
Spiders have their looks against them; they are hairy, have long hairy legs and move quickly. In addition, they often emerge unexpectedly from dark corners, sometimes confronting a human with a hairy, long-legged animal that moves quickly or even displays the chelicerae.
In fact, it is just the other way around; a spider sits quietly in the dark but is suddenly disturbed by light and a large “animal” that is too big to eat and thus is considered an enemy. Not infrequently, a human also behaves as an enemy by attempting to kill the spider. A spider will usually run away or show threatening behavior.
Spiders that grow a bit larger, such as some tarantulas, can bite with their large jaws and because of this, spiders are feared animals. They are used in culture as a metaphor for everything scary and bad. For many people, a confrontation with a spider triggers a startle response, which comes from a reflex and is therefore normal. People who are very afraid of spiders suffer from arachnophobia or arachnophobia. Spiders are also kept as pets in a terrarium to admire them. In some Asian countries, spiders are caught, fried and then eaten.
It is sometimes claimed of spiders that they are swallowed while sleeping. However, this is a sandwich story because a spider is unable to move down the esophagus, let alone walk through it. Also, the human body is very warm for a spider. In addition, spiders’ legs are equipped with many spines so it is also physically impossible to move through the esophagus without becoming entangled in the process.