Scorpions are notoriously creepy. Not only do they have eight spindly legs and several pairs of beady eyes like their spider cousins, but they've also modified their pedipalps into grasping claws for snatching up prey. And the most famous and intimidating scorpion feature? That venomous stinger-tipped tail, of course.
A new study using high-speed video footage of scorpion strikes has revealed that not all species wield their rear weaponry the same way. Scorpion tails vary quite a bit in size and shape, and these diverse weapons – not unlike varieties of swords or knives – are swung with differing techniques, speeds and degrees of precision.
A deathstalker scorpion's defensive strike. © Arie van der Meijden
A moveable stinging backside is pretty unusual in the animal world, and scorpions put it to several uses. When it comes to prey, they'll often grab with their claws and poke around until they've found an ideal spot to inject venom. Males even sting females during mating! But researchers from the CIBIO Research Center in Portugal were most interested in how scorpions use their stingers to ward off predators.
Scientifically speaking, a scorpion's "tail" is not a truly separate appendage like most animal tails – it's actually the last five segments of the body (together called the metastoma), tipped off by a stinging structure called a telson. The venom packed inside has deadly toxins that attack the bodies of the small bugs scorpions prey upon, but it also contains ingredients that are dangerous or even deadly to larger animals that might be looking for a scorpion snack.
"Among the almost 2,500 species of scorpions, there is a lot of difference in the relative size and girth of their 'tail'," says Arie van der Meijden, one of the co-authors of the study. "We wanted to know what these different tail shapes are for."
Previous research has shown that there is a relationship between the shape of a scorpion's metastoma and the potency of its venom. But why all this diversity in the derrière? "We suspected it might have something to do with their defensive behaviour, as that is the most strenuous behaviour they do with their tail," van der Meijden notes.
To investigate the dynamics of stings, the researchers gathered seven African species into the lab. These included West Africa's gigantic (we're talking up to 20cm) emperor scorpion, the famous deathstalker of northern Africa and the Middle East, and the Transvaal thick-tailed scorpion native to southern Africa, which is capable of spraying venom from its stinger.
Encouraging a scorpion to strike can be a bit harrowing for everyone involved, but van der Meijden stresses that safety is a top priority: "We take all precautions to handle the scorpions in a safe and scorpion-friendly way, such as using long rubber-tipped tweezers to handle the scorpions."
And what about the particularly dangerous species? "Only my more experienced students get to work with them!" he adds.
Once all the strikes had been struck and videos recorded, statistical analysis compared the path of the stingers in the different species. The large "fat-tailed scorpions" (genus Androctonus) swung their stingers fast and far, perhaps powered by strongly muscled backsides. But the emperor scorpion's strike was much slower, possibly due to its habit of relying more on its large claws for defence.
Video showing how the trajectory of a deathstalker scorpion strike was digitised. © Arie van der Meijden
Some scorpions swung their tails in very wide arcs, which increased their chances of hitting their target, while also boosting the speed of the strike. Others favoured shorter swings that allowed them to quickly return their stingers to starting position for the next jab.
So, are these different strategies attuned to the potential predators each scorpion might face in the wild? It's a possibility that van der Meijder has considered. "[B]ut currently there is no information available to test this," he explains. "We simply don't know enough about the predators that these scorpions come in contact with."
Across Africa and elsewhere, many animals will happily devour a scorpion meal, from bats and snakes to lizards and even other scorpions. But in order to figure out just how these arachnids deal with such threats – and to understand the evolutionary relationships between the scorpions' various weapons – researchers like van der Meijder need get to know their stingered subjects better.
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This study was published in the journal Functional Ecology.
Top header image: Arie van der Meijden
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