Why aren't spiders bigger?
Walking past some monstrous Halloween decorations on the way to school, my eight-year-old wondered if any real spiders were that big. I decided to look into the web to make sure.
For the Record
The Guinness World Record for the biggest spider by weight (50 g) is the male goliath bird-eating spider (Theraphosa blondi) from the Amazon rain forest. This tarantula has a leg span of about 28 cm. They mostly eat invertebrates, though I have seen some gross videos of people feeding them mice. They do not usually attack people or even eat birds. I don't think I'd want one of these filling my dinner plate, but it is still not in the category of say, Aragog, Hagrid's pet Acromantula in Harry Potter, or Shelob, whom Sam battles in Lord of the Rings. Why not?
To explore this spidery speculation, I emailed some specialists. Spider systematist Wayne Maddison at the UBC Beaty Museum of Biodiversity noted that scaling constrains many aspects of biology because a bigger thing of the same shape has more volume compared to its area.
Spider myth debunker Rod Crawford at the Burke Museum in Seattle said, "My best guess is that the biggest factor is respiration." Spiders take in oxygen and get rid of carbon dioxide like us, but they use tracheal tubes and "book lungs" that depend on diffusion through small openings which is much slower than our breathing system. And their blood is copper-based which isn't as efficient as our iron-based hemoglobin. So unless they did a complete overhaul of their respiratory system, a giant spider wouldn't be able to get enough oxygen to move around.
This could be related to why some giant dragonflies were able to fly around 300 million years ago, when the atmosphere contained more oxygen (35% compared to 21% now). Experiments on insects grown in higher oxygen showed that some species got bigger but others did not, perhaps because their bodies had other priorities.
Arthropods like spiders, insects, and crustaceans, have exoskeletons. This works well for small things, but as it gets bigger, tubes become prone to buckling and an exoskeleton becomes relatively heavy. This may be why the largest arthropods (crustaceans) are in the ocean, where the water helps support their bodies.
UBC biomechanics prof John Gosline shared some studies from his lab on how dragline strength changes with body size. The safety factor of the lines decreases as spiders get bigger. Really big spiders might not want to crawl around without a safety line.
Not being something can be trickier to explain than being, so this is just a start. in fantasy world, spiders are gigantic, creepy things. At Science World, they are little, cool things (except, perhaps, to certain arachnophobic curators).