When Giant Bugs Attack! Or Not?
While living in Southern Georgia and working towards a Masters in environmental science, I enrolled in a course on insect ecology. This was quite fitting seeing as how Georgia has an array of insect and arachnid species, both annoying and interesting (including some rather large spiders that preferred the comforts of indoor habitats… like our house). My husband isn’t particularly fond of spiders (especially ones he considers “gigantic”), which made for some entertaining memories (spider on the shower curtain while you’re bathing? Cue the not-so-manly scream and string of profanities!). Being from Colorado, we didn’t see many bugs, so it got me thinking about how big bugs can actually get.
I’ve had this question in my head since I watched my first sci-fi moving starring a giant insect as the villain. I believe it was titled Them!, and involved giant, mutated, man-eating ants. As a side note, why is it that most giant bug movies involve a botched government nuclear project? Anyway, I only remember this movie because I had nightmares for weeks about being eaten by giant ants. This was, of course, perpetuated by my older brother who felt the need to torment me every time I saw an ant. Thankfully, I no longer have an irrational fear of being attacked by giant insects, and science can back me up on this one.
To answer this question, I will use one of my favorite, guilty-pleasure, sci-fi B-movies titled Big Ass Spider! as an example. I know what you’re thinking: “But spiders aren’t insects!”. While that is true, the same principles apply regarding size restraints (and let’s face it, more people are afraid of spiders than ants!). As a quick synopsis of the movie, scientists inadvertently create a spider the size of a house through “crossbreeding microbial DNA samples found in Martian soil, and infusing it with mega-growth hormone”. This concoction was originally meant for a fruit plant in order to enlarge the plant in an attempt to combat world hunger, but they didn’t realize that there was a spider living within the fruit plant when they infused it with the formula. This brings to mind the saying; “The road to hell is paved with good intentions” (author unknown, though likely Saint Bernard of Clairvaux). The spider quadrupled in size within 24 hours, and continued to grow exponentially to the point of quadrupling its size again within another 4 hours. This miraculous, terrifying spider had the ability to spray acid that could melt off a human face, was strong enough to penetrate a military tank with one of its long legs, had an exoskeleton so strong that not even bullets could penetrate it, and could move and jump incredibly fast and far.
The Science: While entertaining, this scenario is not backed by scientific principles (surprise, surprise!). According to National Geographic, the largest spider (arguably) is the Goliath Bird-eater Tarantula (Therapnusa blondi). With a leg span of around 11 inches, this arachnid is quite small in comparison to the spider portrayed in “Big Ass Spider!”. Spiders and insects are small in scale for various possible reasons, and discussing the inaccuracies of the movie will help to understand a few of these reasons:
The surface area to mass ratio comes into play as the spider is quadrupling its size. The surface area (what we commonly refer to as “size, but refers to the total area that the surface of the outermost portion takes up) increases to the square, but the mass (commonly referred to as “weight”, which is fine if you stay on Earth, though the two are very different elsewhere) increases to the cube. As the mass increases substantially, the spider becomes much more susceptible to the forces of gravity, which poses various issues to the survival of the spider:
First, the force of gravity pushing down on the growing spider would have made it impossible for the spider to move on their long, thin legs. Think of some of the largest organisms on Earth (such as an elephant or rhinoceros) and how thick their legs are. The limbs of the spider would have had to evolve quickly to become very thick and short in order to carry the extra mass, and it would not be possible for the spider to have the ability to run at great speeds or jump great distances as was portrayed in the movie.
In addition, as the spider increases in mass, the exoskeleton (surface area) would, in turn, become thinner. As the protective exoskeleton becomes thinner, the force of gravity acts upon the giant mass, forcing it to collapse in on itself (especially right after molting, before the exoskeleton has a chance to harden). In the movie, the spider’s exoskeleton was bulletproof, when in reality, it would be very weak (to the point that minor scratches can cause severe damage to it).
Another, and likely the most important, limiting factor to size in insects and arachnids is their mode of oxygen intake. Arachnids, like insects, utilize passive respiration in the form of diffusion to take in and disperse oxygen throughout their bodies. With our current atmosphere, this is a slow process compared to human breathing, so the organism must be small in order to circulate oxygen throughout the body efficiently using diffusion. In essence, the parts of an insect body that are more than half an inch from the air will always be low on oxygen, which is why most insects aren’t thick creatures. The gigantic spider in the movie would not have been able to take in enough oxygen to supply all of the enormous number of cells inside its body with a constant supply of oxygen. It would be impossible for the spider to survive unless it somehow evolved giant lungs while it was growing exponentially.
*Here’s a fun factoid: about 300 million years ago (give or take), some insects WERE much larger due to the oxygen-rich atmosphere (over 30-35% oxygen compared to our measly 21ish%) during the Carboniferous period. We are talking an organism that looks like a dragonfly, but has a wing span of over 2 feet (aptly named the griffinfly) and happened to be a predator. Though that size is still short of the house-sized creatures in our beloved sci-fi flicks, it’s still enough to give you nightmares!
While there are other possible reasons that bugs can’t become the size of houses, I think I have made my point. We will not have to worry about being eaten by giant spiders or gargantuan ants any time soon. However, these plotlines can be quite entertaining, and I hope I haven’t ruined the thrill-factor for anyone!
References (because old habits die hard):
Douglas, G. (Director). (1954). Them! [Motion picture]. USA: Warner Bros.
Mendez, M. Big ass spider! [Motion picture]. (2013). U.S.A.: Epic Pictures Group.
National Geographic. (n.d.). World's Largest Spider. Retrieved January 23, 2015, from http://video.nationalgeographic.com/video/tarantula_goliath