Arachnolingo 2: Venomous, medically significant, dangerous. Part I: What does "venomous" mean?

This is part 1 of an article that was so long it became a trilogy. You can click here for part two, about what "medically significant" means and how it differs from "venomous" and "dangerous", and here for part three, which explains why it's important to understand the difference. 

"-Is this spider venomous?
-It's not medically significant."

When you just want to know if you're in any danger around an animal you just found in or near your home, this answer can easily seem needlessly complicated, and even frustrating.
It's tempting to assume that the person saying "medically significant" just wants to sound clever by using big words when they could just use simpler terms such as "venomous" or "dangerous".
However, these words are not interchangeable, and it's important to avoid mixing them up. Here's why.

Almost all spiders, including this trapdoor spider (Nemesia sp.) are venomous. However, like most spiders, this one is not medically significant.

 

What does "venomous" mean?

It's actually pretty simple or extremely complicated, depending on the level of detail you want to go into. The broadest definition of "venomous" is "an organism which secretes and uses venom". Simple, but unhelpful, as it directly leads to the much more complicated question of "what is venom?". That one goes into deep levels of complexity, and is still not fully agreed upon by scientists worldwide.

The common definition of "venomous" in the English language, the one most often found in wildlife documentaries and other sources intended for the general public, is based on the delivery mechanism: a venomous animal has to inject its poison into the target organism's system, while species which use toxic substances but don't inject them are poisonous. 

"Venom is injected, poison is ingested". Is that the definition of venom? Sounds simple enough...
Too simple, actually, to fully reflect the diversity of toxic substances and their uses in the natural world. Its main issues can be summed up with the following examples:
- a spitting cobra's venom is both cytotoxic and neurotoxic, and, injected through a bite, can cause extensive tissue damage. However, spat in the eyes of a mammal, it also immediately attacks the corneal tissue and causes extreme irritation by mere contact with it, without entering the blood flow (cornea is avascular tissue, it does not have blood vessels). Therefore, spitting cobra venom can have marked toxic effects, even when it is not injected.
- Indigenous G/ui and G//ana people of central Kalahari, in Botswana, hunt medium-sized to large mammals using arrows poisoned with the raw, unprocessed contents of the banded orb weaver's (Argiope australis) abdomen. Interestingly, it's not the spider's venom (harmless to mammals) which is used, but the guts and haemolymph, which seems to act as a poison potent enough to kill an animal within two to four hours. The absence of precautions to avoid contact of the poison with the hunter's skin, eyes or mouth suggests it is safe if ingested or absorbed through the skin, and has to enter the blood flow to take effect.
- European and North American water snakes (Natrix spp. and Nerodia spp.), along with some other genera of "nonvenomous" snakes, nonetheless have a Duvernoy's gland, whose secretions are toxic to animals such as mice, and are believed to help subdue their (mainly fish and amphibians) prey. While these snakes have no fangs nor grooved or enlarged teeth, or muscles to contract the glands and deliver their contents, the secretions continuously mix with the saliva and passively enter the prey's system when the snake bites and holds on. Thus, these secretions are now commonly recognised as venom, even though the glands aren't connected to any type of specialised delivery apparatus.

While its venom is harmless to humans and other large mammals, Argiope australis is surprisingly used by some Indigenous people of the Kalahari to poison their arrows; they do not use its venom, but the contents of its abdomen, which seem to cause deadly poisoning when entering an animal's blood flow

As these few examples show, the black-or-white, injected or ingested, distinction of "venom" and "poison" is not enough for biologists to work with.
Just like everything in the living world, the venomous apparatus, and the venom, evolve constantly; there's a huge diversity of substances chemically or functionally akin to venoms, and also an enormous range of different structures to deliver those substances.
Both venom and the delivery apparatus generally evolve from pre-existing substances (in vertebrates, it's often saliva or skin secretions) and structures (such as teeth, spines or fin rays) that get, with time and natural selection, modified for use as a hunting or defensive weapon.
Therefore, at least initially, these features (the venom and the delivery apparatus) evolve independently; venom-like chemicals can exist without a specialised delivering apparatus (case of the Natrix water snakes), or even without anything to inject them at all (case of toad parotoid gland secretions, for instance).

Thus, a definition of "venom" solely based on its mode of delivery would lump together substances that don't have much in common besides how they are delivered, while artificially excluding secretions that have everything in common with most of those clearly identified as venoms, except the way they're delivered. That's why scientists need a delimitation that gets more specific than that.

Although there are several definitions of venom, which, depending on authors and disciplines, vary slightly in the details, they all combine a few common criteria:
- venoms are complex substances, made up of a variety of components (toxins), including proteins.
- venom is produced by the animal which uses it (autogenous), through a dedicated and specialised secretory apparatus.
- venom has a harmful effect on other living organisms (at least on some of them) and is produced for use as a weapon against them, be it to help subdue potential prey, defend themselves against potential predators, or both.
- in most cases, there is some kind of apparatus (which can be a simple as a spine or claw) which helps delivering the venom into the target's system by creating a wound. However, if a substance fulfils the previous criteria but not this one, it can still be regarded as venom. Toad secretions, for instance, are generally classified as venom¹, although they aren't equipped with anything to inject them.
Meanwhile, dart frogs do not secrete their own poison, but store and concentrate toxins they find in their food; it does therefore not fit the definition of venom, although it is used in pretty much the same way as toads use theirs.

The difference between "poisonous" and "venomous" in common language is thus not in line with the current definitions of "venom" used by biologists; poisonous animals with venom are a thing!
How simple is that now?

According to the colloquial distinction between "venomous" and "poisonous", toads (family Bufonidae) are poisonous; yet their poison is classified as venom¹. Simple, right?

This makes things very complicated to the layperson; actually, they're even complicated to scientists who study them, as the exact definition of "venom" is still debated in the scientific community.
The good news is, while does matter to biologists, none of that is very important in everyday conversations.
Some people will never miss an opportunity to smugly correct those who say "poisonous" instead of "venomous", and act like it's a very big deal, but the truth is, this distinction is actually of little concern to non-biologists.
Even scientists themselves are not that strict about it: the same secretions will sometimes indifferently be referred to as "poison" or "venom" in the literature, and even phrasing as "heretical" as "poisonous snake bite" will sometimes be tolerated in scientific publications.

The important point in all this is to understand that none of the definitions of "venomous", "poisonous" or "venom" are based on any notion of danger to humans. As Scott Weinstein explains so well, "it is important to again re-emphasize that the coincidental medical effects of snake venoms should have no role in their definitions, as these were evolved long before humans".
To put it shortly, venomous does NOT mean dangerous to humans.
An animal can very well be venomous and completely harmless to humans; this will not challenge its classification as a venomous species in any way. Being harmless doesn't imply the animal is nonvenomous, and being venomous does not necessarily mean it is harmful to humans.
Actually, venom is incredibly widespread in the animal kingdom: nearly all the major animal groups include at least some venomous representatives.
While snakes, spiders, scorpions and many insects are famous for being venomous, many other lesser-known animals, such as some worms, snails, mites, pseudoscorpions, corals, and even some mammals also are.

Pseudoscorpions (which are a different order from true scorpions and are not closely related to them) are venomous, but they're also completely harmless to humans

All spiders, except the family Uloboridae, and the two species in the genus Holarchaea in the family Anapidae, which seem to have lost venom glands, are venomous. All scorpions are venomous.

The family Uloboridae appears to be the only spider family (out of 135) to lack venom glands

However, only about 1% of the 50 000+ known spider species, and less than 10% of the ~2200 described scorpion species worldwide are equipped with venom that can cause clinically serious symptoms in humans.
Actually, bites or stings from most venomous animals pose little to no threat to human health.

Less than one in ten species of scorpions is capable of causing serious envenomations in humans, and almost all of them, such as this Buthus occitanus, belong in a single family, the Buthidae

The reason for that is simple: venom is a weapon used by animals to subdue prey, protect themselves against predators, or both. That means the main selective factor driving the evolution of venom and venomous apparatus in a species is its efficiency against its favourite prey items and/or its main predators.
There is no living venomous animal species whose favourite prey or main natural predator is human. Therefore, efficacy against humans isn't a driver of selection in the evolution of their venoms; there is no known instance of an animal species whose venom evolved specifically to harm or kill humans.

The thing is, resistance and sensitivity to a toxic substance varies a lot from one group of living organisms to another; what's deadly to a fly can be harmless to a human, and vice versa.
It is, for instance, a well-known fact that slugs can safely eat mushrooms so toxic to humans that a few grams are enough to kill an adult.

To humans, the poisonous deathcap (Amanita phalloides) is highly toxic, and even a small piece of one of these mushrooms would be enough to kill an adult. However, it is not so toxic to slugs, which can happily and safely munch on it.

The vivid colours of the elegant grasshopper (Zonocerus elegans) warn predators that it is poisonous, as it stores toxins from the plants it feeds on. However, it is also consumed by some human populations who enjoy its pungent taste, apparently without perceptible harmful effects.

Same goes for venoms. Studies have shown, for instance, that widow spider (Latrodectus sp.) venom is considerably more toxic (in mg of venom per kg of animal) to mice than to frogs, much more to flies than to cockroaches, and that guinea pigs are much more sensitive to it than mice.

To us humans, that means that, as we are not the intended target, most animal venoms only have mild effects on us.

Scorpions feed mainly on insects and other arthropods; therefore, most species are equipped with venom that will kill insects rapidly, but will only have mild, local effects on humans and other larger mammals.

Species whose venom is significantly harmful to humans are generally those that hunt mammals on a regular basis or have frequent hostile interactions with them, or those whose venom contains broad-spectrum toxins, active on a very wide variety of animal species.
In some rare cases, it can also be an uncanny coincidence, where humans accidentally happen to be particularly sensitive to a toxin while not being its intended target at all: this is, for instance, the case of the δ-hexatoxins found in the Sydney funnel-web spider's (Atrax robustus) venom, which induce strong pain in its small mammalian predators (rodents and marsupials), but, as a result of an evolutionary "accident", are highly toxic to primates (which didn't evolve alongside them), including humans.

As the vast majority of venomous animals species do not fall in any of these categories, most of them do not represent a significant risk to human health and life.

It is, therefore, critically important to avoid mixing up "venomous"(or "poisonous") with "harmful to humans", and, conversely, not to understand "nonvenomous" as "harmless". These words are not synonyms. Not even close.

Nonvenomous does NOT mean harmless

 

But what about the animals whose venom does represent a risk to human health? How should we call them?

Find out in part two.⏹

Except when the source is explicitly cited, the images illustrating this blog are mine and are not free to use without permission.

References are integrated into the text of the article; the words in blue are clickable and will redirect you to the sources of the information.

¹ The source for this information is a search in Google Scholar, with the use of the operator " " to restrict results to articles explicitly containing the terms "toad poison" or "toad venom" (because a simple search of the terms "toad poison" without the quotation marks will include results containing the broader term "poisoning" which can apply to both poison and venom). The search for the terms "toad poison" yielded 553 results, while "toad venom" yielded 3500 results, showing that biologists studying toxic secretions from toads much more commonly refer to them as venom rather than poison.