Vertebrates (like humans) have what is termed a “closed circulatory system”. Nutrients and oxygen are carried to all the body tissues by the blood, and the blood travels inside a system of tubelike vessels: arteries carry oxygenated blood away from the heart out to all the body cells, which are fed by a network of tiny capillaries. The depleted blood is then carried back to the heart by veins, and the heart sends the blood over to the lungs to become oxygenated and start the whole loop all over again. Vertebrate hearts have to be powerful enough to be able to pump the blood at suitable pressure so it constantly circulates around the body. Since the blood is completely contained within the network of blood vessels, this is termed a “closed” system.
Spiders, however, like most invertebrates, have an entirely different process. First of all, they do not have “blood”—they have “hemolymph”. In vertebrate blood, oxygen is carried around in the red blood cells by a protein called hemoglobin, which is based on iron. In spider hemolymph, however, there are no red blood cells, and oxygen is transported by a protein called hemocyanin which is based on copper (and that gives hemolymph a bluish-greenish color). Although hemocyanin is not as efficient as hemoglobin, it does the job well enough to power a small body like a spider.
(Not all vertebrates use hemoglobin—there are some Arctic ice-fish who have no red blood cells. Some invertebrates do use a form of hemoglobin, though, including the common earthworm. And some oddball invertebrates use more exotic proteins and have yellow or purple blood. But that would be a topic for another diary.)
In addition to having a different blood protein, the spider’s circulatory system is organized entirely different from the vertebrate. Instead of a closed system with blood flowing inside arteries and veins, spiders have an “open” circulatory system, in which there are no blood vessels. Instead, the inside of a spider’s body consists of a single hollow chamber that is filled with hemolymph, which floods the entire interior and bathes all the organs. This allows the hemocyanin to deliver oxygen directly to the body tissues. And instead of the vertebrate’s high-pressure heart which pumps blood through narrow vessels, spider hearts are low-pressure pumps, essentially a tube of muscle, whose job is simply to swish the hemolymph around inside the body cavity, allowing it to pick up oxygen from the spider’s book-lungs and carry it to the body tissues.
The spider’s legs, meanwhile, consist of long jointed tubes. Unlike vertebrate limbs, which have a hard core of bone that is surrounded by muscles on the outside that can manipulate them, a spider’s legs are hard exoskeleton on the outside with the manipulating muscles on the inside. But there’s another twist: spiders have only one set of muscles in their legs, which pulls the joints together to fold the limbs inward. To open its joints and push the legs outward, the spider uses a set of muscles in its body to push the hemolymph into the legs, thereby opening the joints and straightening the legs through hydraulic pressure, just like the cylinders on a hydraulic piston. So when the spider walks, it is using a choreographed sequence of muscles to pull its legs in and then allowing its own blood pressure to push them out.
When the spider dies, however, the muscles in the legs involuntarily contract, pulling the legs inward. Since the dead spider no longer has any blood pressure, there is no flow of hemolymph to straighten the legs again, and the spider curls into a tight ball as its single set of leg muscles contract. And that is why you always find a dead spider in that characteristic position.
So, what about Cockroaches—we seem to always find them dead on their back. Is that for the same reason?
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A recently-departed member of my pet Central American Cockroach colony, in typical “dead cockroach” position—flat on his back with legs in the air. The individual bugs come and go, but the colony lives on forever.
Nope. While insects like Cockroaches do have an open hemolymph circulatory system like a spider, unlike spiders they have a second set of muscles that pulls the leg joints out and they do not depend upon hydraulic pressure for that, which is why bugs can run so fast. The reason they are almost always looking at the sky when dead is because they have a relatively high center of gravity, and as the Cockroach dies (especially if it is killed by a chemical insecticide) muscular spams will usually extend its long legs wildly and push it over onto its back. Once there, the rounded surface of the Roach’s thorax is stable and tends to keep it there. If the dead bug is pushed or blown around, it is more likely to just rock back and forth or spin around on its back rather than flip over onto its belly. And that is how we find them on the kitchen floor in the morning.
And now it is your turn—what’s curled up today in your neck of the woods?
We are always looking for new people to produce Daily Buckets, where we practice “backyard science” by reporting and recording what we have seen in the natural world around us (and no matter where you are–even in the middle of the city–there is nature around you). You don’t need any science degrees or training or in-depth experience to be a backyard scientist, and you don’t need any expensive camera equipment or photo-editing software to do Daily Buckets. All you need is a pair of eyes and ears, a phone with a camera, and a lot of curiosity and enthusiasm.