There's more to arachnids besides Spiders (Araneae) and Scorpions (Scorpiones). These two groups, along with the Mites and Ticks (Acari) are among the most familiar and famous arachnids, but there are several other less well-known groups, some of which are known to occur in Singapore, like the Whip Scorpions (Thelyphonida or Uropygi). Containing about 100 species, found mostly in tropical and subtropical climates in Asia, the Americas, and a single species in West Africa, these nocturnal creatures dig burrows in the soil or beneath logs, rotting wood, and rocks. They are mostly forest inhabitants, although one species, Mastigoproctus giganteus, is known to inhabit deserts and other arid landscapes in the southern United States.
Like Scorpions, Whip Scorpions are predators, feeding on insects, spiders, and other arthropods, which they seize and crush with their pedipalps ('pincers'). They have the classic arachnid body plan - four pairs of legs, a cephalothorax (head and thorax), and an abdomen. Unlike Spiders and Scorpions though, the Whip Scorpion walks on three pairs of legs; the first pair of legs are elongated, and serve as sensory appendages, much like the antennae of insects. As it moves about, the Whip Scorpion taps the ground with its first pair of legs, which provides it with information about its surroundings, and helping it to detect any prey nearby.
Whip Scorpions derive their name from the long, thin flexible 'tail' at the tip of the abdomen. Unlike in the Scorpion, there is no stinger. If threatened by another animal, the Whip Scorpion will rear up and spread its pedipalps, threatening to pinch anything that comes too close. Alternatively, it can turn around and spray chemicals from a gland at the base of its tail. While this chemical discharge, which contains acetic acid, is harmless to the skin, it can cause severe discomfort if it manages to enter the eyes, mouth and nostrils. And so the Whip Scorpion repels predators with a concentrated vinegar spray; the strong smell of vinegar after a close encounter with a Whip Scorpion has led to it also being called the Vinegaroon.