The term “venom” is used to refer to any poison that certain animals (or, I suppose, even some plants[1]) inject into their victims. This couples a toxin with some sort of injection system, like a stinger, or a hollow fang, or poisoned hairs. While there are a lot of venomous animals in the world, arthropods (and particularly insects) account for the vast majority of the ones that humans are actually likely to encounter. Which raises a bunch of questions:

1. What benefits do arthropods get from venom production that makes it so popular? and
2. How does one go about evolving venom?

Let’s start with the benefits. There are basically two, one if you are the hunter, and one if you are the hunted.

(a) For the hunter, venom allows you to tackle much bigger prey with minimal risk to yourself. Without venom, you need to be able to physically overpower your prey to keep it from squirming free while you eat it to death (like a praying mantis does).

If you have venom, on the other hand, you just need to get in there and give your prey a quick shot of the juice, then just keep it from getting away until your presumably fast-acting toxin either paralyzes or kills it.

And, if your venom doubles as digestive juice, it can dissolve the contents of your prey, allowing you to suck out its insides.

Which is actually quite a benefit when you think about it, because this means that you don’t have to physically pry off its exoskeletal armor to get to the meat underneath.

So, a lot of fairly fragile hunters (like spiders, scorpions, centipedes, predatory “true bugs”, and parasitic wasps) have venoms specifically to help give them an advantage over prey that they otherwise wouldn’t be able to tackle.

For this application, you want your venom to be fast-acting, and maybe an aid to digestion. Causing pain is largely irrelevant.

(b) For the hunted, your goal becomes defensive. It is a way to fend off predators larger than you. Sometimes much larger (like, say, mammals blundering into your nest). Here the risk to yourself is much less of a concern – when something is trying to eat or squash you, all you want to do is make things as unpleasant as possible for your aggressor. So, even if you die or are horribly maimed, at least the predator will think twice about trying to eat any of your relatives in the future. This is particularly useful if you live in a colony, where your sacrifice can save your relatives.

For this application, lethality is overrated. It is hard to produce enough toxin to actually kill an animal that is, say, 10,000 times your size. But pain! Ah, pain we can provide! It takes much less toxin to kick the old pain sensors into overdrive than to, say, stop their heart.

So, it looks like the big reason why venom is so popular with arthropods is that they are so small, and the need for dealing with things that are bigger comes up a lot for them. While the two uses for venom are superficially similar (and it is possible to use hunting venoms for defense, and vice versa), you do tend to end up with significantly different venom characteristics depending on who it is you intend to use it on most of the time.

Evolution of Hunting Venoms
When hunting, the key thing is that your prey is going to be your food. Which means you are going to digest it. And to digest, you need chemicals to break their bodies down to simpler molecules. So, everybody is already producing digestive juices of various sorts right from the start.

It is pretty common to have saliva as a “pre-digestion” juice to get things started, before the food gets into the stomach where digestion can get going in earnest. Since the saliva is produced in the mouth, it is already right handy for when you start to eat your prey. As a result, the majority of arthropods that use venom as a hunting aid use saliva as the base.

One point I note is that, if you consider the arthropods that use modified saliva as a hunting venom, they all seem to have piercing-sucking mouthparts – spiders have the fangs in their celicerae, which both inject venom and suck up juices, and true bugs have a single piercing-sucking mouthpart for similar purposes. This suggests that they may always have been sucking juices, but without a saliva that really dissolves the flesh well, the yield of food would not have been very good from any given prey item. Even now, I’ve noticed that some spiders and predatory bugs are much better at dissolving the innards of their prey than others. Many of them leave a recognizable corpse, while some others will leave nothing but a pile of crumbled exoskeleton. So the potency of the venom is likely to be directly proportional to its efficiency as a digestive juice.

The arthropods that have hunting venoms are the ones that will actually “bite”, using their mouths. They may bite humans, but the occasion doesn’t actually arise much and they usually won’t be going out of their way to do it (I have yet to be bitten by a spider, myself, and I’ve certainly given them both cause and opportunity often enough). And the pain from their bites is caused by direct tissue damage as their venom digests the immediate skin. This is why a lot of spider bites aren’t generally all that painful,[2] at least not compared to stinging insects.

There are also quite a number of arthropods that use venom in hunting that is not derived from their saliva. For example, centipedes have “poison claws” on either side of their heads that are modified legs;

pseudoscorpions have venom in their claws;

scorpions have a venom gland and injector at the tip of their elongated abdomen; and predatory wasps also have a stinger at their abdomen tips.

These venoms don’t have a digestive function, they are just intended to make the prey stop moving as quickly as possible. Some of these non-saliva venoms are also quickly lethal, killing the prey so that it can be eaten on the spot. Others, though, particularly the venoms used by wasps, are intended just to paralyze the prey without killing it. The wasp then lays its egg on the still-living prey, which keeps it from spoiling before the wasp grub can finish eating it. The key thing here is that the objective is to kill or paralyze. They are still likely to be painful, but only because the pain is a side effect of the lethal or paralytic effects. Causing pain for its own sake is only useful when you start using venom in a defensive role.

Evolution of Defensive Venoms
Defensive venoms don’t seem to be mostly associated with the mouthparts, they are usually in a stinging organ. In the case of the most common group of stinging insects, they Hymenoptera (bees, wasps, and ants), the stinger is derived from the ovipositor. The ancestral Hymenoptera evidently actually used its ovipositor to cut holes in plants and then laid eggs through it, the way that sawflies still do. The branch of the Hymenoptera that became ichneumon and braconid wasps became parasitoids of other arthropods and developed long, complex ovipositors to inject their eggs, but they still do pass their eggs through them.

The ones that still use their ovipositor to oviposit are the harmless ones, or at least the ones that aren’t going to hurt much. However, some of them started also injecting venoms into their prey along with their eggs. And over time they separated the functions so that the former ovipositor became the venom injector, while the egg was actually laid separately.[3]

Once the egg-laying and venom injecting functions were separated, the potential to use the venom for purposes other than hunting came up. In addition to using the venom on their prey, they could also use it to fend off predators. This opened the door to the social insects. See, a big colony of insects has the disadvantage that it is a concentrated source of food that predators would happily seek out to eat if it isn’t defended. But, once the hymenoptera had stingers, that all changed. If the non-reproducing workers are armed with this defensive weapon, then they can make this otherwise-attractive nest suddenly become extremely unattractive. Now, rather than just being a giant sign to predators saying “Eat Me!”, a big nest of social hymenopterans is instead a huge warning saying, “If you try it, you’ll be sorry!”

And for this function, pain suddenly becomes the primary objective. The paralytic, toxic, and digestive functions suddenly all go by the wayside, and the venoms become chock-full of various types of “Liquid Pain”: in bees, mostly Melittin, with smaller amounts of Apamin, phospholipase, hyaluronidase, histamine, tertiapin, and dopamine which have various effects like increasing blood flow to spread the venom, causing damage to nerves which causes pain signals, and inducing allergic responses to prolong the period of pain and discomfort beyond the time when the venom effects would otherwise have dissipated. The effects of wasp and ant venoms are similar, but they have significantly different compositions, and so if you develop an allergic reaction to bee venom, this does not necessarily mean that you will also be allergic to wasp or ant venoms (or vice versa).

And one last point, concerning allergic reactions: from the point of view of a bee, wasp, or ant, inducing an allergic reaction with their stings is an outcome greatly to be desired. It basically amounts to persuading their victim’s immune system to over-react, and start doing damage itself beyond anything the venom could possibly have done on its own. If the stinging insect is lucky, the allergic reaction could even be severe enough to be fatal. However, they can’t depend entirely on the allergens, for a couple of reasons. First, nobody is allergic to anything at first. You have to be exposed to the allergen at least a few times to prime your immune system, and so the stinging insect needs to still have the pain-making compounds in their venom to take care of first-time nest attackers. The other problem with depending on the allergic reaction is that it is not reliable. The victim has to get stung on a particular schedule, just often enough for the immune system to recognize the venom as a thing, but not so often that the immune system figures out what the venom is and learns to ignore it. So only some people develop an allergic reaction, and the reactions vary all over the place from just non-life-threatening local swelling (the most common reaction) to full anaphylactic shock (which is much, much rarer, but can be fatal if not treated).

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[1] While the list of poisonous plants is very long, and contains an alarmingly large number of plants that we actually eat anyway, most of them deliver their toxins by being eaten, and so aren’t actually venomous in the strict sense of having injectable poisons. There are a number that actually inject venoms through stinging hairs, though, of which the most well-known are the nettles. There are also a number that just ooze irritants onto your skin when you touch them, like Giant Hogweed and Poison Ivy. Although, surprisingly, the general opinion among botanists is that urishiol (the irritant in poison ivy sap) is not really of defensive benefit to the plant (it doesn’t appear to affect non-primate mammals particularly, and deer eat poison ivy with no difficulty). It is a chemical to aid in water retention, that just happens to be highly sensitizing to the human immune system.

[2] I’m reading an interesting book at the moment, The Brown Recluse Spider, by Richard Vetter. It’s basically all about how the things that people think they know about the infamous Brown Recluse are actually not so. For example, around 90% of Brown Recluse bites are only slightly painful, and heal up within a couple of days. The other 10% of bites, the ones that lead to necrotic ulcerating wounds and maybe potentially-lethal systemic reactions, are just the ones that get all the press. And then there is the point that every year there are hundreds of reports of “brown recluse spider bites” in places like Canada, where the brown recluse has never been found.

[3] Incidentally, this is why male bees, ants, and wasps are harmless: since the stinger is derived from an ovipositor, it is a sex-linked characteristic, and the males don’t have stingers. So, if you have access to a hive of bees and want to cause a ruckus without any actual chance of anyone getting stung, just go in and catch a bunch of drones (you can tell which ones they are, because they are significantly bigger than the workers, and have eyes covering almost their whole head). Then let your drones loose at a party somewhere, and see how people react.