Spiders (order Araneae) are air-breathing arthropods that have eight legs, chelicerae with fangs generally able to inject venom, and spinnerets that extrude silk. They are the largest order of arachnids and rank seventh in total species diversity among all orders of organisms. Spiders are found worldwide on every continent except for Antarctica, and have become established in nearly every land habitat. As of August 2022, 50,356 spider species in 132 families have been recorded by taxonomists. However, there has been debate among scientists about how families should be classified, with over 20 different classifications proposed since 1900.
Anatomically, spiders (as with all arachnids) differ from other arthropods in that the usual body segments are fused into two tagmata, the cephalothorax or prosoma, and the opisthosoma, or abdomen, and joined by a small, cylindrical pedicel. However, as there is currently neither paleontological nor embryological evidence that spiders ever had a separate thorax-like division, there exists an argument against the validity of the term cephalothorax, which means fused cephalon (head) and the thorax. Similarly, arguments can be formed against use of the term abdomen, as the opisthosoma of all spiders contains a heart and respiratory organs, organs atypical of an abdomen.
Spiders have developed several different respiratory anatomies, based on book lungs, a tracheal system, or both. Mygalomorph and Mesothelae spiders have two pairs of book lungs filled with haemolymph, where openings on the ventral surface of the abdomen allow air to enter and diffuse oxygen. This is also the case for some basal araneomorph spiders, like the family Hypochilidae, but the remaining members of this group have just the anterior pair of book lungs intact while the posterior pair of breathing organs are partly or fully modified into tracheae, through which oxygen is diffused into the haemolymph or directly to the tissue and organs.The tracheal system has most likely evolved in small ancestors to help resist desiccation.The trachea were originally connected to the surroundings through a pair of openings called spiracles, but in the majority of spiders this pair of spiracles has fused into a single one in the middle, and moved backwards close to the spinnerets. Spiders that have tracheae generally have higher metabolic rates and better water conservation.Spiders are ectotherms, so environmental temperatures affect their activity.
Spiders range in body length from 0.5 to about 90 mm (0.02–3.5 inches). The largest spiders are the hairy mygalomorphs, commonly referred to as tarantulas, which are found in warm climates and are most abundant in the Americas. Some of the largest mygalomorphs include the goliath bird-eating spider (Theraphosa leblondi or T. blondi), found in parts of the Amazon, and the pinkfoot goliath (T. apophysis), limited to southern Venezuela. The smallest spiders belong to several families found in the tropics, and information about them first became known in the 1980s.
Anatomically, spiders (as with all arachnids) differ from other arthropods in that the usual body segments are fused into two tagmata, the cephalothorax or prosoma, and the opisthosoma, or abdomen, and joined by a small, cylindrical pedicel. However, as there is currently neither paleontological nor embryological evidence that spiders ever had a separate thorax-like division, there exists an argument against the validity of the term cephalothorax, which means fused cephalon (head) and the thorax. Similarly, arguments can be formed against use of the term abdomen, as the opisthosoma of all spiders contains a heart and respiratory organs, organs atypical of an abdomen.
Their abdomens bear appendages that have been modified into spinnerets that extrude silk from up to six types of glands. Spider webs vary widely in size, shape and the amount of sticky thread used. It now appears that the spiral orb web may be one of the earliest forms, and spiders that produce tangled cobwebs are more abundant and diverse than orb-weaver spiders. Spider-like arachnids with silk-producing spigots (Uraraneida) appeared in the Devonian period about 386 million years ago, but these animals apparently lacked spinnerets. True spiders have been found in Carboniferous rocks from 318 to 299 million years ago, and are very similar to the most primitive surviving suborder, the Mesothelae. The main groups of modern spiders, Mygalomorphae and Araneomorphae, first appeared in the Triassic period, before 200 million years ago.
Female spiders generally are much larger than males, a phenomenon known in animals as sexual size dimorphism. Many female orb weavers, such as those in the families Tetragnathidae and Araneidae, show extreme size dimorphism, being at least twice the size of males of the same species. The extreme difference in body size appears to have arisen through selection processes favouring fecundity in females and “bridging” locomotion in males. Bridging is a technique used by spiders for orb web construction; the spider produces a silk thread that is carried by the wind and becomes attached to an object, forming a bridge. Small, light males can build and traverse silk bridges more rapidly than larger, heavier males can. Scientists suspect that this gives small males more mating opportunities, thereby favouring selection for their small size.
Although many spiders produce venom for use in capturing prey, few species are toxic to humans. The venom of the black widow (genus Latrodectus) acts as a painful nerve poison. The bite of the brown recluse and others of the genus Loxosceles may cause localized tissue death. Other venomous spiders include the tarantula-like funnel-web spider (genus Atrax) of southeastern Australia and some African members (baboon spiders) of the family Theraphosidae of Africa and South America. In North America Cheiracanthium mildei, a small, pale spider introduced from the Mediterranean, and the native Cheiracanthium inclusum may enter houses in late fall and are responsible for some bites. Occasionally tissue death at the site of the bite occurs. Some American tarantulas throw off abdominal hairs as a defense against predators. The hairs have tiny barbs that penetrate skin and mucous membranes and cause temporary itching and allergic reactions.
Their abdomens bear appendages that have been modified into spinnerets that extrude silk from up to six types of glands. Spider webs vary widely in size, shape and the amount of sticky thread used. It now appears that the spiral orb web may be one of the earliest forms, and spiders that produce tangled cobwebs are more abundant and diverse than orb-weaver spiders. Spider-like arachnids with silk-producing spigots (Uraraneida) appeared in the Devonian period about 386 million years ago, but these animals apparently lacked spinnerets. True spiders have been found in Carboniferous rocks from 318 to 299 million years ago, and are very similar to the most primitive surviving suborder, the Mesothelae. The main groups of modern spiders, Mygalomorphae and Araneomorphae, first appeared in the Triassic period, before 200 million years ago.
All spiders are predators. Because of their abundance, they are the most important predators of insects. Spiders have been used to control insects in apple orchards in Israel and rice fields in China. Large numbers of spiders have also been observed feeding on insects in South American rice fields and in fields of various North American crops. Modern pest-management strategies emphasize the use of insecticides that do the least damage to natural predators of insect pests.
Although many spiders produce venom for use in capturing prey, few species are toxic to humans. The venom of the black widow (genus Latrodectus) acts as a painful nerve poison. The bite of the brown recluse and others of the genus Loxosceles may cause localized tissue death. Other venomous spiders include the tarantula-like funnel-web spider (genus Atrax) of southeastern Australia and some African members (baboon spiders) of the family Theraphosidae of Africa and South America. In North America Cheiracanthium mildei, a small, pale spider introduced from the Mediterranean, and the native
Certain species of orb weavers (Araneidae), tarantulas (Theraphosidae), and huntsman spiders (Sparassidae) and members of family Nephilidae are suspected predators of bats, especially species of vesper bats (family Vespertilionidae) and sheath-tailed bats (family Emballonuridae). Birds have also been known to become trapped in spider webs, and in some instances spiders have been observed feeding on birds. These reports have led scientists to propose that flying vertebrates may be an important source of prey for certain species of spiders.
The bodies of spiders, like those of other arachnids, are divided into two parts, the cephalothorax (prosoma) and the abdomen (opisthosoma). The legs are attached to the cephalothorax, which contains the stomach and brain. The top of the cephalothorax is covered by a protective structure, the carapace, while the underside is covered by a structure called the sternum, which has an anterior projection, the labium. The abdomen contains the gut, heart, reproductive organs, and silk glands. Spiders (except the primitive suborder Mesothelae) differ from other arachnids in lacking external segmentation of the abdomen and in having the abdomen attached to the cephalothorax by a narrow stalk, the pedicel. The gut, nerve cord, blood vessels, and sometimes the respiratory tubules (tracheae) pass through the narrow pedicel, which allows the body movements necessary during web construction. Among arachnids other than spiders, the tailless whip scorpions (order Amblypygi) have a pedicel but lack spinnerets. Spiders, like other arthropods, have an outer skeleton (exoskeleton). Inside the cephalothorax is the endosternite, to which some jaw and leg muscles are attached.
Some spiders have a cribellum, a modified spinneret with up to 40,000 spigots, each of which produces a single very fine fiber. The fibers are pulled out by the calamistrum, a comblike set of bristles on the jointed tip of the cribellum, and combined into a composite woolly thread that is very effective in snagging the bristles of insects. The earliest spiders had cribella, which produced the first silk capable of capturing insects, before spiders developed silk coated with sticky droplets. However, most modern groups of spiders have lost the cribellum.[13]
Even species that do not build webs to catch prey use silk in several ways: as wrappers for sperm and for fertilized eggs; as a "safety rope"; for nest-building; and as "parachutes" by the young of some species.
As with other arthropods, spiders' cuticles would block out information about the outside world, except that they are penetrated by many sensors or connections from sensors to the nervous system. In fact, spiders and other arthropods have modified their cuticles into elaborate arrays of sensors. Various touch sensors, mostly bristles called setae, respond to different levels of force, from strong contact to very weak air currents. Chemical sensors provide equivalents of taste and smell, often by means of setae.[25] An adult Araneus may have up to 1,000 such chemosensitive setae, most on the tarsi of the first pair of legs. Males have more chemosensitive bristles on their pedipalps than females. They have been shown to be responsive to sex pheromones produced by females, both contact and air-borne.[30] The jumping spider Evarcha culicivora uses the scent of blood from mammals and other vertebrates, which is obtained by capturing blood-filled mosquitoes, to attract the opposite sex. Because they are able to tell the sexes apart, it is assumed the blood scent is mixed with pheromones.[31] Spiders also have in the joints of their limbs slit sensillae that detect force and vibrations. In web-building spiders, all these mechanical and chemical sensors are more important than the eyes, while the eyes are most important to spiders that hunt actively.[13]
Like most arthropods, spiders lack balance and acceleration sensors and rely on their eyes to tell them which way is up. Arthropods' proprioceptors, sensors that report the force exerted by muscles and the degree of bending in the body and joints, are well-understood. On the other hand, little is known about what other internal sensors spiders or other arthropods may have.
Eight eyes (E) are typically placed in two rows, on the front of the carapace. The rows are described as anterior (A) or posterior (P) and then the position within the row is lateral (L) or median (M). The AME or direct eyes, differ markedly in structure from the other indirect eyes (ALE, PLE, PME). The direct eyes appear dark, whereas the indirect eyes usually have a layer of light reflecting crystals, the tapetum, behind the light sensitive retina, giving these eyes a silvery appearance. The tapetum increases visual sensitivity because light entering the light sensitive retinal cells is immediately reflected back through them, so intensifying the image. These indirect eyes are adapted for seeing at low light intensities and their lenses are often enlarged in spiders with good vision. Spider eye lenses are better than photographic lenses in terms of their image brightness (very low F-numbers). However, because most spider eye retinas have relatively coarse-grained mosaics of receptor cells, their resolution of these images is much poorer than in the human eye.
The abdomen has no appendages except those that have been modified to form one to four (usually three) pairs of short, movable spinnerets, which emit silk. Each spinneret has many spigots, each of which is connected to one silk gland. There are at least six types of silk gland, each producing a different type of silk.[13] Spitting spiders also produce silk in modified venom glands.Some spiders have a cribellum, a modified spinneret with up to 40,000 spigots, each of which produces a single very fine fiber. The fibers are pulled out by the calamistrum, a comblike set of bristles on the jointed tip of the cribellum, and combined into a composite woolly thread that is very effective in snagging the bristles of insects. The earliest spiders had cribella, which produced the first silk capable of capturing insects, before spiders developed silk coated with sticky droplets. However, most modern groups of spiders have lost the cribellum.[13]
Even species that do not build webs to catch prey use silk in several ways: as wrappers for sperm and for fertilized eggs; as a "safety rope"; for nest-building; and as "parachutes" by the young of some species.
Silkworm silk is mainly known as a luxurious textile. Spider silk is an alternative to silkworm silk fibers and has much more outstanding properties. Silk diversity ensures variation in its application in nature and industry. This review aims to provide a critical summary of up-to-date fabrication methods of spider silk-based organic-inorganic hybrid materials. This paper focuses on the relationship between the molecular structure of spider silk and its mechanical properties. Such knowledge is essential for understanding the innate properties of spider silk as it provides insight into the sophisticated assembly processes of silk proteins into the distinct polymers as a basis for novel products. In this context, we describe the development of spider silk-based hybrids using both natural and bioengineered spider silk proteins blended with inorganic nanoparticles. The following topics are also covered: the diversity of spider silk, its composition and architecture, the differences between silkworm silk and spider silk, and the biosynthesis of natural silk. Referencing biochemical data and processes, this paper outlines the existing challenges and future outcomes.
This paper focuses solely on spider silk-based hybrids. Therefore, the structure and properties of silkworm silk, as well as those hybrids based on it are not the subject of this review. Differences between structure and material performance regarding silkworm and spider silk are explored, however, to provide a deeper understanding of silk backbone performance in the fabrication of silk-based functional materials. Namely, their organization at the molecular level, interactions to form secondary structures, and various mechanical properties are highlighted. Challenges and approaches to the large-scale production of spider silk-based materials for numerous applications are also reviewed. Additionally, the high complexity of spider silk organization, and the tunability of its properties are revealed. Recent advances and emerging strategies concerning the fabrication and applications of natural or bioengineered spider silk–inorganic nanoparticle hybrid materials are described. Lastly, this paper concludes with the prospects of hybrid spider silk-based materials.
There is strong evidence that spiders' coloration is camouflage that helps them to evade their major predators, birds and parasitic wasps, both of which have good color vision. Many spider species are colored so as to merge with their most common backgrounds, and some have disruptive coloration, stripes and blotches that break up their outlines. In a few species, such as the Hawaiian happy-face spider, Theridion grallator, several coloration schemes are present in a ratio that appears to remain constant, and this may make it more difficult for predators to recognize the species. Most spiders are insufficiently dangerous or unpleasant-tasting for warning coloration to offer much benefit. However, a few species with powerful venom, large jaws or irritant bristles have patches of warning colors, and some actively display these colors when threatened.
Silkworm silk is mainly known as a luxurious textile. Spider silk is an alternative to silkworm silk fibers and has much more outstanding properties. Silk diversity ensures variation in its application in nature and industry. This review aims to provide a critical summary of up-to-date fabrication methods of spider silk-based organic-inorganic hybrid materials. This paper focuses on the relationship between the molecular structure of spider silk and its mechanical properties. Such knowledge is essential for understanding the innate properties of spider silk as it provides insight into the sophisticated assembly processes of silk proteins into the distinct polymers as a basis for novel products. In this context, we describe the development of spider silk-based hybrids using both natural and bioengineered spider silk proteins blended with inorganic nanoparticles. The following topics are also covered: the diversity of spider silk, its composition and architecture, the differences between silkworm silk and spider silk, and the biosynthesis of natural silk. Referencing biochemical data and processes, this paper outlines the existing challenges and future outcomes.
This paper focuses solely on spider silk-based hybrids. Therefore, the structure and properties of silkworm silk, as well as those hybrids based on it are not the subject of this review. Differences between structure and material performance regarding silkworm and spider silk are explored, however, to provide a deeper understanding of silk backbone performance in the fabrication of silk-based functional materials. Namely, their organization at the molecular level, interactions to form secondary structures, and various mechanical properties are highlighted. Challenges and approaches to the large-scale production of spider silk-based materials for numerous applications are also reviewed. Additionally, the high complexity of spider silk organization, and the tunability of its properties are revealed. Recent advances and emerging strategies concerning the fabrication and applications of natural or bioengineered spider silk–inorganic nanoparticle hybrid materials are described. Lastly, this paper concludes with the prospects of hybrid spider silk-based materials.
Spiders occur in a large range of sizes. The smallest, Patu digua from Colombia, are less than 0.37 mm (0.015 in) in body length. The largest and heaviest spiders occur among tarantulas, which can have body lengths up to 90 mm (3.5 in) and leg spans up to 250 mm (9.8 in).Female spiders generally are much larger than males, a phenomenon known in animals as sexual size dimorphism. Many female orb weavers, such as those in the families Tetragnathidae and Araneidae, show extreme size dimorphism, being at least twice the size of males of the same species. The extreme difference in body size appears to have arisen through selection processes favouring fecundity in females and “bridging” locomotion in males. Bridging is a technique used by spiders for orb web construction; the spider produces a silk thread that is carried by the wind and becomes attached to an object, forming a bridge. Small, light males can build and traverse silk bridges more rapidly than larger, heavier males can. Scientists suspect that this gives small males more mating opportunities, thereby favouring selection for their small size.
Female spiders generally are much larger than males, a phenomenon known in animals as sexual size dimorphism. Many female orb weavers, such as those in the families Tetragnathidae and Araneidae, show extreme size dimorphism, being at least twice the size of males of the same species. The extreme difference in body size appears to have arisen through selection processes favouring fecundity in females and “bridging” locomotion in males. Bridging is a technique used by spiders for orb web construction; the spider produces a silk thread that is carried by the wind and becomes attached to an object, forming a bridge. Small, light males can build and traverse silk bridges more rapidly than larger, heavier males can. Scientists suspect that this gives small males more mating opportunities, thereby favouring selection for their small size.
Giant huntsman spider (12-inch leg span)
Goliath birdeater (11-inch leg span)
Brazilian salmon pink birdeater (10-inch leg span)
Brazilian giant tawny red tarantula (10-inch leg span)
Face-size tarantula (8-inch leg span)
Hercules baboon spider (7.9-inch leg span)
Colombian giant redleg tarantula (7-inch leg span)
