Last week I talked about the three basic ways to approach speculative biology, and this week I’m starting with the first: getting inspiration from real animals. For example, bugs. Sci-fi writers love bugs. They’re gross, they’re weird. They’re…tiny.
So why don’t we have giant bugs?
Steve Bein raised this question in regards to his sci-fi work-in-progress and I decided to help him out because
I expect him to help me with the Japanese main character of my work-in-progress that’s just the kind of guy I am.
The main reasons arthropods can’t get any bigger than a lobster (the largest recorded is 20 kilos) is their circulatory systems.* Athropods’ open circulatory systems are very inefficient because they don’t have blood vessels at all. Instead, blood (technically it’s “hemolymph“) is pumped by the heart (“dorsal blood vessel“) into the body cavity, where it sloshes around. If it sloshes close enough to the spiracles, it gets oxygenated. If it sloshes back up to the heart , it gets sucked in and pumped back out. If it doesn’t, it doesn’t.
You could just shove a vertebrate’s closed circulatory system into an arthropod and call it a day. Say the dorsal vessel extended branches out into the body, which branched further and so on until you have a full cardiovascular system with a discrete heart running along the back of the animal. But that’s boring, and you would still have the problem of respiration.
Maybe the animal has book lungs like a spider, or maybe the spiracles of an insect have branched inward, becoming an air-filled tracheal system intertwined with the fluid-filled cardiovascular system of blood. Each leg has its own “cardio-pulmonary complex” associated with it, plus a big one in the belly to feed the organs.
Rather than breathing in and out, these animals breath THROUGH, with air entering the system through spiracles near the head and exiting near the ail. Air is pumped by action of the muscular blood vessels that wrap around the tracheal tubes, or by muscular contraction of the whole abdomen (like a balloon inflating and deflating). Running also generates more flow-through.
Then you run into the second problem, which is structural…
Also, the kind of guy I am is f-ing verbose when I talk about speculative biology, so I’m splitting this post into three. Tune in Wednesday for a discussion of our giant bug’s exoskeleton.
*Given the oxygen in our atmosphere today. In the Carboniferous, there was more oxygen in the air, and the arthropod size limit was higher.