The Web of Life (126-10-1) Spiders

 Spiders

Spiders descended from a scorpion-like ancestor 380 million years ago. By 300 MYA spiders were in their modern form, albeit with much evolutionary potential in front of them. All spiders are carnivorous.

Although many spider species have a wide distribution around the world, most live within a specific habitat, having optimized to living in a niche environment.

Today’s spiders are sorted into 3 families. The most basal are the Liphistiidae. Found in Southeast Asia, China, and Japan, these nocturnal, medium-sized spiders live for many years in tube burrows with trapdoors constructed on top.

The Mygalomorphae appeared during the Triassic, over 200 MYA. Mygalomorphs are hairy and typically heavily built, with large, robust ‘jaws’ (chelicerae) and fangs. Tarantulas are in this group, as are “trapdoor” spiders, which are distant relatives to liphistids.

Over 90% of today’s spider species are Araneomorphae (aka Labidognatha), which arose toward the end of the Carboniferous, ~300 MYA. These “true spiders” have fangs which oppose each other and cross into a pinch. By contrast, tarantulas and their close kin have parallel fangs. Araneomorphs include jumping spiders, cursorial (running) wolf spiders, sheet-weaving dwarf spiders, and the orb-weavers.

Wolf spiders are exemplary of daylight hunters. They visually locate prey and run it down. Many wolf spiders are like cheetahs chasing antelope, with the odd jump thrown in from time to time. Jumping spiders are more ambush oriented, and leaping captures are common.

Wolf and jumping spiders aside, most spiders are short-sighted. Spiders have exquisite tactile senses related to physical vibrations, including sound. Spiders are also pressure, humidity, and temperature sensitive.

Through extensive tactile contacts, spiders create a mental map of their surroundings. Webs rebuilt in the same place require less exploration to establish the best anchor points.

 Spider Brains

Though tiny, spider brains are big relative to their body size (compared to large-bodied animals). A spider’s central nervous system is made up of 2 incomplex nerve cell clusters (ganglia).

The skillful acumen of spiders, including their ability to learn, is an apt illustration that intelligence is not housed in a physical substrate. While animal minds are linked with brains when they exist, nerve and glia cell activity is coincidental, not causal.

○○○

The web functions as an extension of the spider’s exquisitely tuned sensory system. ~ Canadian zoologist Catherine Scott

Spiders have been weaving orb webs for ~200 million years. Not all spiders build webs, nor are webs built just to catch prey.

In the Early Devonian period, when moving from the water to the land, spiders started producing silk to protect their bodies and eggs. The practice gradually turned to hunting: first guide and signal lines, followed by ground and bush webs. Then spectacular aerial webs became popular.

Whereas speciality silks independently evolved, as did novel web designs, the various snares spiders use probably derived from a single original scheme.

 Spider Silk

All spiders spin silk via glands that produce custom blends of proteins with distinct motifs that impart the perfect properties for intended use. The silks emanate from the abdomen through spinnerets. Most spiders have 6 spinnerets, though some have 2, 4, or 8.

Spider silk is comprised of proteins with blocks of recurring alanine and glycine amino acids. Alanine blocks typically pack together in tiny, dense crystals. These are separated by amorphous regions of glycine. Combinations of the proteins and amino acids are precisely arranged in distinct motifs which gives silk its elasticity and toughness.

Spider silk is a unique material: lightweight and stretchy yet stronger than steel. Its production is a chemical engineering marvel.

The proteins (spidroins) from which silk is spun are stored at extreme solute concentration in a gland in a spider’s abdomen. Spidroins have a helical and unordered structure when stored in silk glands. When converted to silk, that structure changes completely, to one that confers a high degree of mechanical stability while being supple.

When a spider wants to spin silk, spidroins pass through a canal where they are converted to gossamer by lowering pH from 7.6 to 5.7 via enzymatic action within the duct.

Silk fabrication happens with water as a solvent at ambient temperature within fractions of a second. The phase change – from solute to solid fiber – is only possible owing to the nature of spidroin. Spider silk dries via mechanical pulling, not evaporation.

Spidroins are large proteins – up to 3,500 amino acids – that contain mostly repetitive sequences. The most important bits for conversion into silk are at the terminal regions, which are unique to spider silk and are selfsame among different spider silks.

One spidroin end pairs up with other molecules at the beginning of the duct, stabilizing as acidity intensifies. Meanwhile, with increased acidity, the other end unfolds to trigger rapid polymerization of the spidroins.

The result is a fiber that contains regions of crystalline and noncrystalline β-sheets, giving silk its unique properties. A β-sheet is a somewhat uncommon protein structure that forms a twisted, pleated sheet at the molecular level.

Spiders use different gland types to produce different silks. Each silk type serves a different purpose: a slick silk safety line, sticky prey-trapping silk, or fine wrapping silk. Some spiders can make up to 8 different silks.

The qualities of the various silks vary by amino acid content. From a mechanical perspective, spider silk varies in having different soft amorphous and strong crystalline components.

The tensile strength of spider silk exceeds that of steel at equivalent weight, with much greater elasticity. Spider silk conducts heat better than most metals.

Spider silk averages around 3 microns thick. A human hair is 20 times thicker (60 microns).

When twisted, ordinary fibers typically spin back and forth around an equilibrium point, eventually returning to the original orientation. Not spider silk, as such uncontrolled spinning at the end of a dropline could cause real problems for a spider making a descent. Instead, spider silk deforms when twisted, cutting down on back-and-forth spinning.

A permanent torsional deformation in spider dragline silk occurs after even small torsional strain. This behaviour is quite different from other materials. Spider dragline silk displays a strong energy dissipation upon the initial excitation (around 75% for small strains and more for a larger strain), which correspondingly reduces the amplitude of subsequent oscillations around the new equilibrium position. ~ Chinese physicist Dabiao Liu et al

The upshot is that spider silk is wondrously designed to allow a spider to steadily descend and dangle on its spun line without untoward spin.

The requirements for spider silk seem almost impossible but are achieved by silk’s amazing microscopic structure. ~ English arachnologist Paul Hillyard

 Spider Webs

Most spiders have poor eyesight and rely almost exclusively on the vibration of the silk in their web for sensory information. ~ English zoologist Beth Mortimer

Spider webs are careful constructions, built with acoustic properties in mind. Spider webs sing siren songs. The unique sonic properties of a web attract flying insects, whose curiosity lands them in a sticky situation.

Spider silk transmits vibrations across a wide range of frequencies. From the sound of a plucked strand, a spider can know the type of prey on the web, the quality and intentions of a prospective mate, and even the web’s structural integrity. Spiders receive nanometer vibrations with organs on each of their legs – slit sensillae – which amplify the delicate web information.

Spiders set out to make a web that ‘sounds right’. ~ Beth Mortimer

Whereas a web made of threads with the same diameter better bears force applied at a single point, such as the impact of insects hitting the web, non-uniformity can withstand more widespread pressure, such as from wind, rain, and gravity.

Carefully calibrated amounts of material are employed to capture insects of different sizes. A spider optimizes a web’s strength for its anticipated prey, and to account for environmental conditions.

Spider webs evolved in their sophistication. Earlier web types were more disordered, such as tangle webs, to which sheet webs were a step up. The epitome of spider web construction is in orb webs. Orb webs consist of 3 structural elements: strong framework threads, strong radial threads which converge at the center, and elastic capture threads which spiral around and are beaded with sticky deposits.

Some orb-weaving spiders decorate their webs with additional, conspicuous designs known as stabilimenta. Stabilimenta independently evolved at least 9 times. Though scarcely understood by arachnologists, these decorations are communication devices with various intentions depending upon species.

Spiders create their webs so that portions may be damaged but the web as whole stays intact: sophisticated fault tolerance.

This is a clever strategy when the alternative is having to make an entire, new web. ~ American biomechanist Dennis Carter

The large Australian golden orb weaver incorporates decaying plant and animal matter into its web. This sit-and-wait predator attracts insects using the odor of the debris. The spider replenishes rubbish bits as necessary to keep the scent at its most alluring.

◊ ◊ ◊

Tarantulas can produce silk from their feet as well as their spinnerets. Foot silk gives tarantulas better cling to surfaces, which is especially useful for this exceptionally large arachnid.

◊ ◊ ◊

Flowers take advantage of electrostatic forces to engender pollination and communicate with pollinators. Through their webs, spiders also enlist rascally electrons to help capture prey.

Insects acquire an electrostatic charge by flying or walking on charged particles. The glue that coats cobwebs electrostatically enlivens threads to grasp at the charged creatures that come close.

 Darwin’s Bark Spider

Darwin’s bark spider spins webs across rivers and small lakes in Madagascar. Orbs may span 2.8 square meters. Silk bridges may run 25.5 meters.

Their web construction techniques are unique, as is their silk, which is the toughest natural material known: twice as strong as other spiders’ silk.

Despite spinning large, bark spiders are not looking to nab big prey: only semiaquatic flying insects; but in large numbers. These huge webs may catch 30 bugs at a time.

Darwin’s bark spider weaves the largest web of any single spider. Some spiders collaborate to weave interlaced webs that cover hundreds of meters.

Though web catching saves the bother of hunting, spinning silk and weaving webs take energy too. Webs wear easily. A Darwin’s bark spider typically eats part or all of its web daily and recycles the silk proteins. The webs of bark spiders are extensive enough that the spiders maintain them, but for less than a week, and then they are replaced.

 Common House Spider

The common house spider has uncommon cleverness in its web construction. While the spider uses the same glue on its silk, it applies the glue differently, depending upon whether its web is intended to catch flying or crawling insects. The velocity of flying insects requires reinforced attachment.

The common house spider is thoroughly domestic. A male and female often share the same web for extended periods.

Several females may make webs close to each other. But they retain a sense of territoriality and are prone to fight if they encounter one another.

Otherwise, house spiders are not aggressive. They are instead quite skittish: afraid of larger foes and inclined to hide if their web is unusually disturbed.

If spooked, a house spider will flee down a silk escape line. If cornered, as a last resort, a house spider may feign death.

 Triangle-Weaver Spider

This spider uses its web like a catapult. ~ American zoologist Sarah Han

The triangle-weaver spider constructs a triangular web. It sits near the web’s vertex, holding that section of the web taut like a bowstring until it senses a prey upon the web. The spider then releases the tension, propelling that part of the web, and itself, at high speed toward the prey. The target is entangled in the propelled web. If not hungry at the moment, the spider wraps the capture in a special wrapping silk for later consumption.

The triangle-weaver is not the only spider to manipulate silk strand tension in a web to attack and subdue prey. Many orb spiders do so for prey that otherwise escape quickly. The ray spider pulls its web into a cone, releasing it in a similar manner to the triangle-weaver. Tangle web spiders construct sticky, gumfooted threads that are held under tension and which sometimes lift prey into the web through springy scaffolding silk.

Using a constructed device as a power-amplified weapon offers several advantages over internal mechanisms. The triangle-weaver’s leg-over-leg loading of the web over repeated cycles of muscular contraction lets the spider store enormous elastic energy per unit muscle mass, amplifying physiological limits. Such a system operates analogous to the ion and water pump-driven systems that power the carnivorous traps of bladderworts and stinging nematocyst cells of cnidarians, but at a much larger scale.

The iterative loading allows the spider to precisely regulate web tension and stored energy as needed, enabling the rapid accelerations and range of motion that successfully jerks silk around intended targets.

The system lets a spider interact with prey from afar, reducing the risk of bodily damage from potentially dangerous insects, and triggering the tangling process immediately after prey contact the web.

○○○

Spiders avoid getting stuck in their own sticky webs with specific adaptations. Spiders work and walk on webs in a way that minimizes adhesive forces, and their legs are covered in branching hairs that have a non-stick coating.

◊ ◊ ◊

Webs are a wondrous way to earn a living, but the lifestyle is limiting. There was a burst of diversification ~100 MYA, as ground-dwellers, such as wolf and jumping spiders, abandoned the chore of spinning and maintaining webs to take up more ambitious ways of snagging prey.

Spider senses attuned to their lifestyle. These spiders have keen senses of touch and smell, hear quite well, and can sense atmospheric electrical charges. Wolf spiders have excellent eyesight, whereas many others are nearsighted.

 Bolas Spider

A bolas spider spins silk but it doesn’t weave a web. Instead, it slings a silk strand across a gap, where it waits, dangling a sticky gob from one leg.

Moths are something of a rare treat for web spiders, as moths’ scaly wings let them slip out of spider webs. A bolas snags moths when they are most vulnerable: on the hunt for love. The spider separately releases the female sex pheromones of 2 different moths precisely when they make their nightly flights. A male flies by, catches the scent, and comes in close for a mating session. Instead, the spider swings his lasso and snags dinner. Specializing in multiple moth species with different time schedules is quite a trick.

 Water Spider

The water spider lives nearly all of its life underwater, including resting, catching and eating prey, mating, egg laying, and overwintering. Water spiders wait for prey to swim past their air-bubble home, then ‘invite’ them in as a meal. The water spider is endemic to mainland Europe, northern Asia, and the British Isles, residing in ponds and other rather still bodies of water.

This spider is also called the diving bell spider, as it builds a silk-based structure which it stocks with air carried down from the surface to form a livable bubble. The silken support is anchored to a submerged water plant. Young spiders do not build air bells – instead taking over empty snail shells which they fill with water.

Capturing air bubbles on the surface is nontrivial. Using a complex movement, a spider traps a bubble using its hind legs and rear of the abdomen. After the first air load is put into place, a spider spends time extending and strengthening a nascent bell before adding more air. A half-dozen trips are typically needed to stock a home bell with enough air.

○○○

Several spiders travel on the breeze. Buoyed by air currents, they soar 4 or more kilometers high in the sky. These arachnids lift a front leg to sense wind speed and electrical potential, letting them know when it’s an apt time to fly. Perched at a suitable high point, they form a silk parachute-glider and balloon away.

At the mercy of the wind, ballooning spiders run the risk of landing on water. They do so with aplomb.

Spiders actively adopt postures that allow them to use the wind direction to control their journey on water. They even drop silk and stop on the water surface when they want. ~ Japanese invertebrate zoologist Morito Hayashi