Q & A
The Spider Man: Matthew Persons, Associate Professor of Biology
The History Channel invited Persons this summer to lend his expertise to an upcoming program on monster spiders. Here, Persons answers Susquehanna Currents’ questions about the likelihood of a monster spider’s existence.
What is the likelihood that a monster spider exists somewhere in the world?
MP: It depends on one’s definition of “monster.” The world’s largest spiders are the tarantulas, and the biggest of these are females of the species Theraphosa blondi. My old Ph.D. advisor had a preserved one stuffed inside a mason jar; it barely fit. To my knowledge, this particular specimen was the largest ever recorded, over a quarter of a pound in size with a leg span exceeding that of a dinner plate (this particular specimen had the label “Bubba” on the jar). Although we can’t rule out the possibility of finding a spider bigger than a T. blondi, the likely heyday for monster-sized tarantulas is probably gone. One person has claimed to have seen a spider the size of a dog.
Why do you say that a giant spider like this is impossible?
MP: It’s not impossible from a physics standpoint, but it’s pretty unlikely. As a cell grows larger, its volume grows at a much faster rate than its surface area, and it can’t diffuse enough oxygen and nutrients to the center of the cell to keep it alive. This is a problem for all animals, but particularly for arthropods because they can’t exchange gases through their skin. Having an armored shell serving as a skeleton for your body is a marvelous thing, but it does a very poor job as a respiratory structure. Also, spiders, like other arthropods, have to shed their exoskeleton to grow. This imparts an upper limit on size and explains why most arthropods are very small.
If a spider the size of a dog did exist, what would it look like?
MP: As arthropods grow in size, their body dimensions grow in disproportionate ways. Large arthropods are not simply photocopy enlargements of smaller ones. The weight-bearing structures grow disproportionately thicker (a term called “elastic similarity”). A large spider would need disproportionately stout legs with a disproportionately thicker exoskeleton. Because of gravity, a very large spider would be in danger of flattening out like a pancake and deforming its legs before its exoskeleton had a chance to harden. The material found in a spider exoskeleton simply cannot support the weight of a spider the size of a dog.
Contributing writers to The 'Grove section are Julie Buckingham '09 and Ryan Rickrode'11.