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Spider Evolution - Adaptations and Ancestors
The Evolution of Spiders
Spiders first evolved around 310 million years ago from earlier arachnid ancestors. They now populate every continent on Earth apart from Antarctica. During the course of their evolution, spiders developed a number of unique adaptations that distinguish them from other species.
Today there about 44,000 species of spider on the planet, with new varieties consistently 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 have been unearthed from fossils. Many of these are extinct ancestors of modern incarnations. Due to their soft exteriors, fossilized remains are prolifically found to be preserved within tree amber. This article will depict a timeline of spider ancestors, and describe their behavioral and anatomical adaptations.
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 popularly known trilobytes were also a type of arthropod.
Arthropods were preadapted for the transition to terrestrial environments; having strong exoskeletons and numerous 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 also developed book lungs to filter oxygen from the air. 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. These include the "chelicerae", which manifest as pincers or fangs. Some chelicerates remained predatory while others became herbivorous or parasitic.
One notable extinct chelicerate is `Megarachne servinei' 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.
Trigonotarbid Palaeotarbus Jerami (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.
Other adaptations include fine, bristly hairs to provide a sense of touch; and slit-sense organs that suggest an auditory ability. These organs consist of thin slits covered by a membrane resembling an eardrum. A hair beneath the membrane detects any vibration. Arachnids also have simple rather than compound eyes. Like human eyes, these include a lens, retina, and cornea. They can be easily adapted to specific environments, allowing arachnids to hunt in all light conditions. Unlike their ancestors, arachnids had 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. 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.
This arachnid also had a tail and lacked a venom gland, setting it aside from all 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.
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.
A young Richard Dawkins explains spider webs.
Modern Spiders (250 mya)
Modern spiders first appeared around 250 million years ago. Depending on jaw construction, they are separated into two groups. 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.
Since 250 million years ago, spider webs have become increasingly sophisticated. This may be attributed to modern spiders evolving spinnerets at the rear of the abdomen rather than the center (like Mesothelae). Natural selection will also determine the success of a particular web design (see video).
Around 140 million years ago, 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 now constitute 25% of all spider species, and they belong to the Araneomorphae suborder.
Timeline of Spider Ancestors
The picture to the right summarizes the timeline of spider ancestors detailed above. It also shows when related species may have diverged from common groupings. The diagram borrows from J. Shultz's analysis of arachnid orders.
Spiders are fascinating predators with a protracted evolutionary history. Despite a genetic and anatomical similarity with 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; though tent caterpillars and fall webworms can produce similar structures. The spider's ancestral timeline exposes these exotic adaptations, and reveals the functional beauty of Darwin's theory of evolution.