You probably only think of spiders as the horrible venomous arachnids that use two of their legs to pry open your eyelids so they can inject your eyeballs with venom while you're sleeping. Turns out? When you look at how they evolved to produce that venom they get even scarier.


The Spider Venom Basics

Spiders are under-studied because, as experts themselves agree, they are terrifying bringers of death that if handled are likely to drop into our clothes and crawl into our underwear. Also there are a lot of them. Conservatively speaking, there are about 38,000 species of spiders, and over their 130-million year history, they have evolved complicated cocktails of venom.


They know how to use that venom precisely. When it comes to envenomation, spiders are more sophisticated than snakes, which generally have to eyeball a prey animal and determine the amount of venom to use in advance of the bite. Spiders can vary amount of venom they use during the bite. Whenever the victim is unexpectedly strong, or perhaps just more prone to struggling, the spider pumps in more poison. It's specially formulated poison, too. Different spiders will work up different cocktails of venom that work best for their particular prey animals.

And Then It Gets Creepy

That's fair enough; everyone's gotta eat. But then there are spiders that have little quirks in their venom. For example, the Australian funnel-web, argued by some to be the most deadly spider in the world, has one chemical in its venom that kills humans, monkeys, baby rats, and fruit flies. The chemical is so specific that it doesn't even harm adult rats — only baby rats, which would not be as dangerous to the spider as adults. The chemical doesn't kill anything else, including anything the Australian funnel-web spider actually eats. It's just a little gift for humans, baby rats, and adult rats that love their babies.

Widow spiders are a little more comprehensible. They have venom that's specific to insects, but they also have a neurotoxin in their venom which works only on vertebrates. (They also, by the way, have one kind of venom that works as a neurotoxin on crustaceans.) The toxin which targets vertebrates, called α-latrotoxin, seems to have resulted from a dramatic genetic change in widow spiders, but no one entirely knows why this change happened. Black widows, the most deadly of the group, have occasionally been documented eating lizards or mice that somehow became entangled in their web, but the vast majority of their diet is insects. It seems like one day the spiders took a dislike to anything with a spine and decided to expend a lot of energy evolving the toxin to kill it, whether its death would greatly help the widow or not.


Perhaps the most horrible example of a randomly effective toxin is the brown recluse spider. Recluse bites are rare, because the spiders tend to scamper off and hide rather than stand and fight, but when they happen they are nasty... eventually. At first, people who have been bitten by brown recluses only feel some itching and pain. Later, often, the tissues around the bite break down. In bad cases the flesh dies away to leave giant canyon-like pits that look like one of the curses of the Old Testament. This is confounding, because not only do brown recluses not eat vertebrates, the toxin doesn't hurt right away. It won't scare mammals off, but it will hurt them a lot well after they've forgotten they were bitten.


The chemical that causes the tissue damage is called sphingomyelinase D, generally shortened to SMD in medical literature. It's shared by the brown recluse in the Americas and their relatives, the assassin spiders of Africa. The two share an ancestor that goes all the way back to the supercontinent of Gondwana. Why did this toxin spring up? And why did it stay in the spiders so long when it does little to help them with prey?

When scientists looked only at the morphology of spiders to determine which spiders were related, they believed that only these two species of spiders developed the particular toxin, but a look at the DNA shows that that's not the case. The common ancestor of four different species of spiders developed SMD. Two lost the ability over the years, but the recluse spiders and the assassin spiders, despite avoiding humans whenever they could, clung to their ability to make people look like they're being ripped apart from the inside by a demon. Why? Just because, that's why.


Spiders Might Have Been the Original Spider-Men

Where did the brown recluse's ancestor get this venom? That's an interesting puzzle. The one clue we have is extremely strange; recluse spiders share a toxin in their venom with a certain bacteria, and with no other known species on earth. The toxin is exclusive to these two species, and scientists believe there might have been some kind of gene transfer between them.


Lateral, or horizontal, gene transfer has been known to happen between creatures. Genes hop between species, and the animal "gifted" with the genes can actually pass those genes down to its offspring. Generally, only bacteria can pass DNA back and forth between each other, but there is one kind of lateral gene transfer that can happen between bacteria and eukaryotic cells. (Eukaryotic cells are cells that have separated parts, like organelles and a nucleus, instead of letting everything float around inside the main cell wall. Your body is made of eukaryotic cells.) Plasmids, little packages of DNA, can transfer between bacteria and animal cells. The most famous recipient of horizontal gene transfer is a sea slug that can photosynthesize after eating algae and absorbing the algae's DNA.

That fact that this spider and this bacteria, and no other creatures, share one particular toxin indicates that a lateral gene transfer might have occurred — we just don't know which way it went. The bacteria might have picked up the genes from the spider. On the other hand, spiders might have been like Spider-Man himself — having a strange new interaction with an odd creature and suddenly getting strange new powers from that interaction. This might explain why they keep developing poisons that work on us instead of their food. If they kill us and eat us, they might one day absorb our power... and the new Spider Age will begin.


Bite Image: Jeffrey Rowland.

[Via Exceptionally Potent Neurotoxin in Black Widow Spider Venom, Glitch in the Evolution of Funnelweb Spider Venom, Binford Spider Lab, Complex Cocktails: The Evolutionary Novelty of Venom, Evolution Mystery: Spiders and Bacteria Share the Same Toxin, Spiders: An Evolutionary Detective's Best Friend]