Venom, it turns out, is not a one-size-fits-all defense mechanism. Certain venomous creatures can modify the toxic stuff they spit out, depending on what they need to use it for. 

The Southern African scorpion, Parabuthus transvaalicus, doesn't just look mean—it's big, too (it can grow up to about 5 inches long). This particular species is known by several names, including the Transvaal thick-tailed scorpion, the Transvaal fat-tailed scorpion, the dark scorpion, and the spitting scorpion. It didn't have to work especially hard to earn that latter tag—these scorpions are capable of spraying venom out of their stingers. (This should go without saying, but it's advisable to stay out of range if you ever run into one.) The good news: humans aren't usually the target. This scorpion tends to feed on small invertebrates, such as crickets and cockroaches. In turn, it is preyed upon by slightly bigger animals, such as mice. 

In 2003, the journal of the United States' National Academy of Sciences published an article about a very special skill this type of scorpion possesses, titled: “One scorpion, two venoms: prevenom of Parabuthus transvaalicus acts as an alternative type of venom with distinct mechanism of action.” 

The paper's author, Dr. Bora Inceoglu, and his collaborators describe an interesting defense strategy that no other scorpion shows: venom economy. Generally speaking, most venoms are "expensive" mixtures of various components, many of them small proteins, which place significant metabolic demands on the organisms that produce them. Scorpions tend to live in dry, arid environments, with not a ton of food around. Because of this, it would make biochemical sense to save “ammunition." 

P. transvaalicus venom glands seem to always be preloaded with a so-called pre-venom, mostly composed of potassium chloride, a simple salt. In itself, this is not a toxic substance—but if it enters an open wound, it can cause quite a bit of pain. Like most salt solutions, this pre-venom looks clear (see below, left side). 

One scorpion, two venoms: prevenom of Parabuthus transvaalicus acts as an alternative type of venom with distinct mechanism of action. Proc Natl Acad Sci USA. 100(3):922-7. (Inceoglu et al. [2003])

When a mouse begins to nibble on one of these scorpions, it will, initially, sting the mouse using its nontoxic pre-venom. The pain inflicted on the predator using this technique is usually sufficient to make it run away. But if a mouse is determined (or hungry) enough, additional stimulation, in the form of bites on the scorpion's body, will induce it to produce the real deal: venom containing lethal toxins (shown as the slightly milky substance on the right side of the figure). 

The economy hypothesis explaining this behavior seems to be correct. Other researchers have reported that upon venom release, P. transvaalicus requires at least 3 days to “reload,” or regenerate, its venom. This regeneration seems to be linked to a higher metabolic rate in the scorpions—meaning, venom is actually a pretty costly investment in an environment where food is scarce. It makes sense to use it wisely. 

Remarkably, this is not the only type of organism with the ability to modulate its venom output. Certain venomous snails can do it too. "Venom economy” may well turn out to be a mechanism used by other animals, too. 

It's comforting knowing that a creature that could spray toxic liquid with wild abandon will probably choose not to, isn't it?