Scuttlebutts

Out of all the iconic Halloween animals, one in particular is so creepy-crawly, it never loses its special spookiness at any point during the year: the spider. They come in all kinds of different sizes, colors, abilities, and, for some species in certain parts of the world, even flavors. (Yes, spiders are edible. No, I probably will never eat one.) But something I wondered about was how these animals move.

A tarantula found while hiking up the Mogollon Rim in late
October, 2018. PC: Jessica Noviello.

One of the most basic things about spiders is that they are arachnids, the biological class that also includes scorpions, ticks, mites, harvestmen (aka "daddy longlegs"), and camel spiders (which are not true spiders, it's just the name). Spiders make up the largest order of arachnids. Usually a rule of thumb for telling an arachnid from the more common insects is the number of legs, since arachnids have eight and insects usually have six. As usually happens in science, the answer is a bit more complex. Some mites, which are classified as arachnids, actually have six legs. Some mite species even have four legs! There are other arachnids who start their lives with six legs, but then grow more legs as they moult.

Another fact about spiders is that they have an exoskeleton, a characteristic they share with insects. It's made out of chitin, a similar flexible material similar to the keratin of human fingernails and hair. An exoskeleton is the support system for an insect or arachnid that exists on the outside of its body, keeping all of the squishy parts inside. We humans have an endoskeleton, which is all of our bones. They serve as the base for our muscles, organs, nervous system, and every other system in our body. The way our bodies move is our muscles pull and contract themselves to move tendons and ligaments, which also make our skeletons move. Our movements can be big, like a jump, or small, like picking a piece of lint off of our clothes.

Spiders don't have true muscles because they have exoskeletons. So how do they move?

The answer is that most spiders use a combination of primitive muscles and hydraulic (fluid) pressure, similar to what many powerful machines use to distribute and lift heavy weight in the human world. First I'll explain the muscles. In humans, there are flexor and extensor muscles that make the surround joints open and close, respectively. Spiders only have flexor muscles, which make their joints turn in. This is why spider legs curl inwards when the spider dies.

To make up for the lack of extensor muscles, the spider uses a hydraulic system. The fluid that spiders and other arachnids (and insects too) use is called hemolymph, which is similar to blood in animals with endoskeletons. This hemolymph is mostly made up of a watery plasma, certain chemicals like amino acids, and hemocytes, which are part of a spider's immune system. This hemolymph surrounds the spider's organs outside of any kind of enclosed circulatory system like what we have, and is called an open circulatory system.

The inside structure of a spider. Used under the Wikimedia Commons License.

A spider does have a heart, though it's not nearly as good at pumping blood quickly as human hearts are. This heart can pump the hemolymph into arteries that simply end, spilling out the hemolymph and allowing it to surround the spider's organs. To get the fluid down to the legs, the middle part of the spider's body, called the cephalothorax, pumps the fluid down the arteries in the legs, working like a bellows or accordion. This is how spiders crawl, scuttle, dart, climb, and jump!

A female zebra jumping spider. Used under the Wikimedia
Commons License.

A final fact about spider locomotion is that spiders are not just limited to the ground. Some spiders use their silk to make "parachutes" that carry them hundreds of miles. The special silk they use to do this is called dragline silk, and is so fine that even a small breeze can tangle it up, forming a "balloon" that the spider can ride. Every gust of wind carries the spider further until it eventually lands somewhere, which can be as much as 200 miles away! This is how arachnologists (scientists who study spiders) think spiders travel among islands, often being the first animals to inhabit new volcanically-formed islands. There are still no mathematical models that accurately describe how far a spider can travel using this method.

Turns out there's a lot to say about how spiders move, which is usually what I discover when I start writing a blog article. Here's a list of fun facts about spiders in general that are guaranteed to shock and inform people at any Halloween party:

1) There are 1,000 named species of tarantula worldwide, and the ones found in the Americas often have hair that is irritating if touched. The hair is used to scare of potential predators and curious humans. Tarantulas from other parts of the world generally have more potent venom in place of hair.
2) As of July 2019, there are over 48,200 spider species named by taxonomists, but how these are classified is still up for debate, as there have been 20+ different classifications proposed since 1900.
3) Spiders legs have seven joints!

The seven joints of a spider's leg. PC: InfiniteSpider.com/Eky.edu

4) One species of jumping spider found in Central America, Bagheera kiplingi, is the only known herbivorous spider. All others are predatory, killing about 400–800 million tons of prey per year. Most of that prey is insects.
5) Almost 1,000 species of spider have been described in the fossil record. The oldest spider found preserved in amber is 130 million years old, and the oldest web fossil (which is a thing?!) is 100 million years old.
6) While most spiders live at most two years (unless killed or eaten, of course), tarantulas can live for decades. There is at least one story of a tarantula even befriending a researcher who visited her burrow for years, which shows they can remember individuals.
7) Female spiders are generally larger and more venomous than male spiders, but it depends on the species. This is an example of sexual dimorphism, where males and females of a species appear physically different.

Sources:
https://www.youtube.com/watch?v=FlKago05Lxg
https://asknature.org/strategy/leg-uses-hydraulics-and-muscle-flex/
https://www.newscientist.com/article/dn9536-how-do-spiders-travel-such-epic-distances/
https://infinitespider.com/spider-legs-work/
https://en.wikipedia.org/wiki/Spider