The strange anatomy of spiders: Why are spiders not insects?

Added: 2026-05-11

[00:00:00]I always thought a spider was one of the simplest things in existence.

[00:00:03]Eight legs, a tiny body, >> [music] >> a web.

[00:00:07]But there are spider species that literally fly.

[00:00:10]They drift through the air on silk threads so fine they are invisible to the naked eye.

[00:00:15]Some have crossed entire oceans this way and have been recorded at thousands of meters of altitude. Spiders are 400 million years old. They are older than dinosaurs, older than trees, flowers, and [music] forests as we know them.

[00:00:28]They have survived more mass extinctions than any other group of predators on Earth, withstanding ice ages, supervolcanoes, and asteroid impacts.

[00:00:37]A spider is not an insect.

[00:00:39]Saying a spider is an insect is like saying a human is a shark.

[00:00:43]Spiders belong to a lineage called chelicerates, >> [music] >> a group that split from the insect line over 500 million years ago, when life on Earth was still learning how to survive out of water.

[00:00:53]Their closest relatives are scorpions, horseshoe crabs, and mites.

[00:00:58]The same mites living in your mattress right now are technically distant cousins of the spider in your room.

[00:01:04]Chelicerates have no antennae and no compound eyes in most species. Instead, they have simple eyes and thousands of microscopic hairs distributed across their entire body that detect the slightest movement of air, a way of perceiving reality that has no equivalent in any other group of animals.

[00:01:20]Between 400 and 450 million years ago, during the Silurian and Devonian periods, the continents were bare rock and [music] primitive plants. No birds, no mammals, no reptiles, only arthropods.

[00:01:33]Among those first land colonizers were the ancestors of spiders. Those proto-spiders had no silk glands or venom. They had eight legs, simple claws called chelicerae, and a body plan that had evolved underwater, but was now facing gravity and dehydration.

[00:01:48]>> [music] >> There are fossils from 305 million years ago that had segmented plates on the abdomen that modern spiders no longer have, and possibly a short tail.

[00:01:56]>> [music] >> They were evolutionary prototypes. For millions of years, evolution kept selecting and discarding until two innovations changed everything: venom and silk.

[00:02:06]Spider venom did not originally evolve to kill. It evolved [music] to digest.

[00:02:11]The first spiders had digestive enzymes in their intestinal system like most animals, but at some point those enzymes began to be produced [music] in glands located near the chelicerae. When injected into prey, they started breaking down tissues from the outside, pre-digesting the food before consuming it.

[00:02:27]Over time, some of those enzymes mutated into neurotoxins that no longer just dissolve tissue, but disrupted [music] nerve signals and paralyzed prey almost instantly.

[00:02:37]Venom did not evolve just once.

[00:02:39]>> [music] >> It appeared independently in multiple spider lineages, each producing different chemical cocktails optimized for specific prey.

[00:02:47]Tarantulas developed venoms designed for insects and small vertebrates.

[00:02:51]Black widows evolved neurotoxins that attack the nervous system of animals much larger than themselves. Australian funnel-web [music] spiders produce compounds capable of killing a human within hours without treatment. But most venoms are not designed to kill. They are designed to liquefy. Spiders bite, [music] inject venom loaded with enzymes, and wait for the inside of the prey to turn into a [music] nutritious soup they suck through their chelicerae.

[00:03:14]Certain tarantulas inject compounds that act as anesthetics, [music] numbing the tissue so the prey does not struggle while digestion begins. Others use venoms that take control of the victim's musculature, forcing it into paralysis while still alive. Spider silk is, pound for pound, [music] stronger than steel and more elastic than rubber. It can stretch up to 40% of its length [music] without breaking and absorb impacts that would destroy most synthetic materials we have ever invented. Spiders have been producing it for over 380 million years.

[00:03:44]Silk did not start out as a trap. It began as a protective wrapping [music] for eggs, produced from specialized abdominal glands called spinnerets.

[00:03:53]Over time, they discovered other uses: safety lines, burrow linings, trail markers, and finally aerial traps.

[00:04:01]A modern spider produces up to seven different types of silk, each made in a different gland, switching between them in real time.

[00:04:08]The strongest silk forms the skeleton of the web. Capture silk comes coated in sticky droplets. Wrapping silk immobilizes the victim in seconds.

[00:04:16]Darwin's bark spider builds webs with a silk stronger than Kevlar, spanning entire rivers with structures reaching up to 25 m. The bolas spider crafts a single thread with a sticky ball at the tip and swings it through the air like a lasso to catch moths.

[00:04:31]Water spiders build underwater diving bells, air-filled silk structures in which they live beneath the surface.

[00:04:37]Dozens of species use silk to fly, releasing threads into the wind and drifting hundreds of kilometers.

[00:04:43]A spider's web is not just a trap.

[00:04:45][music] It is a sensory organ.

[00:04:48]When prey lands on the web, vibrations travel through the threads like signals through a neural network.

[00:04:54]The spider can identify the species of prey by the frequency of the vibrations, estimate its size by the amplitude of the movement, and distinguish without any margin of error between a prey struggling to escape, the signal of a potential mate approaching, or simply the wind.

[00:05:09]Each thread acts like a guitar string tuned to a specific frequency.

[00:05:13]Orb-weaver [music] spiders are capable of locating prey with pinpoint accuracy in complete darkness using exclusively these vibrational signals.

[00:05:21]Circular webs detect flying [music] insects.

[00:05:24]Funnel webs amplify ground vibrations.

[00:05:27]Irregular webs confuse prey while generating multiple signal routes toward the spider.

[00:05:32]Many spiders rebuild their web every day, [music] tear down the old structure, eat it recovering up to 90% of the protein invested, and build a new one optimized for current conditions.

[00:05:41]With a tensile strength of between 1.1 and 1.5 gigapascals and an almost negligible weight, spider silk possesses properties that no synthetic material has managed to replicate at industrial [music] scale. In medicine, it is being tested to produce biological sutures because it is highly biocompatible. The human body does not reject it. It reduces inflammation and is completely biodegradable. Its [music] protein structure can promote cell growth, especially in the regeneration of nerve tissue and skin, opening possibilities for treating spinal cord injuries. In engineering, spider genes have been inserted into bacteria, yeast, and genetically modified goats [music] that produce silk protein in their milk. The resulting materials are studied for ultra-light armor, aerospace cables, and impact resistant materials with prototypes that absorb energy better than Kevlar under certain [music] conditions.

[00:06:27]Biosensors are being developed exploiting the vibrational sensitivity of silk to detect minimal oscillations in the environment.

[00:06:34]Webs also act as passive air filters capturing environmental DNA from nearby insects, allowing biodiversity assessment of an area simply by analyzing the web without [music] collecting any sample directly. The spider converts liquid protein into solid fiber in milliseconds, at room temperature, and without consuming significant energy, a process that industry has not been able to replicate at industrial scale. Most spiders have eight eyes, each covering a different function. Jumping spiders have layered retinas that move internally, scanning the environment without moving the head.

[00:07:04]They perceive depth through chromatic defocus. Different wavelengths of light focus at different distances, and the brain calculates depth from that difference. They also see in ultraviolet. This visual acuity allows them to leap with precision onto prey at distances of up to 50 times their body length.

[00:07:20]Spiders have no extensor muscles in their legs.

[00:07:23]Instead, [music] they pump hemolymph, their blood, into their legs, creating internal hydraulic pressure that forces the joints to straighten.

[00:07:31]If the exoskeleton is punctured, the spider loses pressure and can no longer extend its legs.

[00:07:37]This is why when a spider dies, its legs curl inward, the direct consequence of the hydraulic system shutting down.

[00:07:44]The body is covered in thousands of specialized hairs that detect air movements equivalent to 1/10 the width of a human hair.

[00:07:51]Some species detect the wing beat of an insect before it touches the web.

[00:07:55]Certain hairs respond to sound wave frequencies, allowing spiders to detect sound through the mechanical deflection of structures on their own skin >> [music] >> with no auditory organ whatsoever.

[00:08:05]Trapdoor spiders dig tunnels and build doors covered in soil and vegetation to make them invisible. With an attack [music] reaction time of under 0.03 seconds, net-casting spiders hold a small web stretched between their front legs and throw it when prey passes, extending it to four times its size in complete darkness. Crab spiders change color to blend in with the flowers where they wait for pollinators. Those that mimic ants raise their two front legs to simulate antennae, copy their movements, and infiltrate their colonies to hunt.

[00:08:34]The imitation fools both the ants and the predators that avoid them because of their formic acid.

[00:08:40]Jumping spiders show documented planning behavior. When hunting other spiders, they observe the target's web from a distance for extended periods, analyze its structure and the position of the prey, and [music] then execute a detour approaching from blind angles, taking indirect routes that can last hours.

[00:08:55]In the smallest species, the nervous system occupies 80% of the total body volume, including the legs.

[00:09:01]The most accepted hypothesis is that their cognition is not centralized, but distributed throughout the entire system. The sensors in the legs, the signals from the web, and the visual information are coupled directly to action without passing through abstract representation.

[00:09:15]>> [music] >> In one experiment, caffeine, amphetamines, and LSD were administered to spiders [music] while documenting the changes in their webs. Under caffeine, the web became so irregular it was barely recognizable. With amphetamines, they started multiple webs without finishing any of them. With LSD, repetitive patterns appeared with a certain symmetry, but heavily distorted.

[00:09:34]>> [music] >> This proved that spinning a web is a precise neural control process and not an automatic instinct, [music] given that normal webs have a geometric error margin of under 2%.

[00:09:44]Experiments with jumping spiders in mazes demonstrated that they memorize routes and choose the optimal path toward prey even when the prey temporarily disappears from their field of vision, confirming [music] active working memory.

[00:09:56]All of this with 600,000 neurons compared to the 86 billion in the human brain.

[00:10:02]Spider mating [music] is one of the most extreme processes in the animal kingdom.

[00:10:05]In many species, the female is significantly larger than the male. In the golden silk spider, the male is up to 125 times lighter than the female.

[00:10:13][music] This size difference is the direct result of millions of years of sexual selection, where the size of the female determines her reproductive capacity.

[00:10:20]Before approaching, the male performs elaborate rituals to avoid being mistaken for prey.

[00:10:26]Male jumping spiders perform dances with their front legs, vibrating their abdomen at specific frequencies that females recognize as a courtship signal.

[00:10:34]Wolf spiders tap the ground with [music] their palps, producing vibrational signals that travel through the earth toward the female.

[00:10:40]Some male species bring gifts, insects wrapped [music] in silk, which they deliver to the female during mating to distract her and avoid being attacked.

[00:10:48]Sexual cannibalism is documented in over 60 spider species. In the black widow and the European garden spider, the female devours the male during or after mating in a significant percentage of cases. The male's body provides direct nutrients to the female that improve the quality and quantity of fertilized eggs.

[00:11:05]The male that is devoured contributes more to his offspring than the one that escapes. There are species in which males even actively facilitate the process, positioning their abdomen toward the female's fangs while mating.

[00:11:16]Molting is one of the most extreme processes in spider biology.

[00:11:20]Like all arthropods, they have a rigid external exoskeleton that does not grow.

[00:11:25]To grow, they must abandon it completely.

[00:11:28]Hours before molting, the spider stops eating and becomes motionless. It secretes fluids between the old exoskeleton and the new cuticle forming underneath.

[00:11:37]Then the old exoskeleton splits along a specific line between the two body segments, and the spider begins pulling itself out of its own armor. The process can last anywhere from minutes to hours.

[00:11:46]During that time, the spider is completely vulnerable. Its new exoskeleton takes between hours and days to harden, [music] and until that happens, it cannot use its hydraulic system properly, cannot hunt, and any predator can kill it without resistance.

[00:11:59]The largest tarantulas can take up to 2 [music] days to complete the molt and another 2 weeks for their new exoskeleton to reach the necessary hardness. If something goes wrong during the molt, if a leg gets trapped or the process is interrupted, the spider can [music] be permanently deformed or die.

[00:12:13]Female tarantulas can regenerate lost legs during [music] molting. If they lose a limb in combat, in the next molt, a new one appears, initially smaller but functional, recovering over the following molts.

[00:12:23]There is a deeply ingrained image of spiders as solitary creatures. In most cases, it is true, but there are approximately 25 genuinely social species that form colonies of up to 50,000 individuals.

[00:12:35]Some species from South America build communal colonies with webs covering several square meters, [music] maintained and expanded collectively.

[00:12:42]Within these colonies, spiders cooperate in [music] hunting, immobilizing prey far larger than any individual could handle alone, and in caring for the young.

[00:12:51]The larger ones attack and subdue the prey. The smaller ones wrap and transport it.

[00:12:57]These colonies have something resembling a division of labor that emerges spontaneously from the interactions between individuals without any kind of central hierarchy. There are social species from southern Africa where females collectively care for the offspring of all the mothers in the colony.

[00:13:12]And when the young reach a certain age, the adult females allow themselves to be devoured by them.

[00:13:18]It is not an accident.

[00:13:20]It is an evolutionary strategy in which the mother becomes the first food source for her offspring, maximizing the survival chances of the young at the cost of her own life. Spiders occupy virtually every terrestrial ecosystem on the planet, including somewhere no other complex predator has managed to establish itself. [music] There is a jumping spider species that lives permanently above 6,700 m of altitude on the slopes of Everest, feeding on the small insects the wind carries up to that height. It is one of the largest animals that permanently inhabit that altitude. At the opposite extreme, certain wolf spiders have adapted to living in the Arctic tundra, surviving temperatures as low as minus 40° through antifreeze proteins in their blood that prevent the formation of ice crystals in their tissues. In the deserts of Southern Africa, there are spiders that have developed an unparalleled escape strategy. They hurl themselves down dunes, rolling over themselves at speeds reaching 2 m per second, using their own body as a wheel.

[00:14:13]>> [music] >> In the floodplains of the Amazon, there are species that actively hunt by detecting the waves produced by insects or small fish on the surface, and launching themselves from the banks to catch aquatic prey. The largest tarantula in the world has a leg span that [music] can exceed 28 cm and a weight of up to 170 g. The smallest known spider, found in Colombia, measures just 0.37 mm, small enough to fit comfortably on the head of a pin.

[00:14:38]Between those two extremes, there are over 52,000 described species, and scientists estimate that more than 100,000 may exist in total. The majority still uncataloged. 99% of the 52,000 described spider species are harmless to humans.

[00:14:53]Only between 30 and 40 have venom with real medical relevance, >> [music] >> and annual deaths from spiders are extremely rare.

[00:15:00]Spiders consume between 400 and 800 million tons of insects per year, a figure comparable in mass to the entire annual meat and fish consumption [music] of the human species. Without them, insect populations would grow out of control [music] within weeks, and agricultural, forest, and urban ecosystems would collapse. Some populations are evolving to exploit artificial light, which concentrates nocturnal [music] insects creating high-density hunting zones that did not exist a hundred years ago.

[00:15:25]There is evidence of species evolving to live exclusively indoors hunting household pests. These populations isolated from their outdoor relatives could be in an active process of speciation. Climate change is pushing tropical species into zones that were previously too cold for them creating new competitive dynamics and selective pressures in ecosystems that do not yet know how to respond. Spiders have survived at least five mass extinctions, have watched dinosaurs, trilobites, and large marine reptiles disappear, have endured ice ages that covered entire continents, super volcanoes that darken the sky for years, [music] and asteroid impacts that reshape the planet. If you want to see more content like this, your support means everything. Thank you.