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With eight legs, eight eyes, venomous fangs, and a knack for living just about everywhere, they’re the famous famous arachnids and among the most famous arthropods on Earth, and they’ve been successful for a good 300 million years. This episode, we discuss the diversity and evolution of Spiders.
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Spiders
Spiders are among the most diverse and successful living arthropod groups, with around 50,000 described species living nearly everywhere on Earth but the poles. They belong to a group called the arachnids, alongside scorpions, ticks, and mites, and altogether the arachnids fit within Chelicerata, a group that also includes eurypterids (extinct sea scorpions) and horseshoe crabs.
Though they come in many shapes and sizes, spiders share a unique body plan: two body segments (cephalothorax and abdomen) with four pairs of legs, one pair of leg-like pedipalps, and a pair of mobile fangs. They range in size from less than half a millimeter long to body lengths over 12 centimeters.
Full list of pictured species here from Wikimedia Commons, CC BY-SA 3.0
Spiders are, with few exceptions, terrestrial carnivores that take down prey with the help of their venomous fangs. Some are pursuit predators (like jumping spiders and wolf spiders), others are ambush hunters (such as the well-camouflaged crab spiders), and many make innovative use of another defining feature of spiders: silk, which they produce from special organs called spigots on spinnerets.
Spider silk has many uses: protecting eggs, carrying sperm packets, creating safety lines in case a spider falls, lining nests to protect them from flooding or to form a sensory network for the spider to sense intruders, and of course, many spiders use silk to create webs to capture and hold prey. Other spiders might use webs to create nets to grab prey (like ogre-faced spiders), to make sticky lines they can swing to catch prey (like bolas spiders), or even to carry babies through the air (a technique called “ballooning”). Spiders are not the only animals that make silk, but they are champions at it.
Right: Spider extruding web to tangle prey. Image by Rustedstrings, CC BY-SA 3.0
Spider Evolution
Arachnids were among the earliest arthropods to emerge onto land. The Devonian Period was home to a number of ancestors and cousins of spiders, including Attercopus and a group called trigonotarbids, both of which were very spider-like but don’t seem to have possessed spinnerets. The oldest known true spiders are from the Carboniferous and Permian Periods, including segmented spiders similar to modern-day Mesothelae spiders. Many modern groups of spiders – and modern spider lifestyles, including various modes of hunting and web-use – seem to have originated by the Mesozoic Era.
Left: A pair of Mongolarachne preserved in sediment from China. Image by Paul A. Selden, CC BY-SA 3.0
Right: Spider in amber with prey. Image by Oregon State University, CC BY-SA 2.0
Spider silk clearly originated early in spider evolution, possibly originally used to protect eggs or to retain water around the bodies of early terrestrial spiders. Since spigots are known from Devonian spider-cousins, and spinnerets are known from fossil spiders of the Carboniferous, it’s safe to say spiders have been spinning silk for hundreds of millions of years. Understanding the silk-using habits of extinct spiders is tricky, but paleontologists can gain some insights from the occasional amber-preserved webbing or by studying the body proportions of fossil spiders and comparing them to modern species (orb-weavers vs. funnel-builders, for example).
Learn More
Phylogeny and Classification of Spiders (technical)
Review of the Fossil Record of Spiders (technical)
And just for fun: Spiders in Space!
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If you enjoyed this topic and want more like it, check out these related episodes:
- Episode 62 – Amber
- Episode 99 – Evolution of Insects
- Episode 117 – Crabs and Carcinization
- Episode 82 – Trilobites
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COMMON DESCENT 
Thank you so much for this episode. I have listened to it 2 times and again with my 6 year old because we love spiders so much. How had I never heard of pelican spiders!? Incredible. Would absolutely love more episodes about different kinds of spiders and their wild adaptations!
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