The University of Chicago Magazine
As a zoologist whose research has focused on the biomechanics of marine invertebrates, I am glad to report that Hollywood has not forgotten our spineless relatives. My all-time favorite for quality of the special effects (Ray Harryhausen again!) is It Came from Beneath the Sea (1955), a tale of a gargantuan, radioactive, deep-sea octopus whose hunger compels him to head for the coast.
After snacking on a couple of freighters, the monster discovers San Francisco, where he adds a few police cars, railroad boxcars, and the Ferry Building's clock tower to his diet. The most famous scene occurs as the monster reaches up out of San Francisco Bay, entwines its tentacles around the Golden Gate Bridge, and pulls it down.
From this point on, the creature is strangely passive. It grabs an attacking submarine, but simply holds on, making no attempt to crush or bite it. And it ignores a scuba diver who swims directly in front of its eye, even when the diver shoots a spear into its brain. Finally, the monster is dispatched with explosives and the movie ends. Pulpo, anyone?
What happened at the Golden Gate Bridge to change completely the creature's behavior? The movie's makers may not have had a clue, but I'll offer a very simple explanation. Any time you have a column of water extending vertically, a pressure is generated at the bottom; one atmosphere of pressure (approximately 14.7 pounds per square inch) is produced for each 33 feet of height. The giant octopus, like all macroscopic animals, has a circulatory system extending throughout its body--in essence, pipes filled with water. If the octopus stretched its tentacles vertically while submerged, nothing would happen; the pressure increase on its tentacles would be precisely matched by the pressure in the surrounding water.
However, before pulling the bridge down, the monster extends his tentacles about halfway up the support towers. Their tops stand 500 feet above the deck, itself 220 feet above the high-water mark. (We'll ignore the tide.) At one atmosphere for every 33 feet and an elevation of 470 feet, that's a total pressure of about 14 atmospheres (209 pounds per square inch). With no surrounding mass of water to offset the increase, the full load of this pressure would act to distend the octopus's arteries--arteries that are simply not constructed to withstand such high internal force.
The evidence clearly points to the poor cephalopod suffering a sudden and massive cerebral hemorrhage, just as it rips down the Golden Gate Bridge. Its subsequent passivity now makes perfect sense. With its higher faculties gone, all that's left are peripheral reflexes--grabbing the sub in response to tactile stimulation, twitching when hit with the diver's spear. Rather takes the edge off the finale's "heroic" human actions, doesn't it?
For even higher camp, you can't do better than the Japanese monster movie Mothra (1962). Two 6-inch-tall women are kidnapped from a Pacific island by a showman who plans to make his fortune by exhibiting them. That dastardly deed somehow causes a giant egg to hatch into a gianter caterpillar that swims across the Pacific, devouring everything in its path and getting bigger by the minute. On landfall, the now-huge caterpillar takes a crawling tour of Japan, crushing a large portion of Tokyo and then worming up a radio tower, where it spins a cocoon and emerges a few days later as--you guessed it--a moth with the wingspan of a couple of 747s.
The downdraft of those wings completes the destruction of Tokyo: Buildings blow down, cars fly through the air. The authorities admit defeat and (stop reading here if you don't want the ending spoiled!) the tiny women are brought to the airport where Mothra lands; after the ladies climb aboard, the monster flies off over the Pacific, never to be seen again.
Caterpillars, even normal-sized ones, are peculiar beasts. Unlike adult insects, which have rigid external skeletons, caterpillars have a flexible skin and support themselves with a hydrostatic skeleton--a volume of incompressible fluid that transmits forces and pressures from muscular contraction through the body. In essence, they are animated water balloons with the incompressible fluid (the blood, filling all of the body cavities) surrounded by a tension-resisting skin.
Like the bridge-grabbing octopus in It Came from Beneath the Sea, Mothra should generate some extreme pressures as it crawls up the radio tower. What's more, the tensile stress in a pressurized cylinder (a reasonable model for a giant caterpillar) is directly proportional to its radius; since the caterpillar has a radius of about 30 feet, the stresses are going to be extreme, to say the least. Given that the caterpillar survives its vertical excursion, it would well repay the scientists' efforts to check out the discarded skin after Mothra leaves its cocoon: That skin must be reinforced with something having a tensile strength well in excess of steel.
Enlarging an insect to Mothra's girth raises other interesting problems that don't arise with super-sized cinematic vertebrates like Kong. Take the respiratory system. Rather than inhale air, extract oxygen in the lungs, and transport it in the blood as we do, insects have a tree-like network of trachea that extend through the body and open at one end to the atmosphere. Large insects may actively draw air into the network's outer portions, but the inner region's smaller tubes are not ventilated. Instead, oxygen simply diffuses down the tubes.
It's a remarkably efficient system: Transport of respiratory gases costs the animal nothing, increased demand for oxygen at any location automatically increases the local rate of supply by increasing oxygen's concentration gradient, and the system is easily modified to better deliver oxygen to particular regions. (For example, in the flight muscles of insects where the oxygen demand is extreme, the tips of individual tracheal tubes penetrate the cell membrane, directly delivering oxygen to the mitochondria in the muscle cell's cytoplasm.)
However, because the system is diffusion-based, it does have limitations. The oxygen-delivery rate is directly proportional to the tube's concentration gradient, inversely proportional to its length, and directly proportional to its cross-sectional area. Ah, but demand for oxygen is going to be more or less proportional to the animal's biomass or volume.
As an insect gets larger, demand for oxygen increases in proportion to length cubed, but--if its shape stays constant--rate of supply increases only as length squared. You could decrease demand by having the insect live a more leisurely life, but that's not an option if Mothra is ever going to get off the ground, since flying is a very power-intensive behavior.
The upshot is that Mothra will need a lot more tracheal tubes to maintain a sufficient oxygen supply. Of course, the more of its volume that is tracheal tubes, the less is biomass that needs oxygen, but this implies that, although Mothra may be heavy (because it's big), its density will be very low--about the same as a cotton ball.
This insight into Mothra's physiology eliminates two other problems. Although wearing one's skeleton on the outside has distinct mechanical advantages, large insects are prone to a mode of failure called buckling. If Mothra had really been just a scaled-up moth, its legs would have collapsed when it landed. Second, Mothra's wings are in the same proportion to its body as the moths that bat their heads against the lights outside your door. Total lift generation is proportional to the area of the wings; if mass increased in proportion to volume, Mothra would be grounded. In any case, if Mothra did manage to take off, its wings might produce a sizable downdraft (although not nearly enough to blow over buildings), but its low density implies that it will be forever at the mercy of the Pacific winds.
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