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The Evolution of Spiders and Their Remarkable Adaptations

Thomas is an avid student of evolutionary history with a deep fondness for the natural world.

The Cellar Spider (Segestria Florentina)

The Cellar Spider (Segestria Florentina)

The Evolution of Spiders

Spiders first evolved around 310 million years ago from earlier arachnid ancestors. They currently populate every continent on Earth apart from Antarctica, and there are about 50,000 extant species with new species continually being discovered. As predators, these octopedal creatures consume large quantities of insects, making them a vital component of most land-based ecosystems.

About a thousand species of spider (Araneae) have been unearthed from fossils, and many of these are extinct ancestors of the modern variety. Due to the soft exteriors of spiders, fossilized remains are more likely to be intact if they were preserved in tree amber, as shown below.

Most spider fossils are found in amber.

Most spider fossils are found in amber.

These ancestors are interesting because they show us how spiders and their unusual adaptations (i.e., webs) evolved and diverged from those inherent to other species.

What follows is a timeline of spider ancestors, from the earliest forms of land-based life, to their modern incarnations. Along the way, the evolutionary course of their behavioral and anatomical adaptations is described and explained.

Arthropods (550 mya)

Arthropods were the first group of species to leave the oceans to colonize land. This occurred around 450 million years ago (mya), well before the existence of dinosaurs. The earliest arthropods were marine animals dating back to about 550 mya. They include the Spriggina (pictured) and the Parvancorina. The well-known trilobytes were also a type of arthropod.

Spriggina - one of the earliest arthropods.

Spriggina - one of the earliest arthropods.

Arthropods were preadapted for the transition to land; having strong exoskeletons and (by 450 mya) rudimentary limbs for locomotion. They had an open circulatory system including a heart, and compound eyes utilizing thousands of photo-receptive units.

Those that took to land developed book lungs (from their gills) to filter oxygen from the air. These book lungs are still present in modern spiders and many related species. Indeed, arthropods later evolved into spiders, insects, centipedes, scorpions, mites, ticks, crabs, shrimp, and lobsters.

Chelicerata (445 mya)

Chelicerata is a subgroup of arthropods that diverged around 445 mya. It includes spiders, scorpions, horseshoe crabs, mites, and ticks.

Like arthropods, these creatures had segmented bodies and jointed limbs. Chelicerata are defined as having two segments (the head and abdomen) with myriad appendages, including the "chelicerae," which manifest as pincers or fangs. Some chelicerates remained predatory while others became herbivorous or parasitic.

Megarachne was believed to be a giant spider but it was actually a sea scorpion.

Megarachne was believed to be a giant spider but it was actually a sea scorpion.

One notable extinct chelicerate is `Megarachne servinei' (above), which was once thought to be a giant spider. It was actually a sea scorpion (pictured). Megarachne had a diameter of about 50 cm and died out around 300 million years ago.

Trigonotarbida or "Arachnids" (420 mya)

The earliest known arachnids were called Trigonotarbida (pictured). They looked similar to spiders, but did not possess silk-producing glands. Trigonotarbida appeared between 420 and 290 million years ago.

Arachnids comprise a group of octopedal species including spiders, scorpions, mites and ticks. They possess two chelicerae (fangs) that can look like additional legs. Their long, jointed appendages and improved water conservation meant they were well adapted for quick travel across land.

Trigonotarbid Palaeotarbus Jerami, the oldest arachnid.

Trigonotarbid Palaeotarbus Jerami, the oldest arachnid.

Arachnids added numerous other adaptations now found in modern spiders, such as fine, bristly hairs to provide a sense of touch, and slit-sense organs that suggest a rudimentary auditory ability. These organs consist of thin slits covered by an eardrum-like membrane. A hair beneath the membrane detects its vibrations.

Arachnids also dispensed with the arthropods' compound eyes. Like human eyes, arachnid eyes have a lens, retina, and cornea, which allow them to hunt in a variety of environments and conditions. Unlike their ancestors, arachnids also developed forward pointing mouths, aiding their ability to hunt.

Attercopus Fimbriunguis (386 mya)

Attercopus is the earliest silk producing arachnid, appearing around 386 million years ago. Its silk glands fed tubular, rigid hairs called spigots that were located on the abdomen.

An Attercopus fossil showing a tail (bottom).

An Attercopus fossil showing a tail (bottom).

However, Attercopus was not a true spider because these inflexible spigots were not able to weave webs (they were not `spinnerets'). Indeed, flying insects had yet to evolve, making web production unnecessary. Instead, it probably used the silk to wrap eggs, line nests, or subdue prey.

Attercopus also had a tail and lacked a venom gland, setting it aside from all modern spiders. Despite this, the name `Attercopus' literally means "poison head". These proto-spiders went extinct around 200 million years ago.

Mesothelae Spiders (310 mya)

Mesothelae are the oldest order of true spiders (Araneae), and they evolved around 310 million years ago. True spiders are defined by the presence of silk-producing spinnerets that are capable of weaving webs, and venom glands for disabling prey.

A surviving Mesothelae spider from Japan.

A surviving Mesothelae spider from Japan.

A spider's spinnerets require the suspension of silk-producing spigots on flexible muscles that can quickly aim them across a large angular distribution.

Most spiders have six spinnerets with several spigots on each, and these are typically located towards the rear of the abdomen. However, Mesothelae had eight spinnerets which were centered. Most species of Mesothelae are now extinct, although some still remain in south-east Asia and Japan.

Modern Spiders (250 mya)

Modern spiders first appeared around 250 million years ago. They are separated into two groups depending on the type of jaws they have.

Mygalomorphae have fangs that point straight down. This group consists of heavy spiders and tarantulas that can live for several years.

The other more populous group is called Araneomorphae, which have fangs that cross over like pincers. They typically live for one year and are much smaller than Mygalomorphae.

An Australian Orb Weaver Spider.

An Australian Orb Weaver Spider.

Over the past 250 million years, spider webs have become increasingly sophisticated. As Richard Dawkins explained in the video above, natural selection determines the success of a particular web design. Furthermore, modern spiders having spinnerets at the rear of the abdomen rather than the center (like Mesothelae) permitted greater versatility.

Around 140 million years ago, modern spiders began to climb shrubbery and trees to produce elaborate "orb" webs. This allowed them to catch the growing number of flying insects. Orb weavers (pictured), which belong to the Araneomorphae suborder, now constitute 25% of all spider species, demonstrating the success of this method.

Timeline of Spider Ancestors

The diagram below summarizes the timeline of spider ancestors detailed in this article. It also shows when related species likely diverged. The diagram utilizes data from J. Shultz's analysis of arachnid orders.

A timeline of spider ancestors, beginning 550 million years ago (mya).

A timeline of spider ancestors, beginning 550 million years ago (mya).

Notably, spiders have not changed much in the last 250 million years, suggesting they are already well-adapted to a range of environments.

Despite a genetic and anatomical similarity to crabs, insects, and scorpions, the biological distinctiveness of spiders makes them both noticeable and successful within terrestrial ecosystems. Indeed, the ability to spin webs is largely unique within the animal kingdom, although tent caterpillars and fall webworms can produce similar structures.

The spider's ancestral timeline reveals the development of these exotic adaptations, which ensure their evolutionary success, and which suggest they will populate the Earth long after humans are gone.

More Information

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

Comments

Thomas Swan (author) from New Zealand on October 28, 2014:

Thank you to the two guest commenters for your informative and enthusiastic words!

Anonymous on October 27, 2014:

spiders are awesome

anonymous on September 29, 2013:

There is much disagreement about dating the origins of spiders. Some sources say that the first spiders appeared as long ago as 405 million years ago in the early Devonian period, Paleozoic era. These early spiders would have eaten Trilobites and early Insects such as Springtails. The Mygalomorphs are said to be very primitive and very ancient. These are Tarantulas and Trapdoor spiders. Tarantulas are said to have existed anywhere from 150 million years ago in the Jurassic period, Mesozoic era to as much as 405 million years ago in the Devonian period. Rosamygale is a Tarantula fossil in France dated at 235 million to 240 million years ago in the Triassic period, Paleozoic era. Eurypterids or Sea Scorpions also thrived in the Paleozoic era and true Scorpions appeared 440 million years ago in the Silurian period, Paleozoic era. Tarantulas, Trapdoor spiders, the Mesothelae spiders, Scorpions, and Camel spiders are all ancient living fossils. They existed with Trilobites, Dinosaurs, and prehistoric mammals and they still exist today. Spiders are super ancient and are a window to a time when earth was home to large arthropods.

Thomas Swan (author) from New Zealand on May 23, 2013:

Thanks Gcrhoads64 and grand old lady! It's hard to not be interested in these unique creatures, though I'm with you (grand old lady) in being slightly scared of them. If a spider with a body wider than my thumb scuttled across the floor, I'd be a bit freaked out!

Mona Sabalones Gonzalez from Philippines on May 22, 2013:

Very interesting information. I will admit to not being a spider lover, but your article helped me to appreciate them more:)

Gable Rhoads from North Dakota on May 22, 2013:

I find spiders endlessly fascinating. Great article!

Thomas Swan (author) from New Zealand on May 22, 2013:

Thanks JMcFarland, your kind words and vote is much appreciated!

Elizabeth from The US of A, but I'm Open to Suggestions on May 21, 2013:

This was an awesome and fascinating hub. Thank you so much for sharing. Voted up