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Spider

Araneae

Spiders, classified under Araneae (the largest order of Arachnids), represent a diverse group of air-breathing arthropods known for their distinctive anatomical features. They occupy a significant position within the animal kingdom with their characteristic eight limbs, chelicerae equipped with venomous fangs, and specialized spinnerets capable of producing silk.

Widespread across the globe, spiders have established themselves in almost every terrestrial habitat, ranging from dense tropical rainforests to arid desert landscapes on all continents except Antarctica. According to the World Spider Catalog, about 51,673 species are documented across 135 families, ranking seventh in the total species diversity among all scientific orders.

Description

Size

Spiders exhibit a remarkable diversity in size, ranging from the tiniest Patu digua, measuring less than 0.37 mm (0.015 inches) in body length, to the largest tarantula, Theraphosa blondi (Goliath Birdeater), measuring up to 13 cm (5.1 inches).

Coloration

Three major biological pigments, ommochromes, bilins, and guanine, add a range of colors to spiders worldwide.

While bilins result in the green coloration of Micrommata virescent, the white markings of the European garden spider (Araneus diadematus) are due to the presence of guanine in granulocytes (specialized storage cells). In certain spider genera like Tetragnatha, Leucauge, Argyrodes, or Theridiosoma, guanine also creates a silvery appearance.

Structural colors, developing from the interaction of light with spider body parts, are observed in some species. These colorations occur due to diffraction, scattering, and light interference, often involving modified setae or scales. For example, the reflective bristles on the white prosoma of Argiope and the modified cuticle areas in Lycosa and Josa act as light reflectors. Notably, male peacock spiders from Australia, belonging to the genus Maratus, are a vibrant show of structural colors.

While many spiders maintain a fixed color throughout their lives, some groups exhibit variable coloration in response to environmental pressures, internal factors, and dietary choices. For instance, in Theridion grallator, the abdomen changes to orange when the spider consumes certain species of Diptera and adult Lepidoptera but turns green when it consumes Homoptera or larval Lepidoptera species.

Environmental factors also induce color changes that are either morphological (occurring over several days) and involve pigment synthesis and degradation or physiological (occurring almost instantly), which involve shifting pigment-containing cells. An example of morphological color change is background matching, where spiders like Misumena vatia adjust their body color to camouflage with their surroundings, making them less visible to their prey. Similarly, physiological color change is observed in Cyrtophora cicatrosa, a spider that instantly shifts from white to brown.

Body Plan

Like all arthropods, spiders have segmented bodies with jointed limbs covered in a tough cuticle made of chitin and proteins. Their heads are composed of several segments that fuse during embryonic development.

As chelicerates, their bodies are divided into two segments (tagmata) that serve similar functions. The front part, known as the cephalothorax or prosoma, is a fusion of segments that, in insects, would form separate head and thorax regions. The rear section is called the abdomen or opisthosoma. The cephalothorax and abdomen are linked in spiders by a small cylindrical section called the pedicel.

Of all arthropods, the pattern of segment fusion is distinct in chelicerates, like spiders, where the first head segment, which would typically develop into antennae in other arthropods, disappears early in development, resulting in chelicerates lacking antennae. Instead, chelicerates have a pair of chelicerae as their only appendages ahead of the mouth and lack true ‘jaws’ in themselves. Following the mouth, the first pair of appendages, called pedipalps, serve various functions across different chelicerate groups.

A spider’s chelicerae consist of two upper sections culminating in venomous fangs. The upper sections of chelicerae often feature thick ‘beards’ that help filter solid particles from their food since spiders only consume liquids.

Locomotor Organs

Like many arthropods, spiders have intricate limb structures. Each of their eight legs consists of seven main segments:

  1. The coxa is closest to the spider’s body (cephalothorax) that attaches the leg to it.
  2. The trochanter is the short segment following the coxa that acts as a hinge for the rest of the five segments.
  3. The femur is a long segment next to the trochanter and is crucial for leg movement.
  4. The patella, commonly known as the ‘knee’ of the spider’s leg, acts as a hinge for the following three segments.
  5. The tibia follows the patella and connects it to the metatarsus.
  6. The metatarsus is the segment that connects the tibia to the tarsus.
  7. The tarsus is the last segment of the spider’s leg that ends in a claw. Depending on the spider’s family, the claw can have two or three pointed ends.

For movement, spiders use a combination of muscles and hydraulic pressure.

Physiology

Circulatory System

Spiders are coelomates containing hemocoel, a large cavity covering most of the body’s length and allowing blood to flow through it. The heart, a tube-shaped structure in the upper part of the abdomen, contains openings (ostia) along its sides that allow blood to enter from the hemocoel but prevent its return until the blood reaches the front end of the heart. The purified blood is discharged into the hemocoel through one artery opening at the posterior end of the abdomen and also by branching arteries passing through the pedicle and entering the cephalothorax. 

Thus, spiders’ circulatory system is open and better developed in species with book lungs than those with bundles of tracheae extending to various body parts. Spiders with book lungs also contain hemocyanin, a respiratory pigment that aids in the efficient transport of oxygen.

Respiratory System

These arthropods have book lungs, trachea, or both, facilitating gas exchange. While Mygalomorph and Mesothelae spiders have book lungs, only small ancestors have evolved tracheal systems. Most commonly found are the book lungs, which are paired organs composed of 10 to 80 hollow leaves that extend into a blood sinus separated by tiny, hardened columns. The less common tracheal units are tubes originally connected to the surroundings through openings called spiracles, but in most spiders, they have fused into a single one close to the abdomen.

It is found that spiders containing trachea have higher metabolic rates and conserve water better than the ones with book lungs.

Digestive System

Like most arachnids, spiders have a narrow gut with two filters to allow only liquified food to pass. The bases of the chelicerae and pedipalps form a distinct pre-oral cavity for holding the food while processing.

Broadly, there are two systems for their external digestion. While some spiders pump digestive enzymes from the midgut into the prey and then suck its liquified tissues, many grind the prey using their feeding apparatus while flooding it with enzymes. After the pulp has moved into the stomach, it is pumped deeper into the digestive system for further digestion. The midgut contains digestive compartments called ‘ceca’ that finally extract or assimilate the nutrients from the food consumed.

Excretory System

Most spiders are uricotelic, converting nitrogenous waste into uric acid, which is then excreted as dry material. The waste is extracted from the hemocoel by little tubules called ‘Malpighian tubules’ and dumped into the cloacal chamber, from which it is released through the anus.

Though expelling waste in the form of uric acid has evolved in terrestrial spiders as an adaptive strategy to conserve water, some primitive spiders have not adopted this method. Specifically, spiders belonging to the Mesothelae and Mygalomorphae groups have retained the ancestral nephridia, which use large amounts of water to excrete waste as ammonia.

Nervous System

The basic arthropod nervous system consists of nerve cords running below the gut and paired ganglia acting as local control centers in all segments. The brain is formed by the fusion of ganglia for the head segments ahead and behind the mouth, resulting in the esophagus being encircled by this conglomeration of ganglia.

Spiders, however, have a more centralized nervous system than arthropods. All ganglia of segments behind the esophagus are fused in them, so the cephalothorax is mostly filled with nervous tissue. In Mesothelae, the abdominal ganglia and the cephalothorax’s rear part remain separate, reflecting a less centralized arrangement.

Sense Organs

Spiders typically possess four pairs of eyes located on the top-front area of the cephalothorax. Most arthropods have a primary pair of ocelli, also known as pigment-cup ocelli, that only detect light direction using shadows cast by the cup’s walls. However, in spiders, they are also capable of forming the image.

Unlike the primary pair of eyes, the secondary pair detects light reflected from the tapetum lucidum, a reflective layer immediately beneath the retina. The primary and secondary pairs differ in structural variations and functionalities. Unlike the primary pair, the secondary pair lacks eye muscles and is immobile. The latter also contains light-sensitive rhabdomeres, pointing away from the incoming light source. 

The visual acuity of some spider varieties is even better than most insects with excellent vision. This heightened sensitivity is achieved through the following:

Many species of spiders have a reduced number of eyes. For example, Periegops suterii have six eyes, with a pair absent on the anterior median line, whereas members of the Caponiidae family have as few as two. Again, some cave-dwelling spiders lack eyes or possess vestigial eyes incapable of vision.

Other Senses

Like all arthropods, spiders have adapted their cuticles into sophisticated sensory arrays that help them gather information from their surroundings. 

  1. Tactile sensors, primarily in bristles or setae, detect a range of forces, from strong contact to very subtle air currents.
  2. Chemical sensors, often located on setae, provide them with taste and smell. For instance, adult Araneus spiders have up to 1,000 chemosensitive setae, mostly located on the tarsi of their first pair of legs. Male spiders typically have more chemosensitive bristles on their pedipalps than females, as they are responsive to female sex pheromones.
  3. Scents are used by certain spider species, like the jumping spider Evarcha culicivora, to attract potential mates. They utilize the smell of blood from mammals and other vertebrates (obtained by capturing blood-filled mosquitoes) to seduce the opposite sex. This behavior suggests that the scent of blood is mixed with pheromones, allowing spiders to distinguish between sexes.
  4. Slit sensilla in the joints of their limbs help spiders detect prey movements or disturbances in their surroundings.

In web-building spiders, these mechanical and chemical sensors are often more important than their eyes, whereas spiders that actively hunt rely more heavily on their vision. The diversity of sensory reception demonstrates the remarkable adaptability of spiders in utilizing various sensory modes to navigate their environments effectively.

Taxonomy

Araneae are a monophyletic group, representing a recent common ancestor and all its descendants. Despite ongoing debates about their closest relatives, spiders possess several unique evolutionary traits distinguishing them from other arachnids.

Unlike most chelicerates that bear features of the ancestral arthropod feeding system, spiders lack backward-pointing mouths and gnathobases (jaw-like modifications at the bases of their legs.) Moreover, they are distinguished from other Arachnid groups by the presence of spinnerets and modified pedipalps used for transferring sperms.

According to the World Spider Catalog, Araneae is broadly divided into two suborders and 135 families.

Types of Spiders

Distribution

Spiders have a global distribution found on almost every continent except Antarctica.

Habitat

They inhabit many terrestrial habitats, from tropical rainforests to deserts, grasslands, forests, wetlands, and even artificial structures like houses, sheds, and gardens.

Diet

Although spiders are primarily carnivorous, the jumping spider, Bagheera kiplingi, often acquires most of its nutrition from solid plant matter. Some juveniles from the families Anyphaenidae, Corinnidae, Clubionidae, Thomisidae, and Salticidae often feed on plant nectar containing sugar, amino acids, lipids, vitamins, and minerals. Studies suggest that spider species that do not otherwise maintain a herbivorous diet live longer when fed with nectar.

Some spider species also scavenge on dead arthropods and their shed exoskeletons. They also feed on web silk and pollen grains in the wild, whereas in captivity, they eat bananas, marmalade, milk, egg yolk, and sausages.

Behavior

Silk Production

Spiders spin silk with the help of one to four (usually three) pairs of specialized spinning organs called ‘spinnerets.’ These organs emit silk through multiple ‘spigots,’ each connected to a distinct silk gland. At least six types of such glands were discovered, each producing a different kind of silk. Venom glands are sometimes modified to produce silk, like in a few species of spitting spiders.

Some spiders possess a ‘cribellum,’ a modified spinneret with numerous spigots (up to 40,000), each producing a single very fine fiber. These fibers are pulled out using the ‘calamistrum,’ a comblike set of bristles on the jointed tip of the cribellum, and assembled into a wooly thread effective for catching prey. Silk is also used as a ‘wrapper’ to protect their sperm and fertilized eggs, for nest-building, and as a ‘parachute’ for the young.

Socialization

Some spider species exhibit social behaviors, forming colonies with varying degrees of complexity. Anelosimus eximius, for example, can form colonies of up to 50,000 individuals, while sociality is observed across all known species in the Americas and, to some extent, those in Madagascar. 

Communal behaviors are also seen in Philoponella species (Uloboridae), Agelena consociata (Agelenidae), Mallos gregalis (Dictynidae), and the herbivorous Bagheera kiplingi. Even the cannibalistic widow spiders of the Latrodectus genus have been observed forming colonies in captivity, sharing webs, and cooperating in feeding.

Web Making

Spiders prepare diverse webs that help in capturing prey, egg-laying, and mating. Members of the family Araneidae build orb webs, characterized by a circular shape with radiating lines, while Theridiidae spiders make irregular three-dimensional cobwebs. Some more complex webs, like funnel webs, are common among the families Agelenidae and Linyphiidae. Additionally, water spiders of the species Argyroneta aquatica build striking underwater ‘diving bell’ webs filled with air for prey capture and mating.

Capturing Prey

Spiders employ diverse hunting techniques to capture prey, often utilizing their skilfully constructed sticky webs.

Lifespan

Though most spiders live for only one or two years, some tarantula species can survive over 20 years in captivity. An Australian female trapdoor spider was recorded to live 43 years in the wild.

Reproduction and Life Cycle

Spiders undergo sexual reproduction, and fertilization is internal. However, the mode of sperm transfer is indirect, and the sperm is not inserted directly into the female’s body by the male’s genitals but through an intermediate stage.

Unlike most terrestrial arthropods that produce pre-packaged spermatophores, male spiders create small sperm webs onto which they deposit their ejaculate. They then transfer the sperm to specialized syringe-like structures called ‘palpal bulbs’ found on the tips of the pedipalps of the males. On identifying a receptive female, males may engage in courtship rituals to attract her and initiate copulation.

There are different patterns of courtship rituals that the males perform to attract the large females, especially if they tend to prey on smaller males before fertilization. In web-weaving spiders, the patterns of vibrations in the web form an integral part of the mating ritual, whereas in spiders that actively hunt, courtship is characterized by the patterns of touch on the female body. Jumping spider males engage in elaborate jumps and dance to attract their potential mates. If the courtship is successful, the males inject their sperm from the palpal bulbs into the female through the openings on the underside of her abdomen.

There are two distinct parts in the female reproductive tract of a spider:

  1. Haplogyne or non-entelegyne is the ancestral arrangement consisting of a single genital opening, leading to two seminal receptacles or ‘spermathecae’ where females store sperm.
  2. Entelegyne is the advanced arrangement with two further openings leading directly to the spermathecae, creating a ‘flow-through’ system rather than a ‘first-in-first-out’ one.

Eggs are primarily fertilized during oviposition, as the stored sperm is released from the spermathecae. However, in species like Parasteatoda tepidariorum, the females activate the dormant sperm before oviposition, enabling them to migrate to the ovarian cavity for fertilization.

The only known instance of direct fertilization in spiders is of an Israeli spider, Harpactea sadistica, in which the male penetrates his pedipalps through the female’s body wall and injects his sperm directly into her ovaries (traumatic insemination.)

The females lay up to 3,000 eggs in silk egg sacs, maintaining a relatively constant humidity level for the growth of the eggs. While some female spiders die after giving birth, many protect the sacs by attaching them to their webs and hiding them in spinnerets and chelicerae.

All spiders pass through their larval stages in the egg sacs and emerge as tiny spiderlings, yet to mature sexually but similar in shape to adults. Some spiders provide parental care, as in the wolf spiders that cling to their mother’s back, while the mother also reciprocates their ‘begging’ behavior by regurgitating food into their mouth.

Like other arthropods, spiders cannot stretch their exoskeleton and thus need to molt into adulthood. In some spiders, males are found to mate with freshly molted females, which are too young to threaten the males.

Surprisingly enough, males of the genus Tidarren often amputate one of their palps before maturation and enter adulthood with one palp. For about four hours, the remaining palp of the Yemeni species Tidarren argo remains attached to the female’s external genital structure (epigynum) and functions independently.

Predators

Spiders have several natural predators, depending on their size and habitat. 

Adaptations

Cryptic Coloration

The exoskeletal colors of spiders help them camouflage against the surroundings and thus escape the eyes of birds and wasps. Depending on the background color, Thomisus onustus, a crab spider, can switch between white, yellow, and pink, while some spiders also have stripes and blotches on their bodies that break up their outlines against their backgrounds. Moreover, a few spider species with venom, large jaws, or irritant bristles often display warning color patches when attacked by an enemy.

Mimicking

A striking example of mimicry is observed in ant-mimicking spiders. They develop slimmer abdomens and false ‘waists’ in the cephalothorax to mimic the discrete body regions of ants. These spiders wave their first pair of legs in front of their heads to imitate the ants’ antennae and disguise their eight legs. Large color patches around one pair of eyes help to conceal their simple eyes, while reflective bristles cover their bodies, resembling the shiny appearance of ants.

Ant-mimicking spiders adjust their behavior to match the target ant species. Many of these spiders adopt a zig-zag movement pattern, while spiders of the genus Synemosyna walk on the outer edges of leaves in the same way as Pseudomyrmex. This mimicry may protect them from predators that hunt visually, such as birds, lizards, and other spiders.

Presence of Urticating Bristles

Many tarantulas that belong to the family Theraphosidae possess specialized ‘urticating hairs’ on their abdomens as a defense mechanism. These urticating hairs are specialized setae with fragile bases and barbed tips. While these hairs don’t contain venom, they can cause irritation and discomfort to predators or perceived threats. The barbs on the hair strands can embed themselves in the skin or mucous membranes of the predator, leading to itching, burning sensations, or even allergic reactions in some cases.

Presence of Scopulae

Spiders that hunt actively bear scopulae, the dense tufts of fine bristles found between the paired claws at the tips of their legs. These specialized structures enable them to navigate challenging terrains and easily exploit vertical surfaces. Composed of bristles with ends split into as many as 1,000 branches, these scopulae give spiders remarkable adhesion power.

Web Modifications 

Some species of spiders, including certain orb-weavers (such as those in the genus Argiope), have been observed incorporating robust threads into their webs as a defense mechanism against predators like wasps. These specialized threads are called ‘barrier’ or ‘signal’ threads.

When a wasp attempts to approach the spider’s web, it may encounter these strong threads, which can impede its progress and alert the spider of a potential threat. The robust threads can entangle the wasp or create obstacles that slow it down, giving the spider time to retreat safely.

Interesting Facts

References Article last updated on 6th May 2024
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